is backpressure hurting your combo?



is backpressure hurting your combo?

Postby mike » October 14th, 2008, 10:01 am

an automotive engine will run best with little or no back pressure on the exhaust system , back pressure ALWAYS tends to reduce the cylinder fill and scavenging efficiency,thus it always going to reduce the potential power production of a correctly tuned engine. you can get a very good idea, as to your exhaust systems back pressure if you have a muffler shop in your area that will test the back pressure in your car for you with a pressure gauge and a DB meter for noise levels., but if you do, you will be one of the very few people that bother to test and deal in FACTS , PROVEN BY TESTING,rather than make guesses like the vast majority.
I'm certainly not picking on anyone here, but I see similar responses all the time,
when I ask what the back pressure in the header collectors reads on a gauge, at peak rpm levels,
and where guys just almost always assume the exhaust is fine simply because they read a chart some place that said for example a dual 2.5" exhaust will handle 400 hp or some other figure.
yes theres other factors at work here, but don,t over look a potentially restricted exhaust with out testing.
when I ask about back pressure at peak rpm levels and get that deer in the head lights look, and I,m amazed that almost everyone assumes that the exhaust must be fine! and so few guys take the time to get out a vacuum/pressure gauge , and plumb in a bung and port adapter and actually measure the exhaust back pressure at peak rpm levels, I can remember only one other guy I've dealt with in the last 45 years who had actually done that and used a fuel/air ratio meter to help tune his car before I got involved and asked about the exhaust back pressure and individual exhaust port temps. which can be very useful in locating tuning problems or fuel/air ratio distribution issues


heres the one I use and recommend
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http://www.professionalequipment.com/ex ... ermometer/
you can,t make intelligent decisions without facts, and you get the facts by testing!
and it should be obvious that testing needs to be done under the current operational conditions at the rpm range that your concerned with.
A stock automotive exhaust might have over 5 psi OF BACK PRESSURE, AT PEAK RPMS WHERE EXHAUST FLOW IS AT MAX,or even more in a few cases, of back pressure in the exhaust system if measured at the collectors at max engine rpm, with a decent accurate pressure gauge, while a good aftermarket exhaust system will roughly cut that in half to the 3-5 psi range. BUT An excellent performance exhaust will get down in the 1-2 psi or LOWER restriction to flow range at max engine rpm. keep in mind that the efficiency of the headers scavenging the cylinders , and helping to draw in the following intake charge,is almost totally dependent on maintaining a very low flow restriction or back-pressure in the collectors ,especially at mid and upper rpm levels, any significant restriction to flow reduces the effectiveness of the headers ability to scavenge the cylinders by allowing the previous exhaust gas inertia mass to help drag in the next intake charge following it into that cylinder as it exits the cylinder thru the tuned headers primary's.
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one of the most common and least tested, factors in most engine build ups is testing for restrictions to building increased horsepower. a huge problem is in restrictive exhaust systems that can not effectively allow the headers to scavenge the burnt gases from the cylinders, a good open collector on a well designed header can reduce the back pressure at the exhaust port to a negative pressure significantly increasing cylinder scavenging out the exhaust, thus helping draw in a fresh intake charge.

is exhaust back pressure killing performance ?
obviously the volume of your engine exhaust increases with the rpm range, so while the stock exhaust may be fine under 5500rpm it surely may be restrictive in the upper rpms, once you've installed better flowing heads and added a cam, after all the engineers at G.M. were matching the stock components not aftermarket parts.
its a fast easy test that needs to be done and more than a few cars running older cats are having a restricted exhaust that's hurting your performance badly, and because the problem tends to build up slowly many people don,t notice the gradual power loss
first thing you'll need to realize is that the TEST POINT needs to be after the header collector and in front of the CAT AND MUFFLERS. next thing you need to know is that to get a valid answer you'll need to have a long connector hose so someone in the pass seat can CLOSELY WATCH the gauge as you accelerate under load (FLOOR THE CAR THRU THE FIRST 1-2 GEARS and watch the pressure surge. readings above 1psi mean somethings partly restricted, readings above 3psi are hurting your performance BADLY ENOUGH THAT YOU SHOULD CORRECT THE RESTRICTION.
you'll occasionally see dyno tests that show something like stock rams horn exhaust manifolds, compared to shorty headers and one or the other shows a slight advantage, the fact they don,t mention is that for headers to work correctly and scavenge the engines cylinders at higher rpms there needs to be very little back pressure , in the exhaust system behind the headers, and if theres almost any restriction the headers can,t scavenge the cylinders efficiently. shorty headers and stock exhaust manifolds effectively use the inertia of the trapped exhaust gases in the stock exhaust to help drag out or scavenge the cylinders but its only effective at low and mid rpm ranges, it tends to be rather restrictive after about 4000rpm in most cars
the test kit is about $50.00
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http://www.ntxtools.com/fuelsystem/tu24a.htm

OR SAVE SOME CASH

http://www.harborfreight.com/cpi/ctaf/d ... mber=93547

YES TAKING THE TIME TO READ THRU THESE LINKS and SUB LINKS IS WELL WORTH THE EFFORT

I found this info posted else ware but its good info
Back pressure: The myth and why it's wrong.

I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?

I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

VI. Conclusion.

SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

Turbo Exhaust Systems:

Some of you asked for a better explanation about restriction in a turbo exhaust, so here 'goes. To sum it all up, on a turbo car, the tighter and more restricted the exhaust housing of the turbo is, the faster you're going to spool your turbo... because the restricted gasses escape through the exhaust housing with more velocity (much like the garden hose description quoted above)... but with this restriction comes the downside. Less exhaust volume will be able to fit through that turbine housing once the turbo is spooled and starts squishing more air through the intake.

This is where the wastegate comes into play. The wastegate is actuated BEFORE the exhaust wheel in the exhaust housing. When it opens, whatever the diameter of your wastegate is gets added to your exhaust piping. Effectively increasing the capacity of the exhaust provided that everything downstream is large enough in diameter to handle it the extra flow. The main reason it does this is to prevent over-spooling the turbo. Once the turbo reaches its efficiency, it doesn't need to flow all the extra gasses through the turbine wheel, so the wastegate allows you to route the exhaust around the turbo... if it can't route enough exhaust around the turbo (restricted wastegate) then too much exhaust gas will be forced by the exhaust wheel and BOOST CREEP will occur as your turbo over-spools.

One way to prevent boost creep is to port the wastegate housing (if you have an internally gated turbo) or to replace the wastegate with a bigger unit. If that still doesn't work, then the problem is likely going to be a restriction in the exhaust downstream from the wastegate. Many overboosted car owners prefer to vent their wastegate dumps to the atmosphere. Not only is it illegal to bypass your catalytic converter, but it's loud as hell, gets your engine bay filthy, and clogs up your K&N like nobody's business; however, it nearly eliminates boost creep. It's a cheap and easy solution that fixes boost creep on a race car.

The best solution to boost creep is to route your wastegate dump past the catalytic converter and back into the exhaust. It will be a custom setup. Nobody makes this. Make it look clean and you'll pass emissions because they don't run your car hard enough to open the wastegate when doing emissions testing. You have to reach full boost for the wastegate to open, and since the exhaust runs through the cat until the wastegate opens, it all gets "cleaned" before it reaches the sniffer. With this setup, the wastegate will also be much quieter because it still runs through the muffler, and you won't trash your engine bay with black caustic funk.

Putting a separate catalytic converter on your wastegate dump is a stupid idea because you'll never get it hot enough to "light off" and start converting the carbon monoxide... so don't get any crazy ideas and create unnecessary exhaust restrictions.

Once compressed air comes into the mix with an engine, exhaust tuning has much less to do with making power. So what if you can milk another 3-5 hp out of a car with a tuned exhaust... the benefits of making an engine sustain an insane final compression ratio (boosted air x compression in the combustion chamber) has much more affect on making power if you can just get rid of the extra gasses it produces. Bigger is almost always better on a turbo setup. The only place where it isn't good is on the exhaust wheel where too big can = no chance of spooling your oversized turbo any time this year.

http://www.uucmotorwerks.com/html_produ ... uemyth.htm

ITS NOT RARE TO LOOSE 20% or more of your potential hp to a restricted cat on your exhaust, or restrictive mufflers or exhaust pipes that are too small and restrictive, no header can effectively scavenge your engine unless its got less than about 2 psi of back pressure at the peak rpm range, on most muscle cars youll want a 2.5"-3" diam. exhaust system with an (X) pipe as close to the header collectors as clearances under the car allow as a MINIMUM size exhaust and its been my experience that is RARE for a 3" dual exhaust with an (X) pipe to NOT allow a hot combo with over 400hp to produce some mid and upper rpm power gains.
if the exhaust pipes too small in cross sectional area the pipe acts like a long rather restrictive collector on the headers, this may be good for low rpm cylinder scavenging, but it usually kills peak hp potential, its best to tune the engines intake and exhaust to match the gearing and cam timing power curve. in an ideal set up the exhaust system beyond the header collectors has a very low restriction to flow, and there will be an (X) pipe to effectively double the exhaust cross sectional area to significantly lower resistance to exhaust flow restriction still further, to increase the header,s ability to scavenge the cylinders efficiently in the intended power & rpm band, keep in mind your goal is or should be in most cases to maximize the torque in the intended rpm range that your engine combo produces

Id highly suggest adding one of these 3" (X) to most exhaust systems
http://store5.yimg.com/I/exhaustpros_1769_9241795
looking thru an (X) pipe
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A CORRECTLY TUNED SET OF HEADERS , MATCHED TO A CORRECTLY DESIGNED CAM TIMING HAS A SIGNIFICANT EFFECT ON INTAKE FLOW AND CYLINDER SCAVENGING EFFICIENCY, EXHAUST SCAVENGING CAN BE 5 TIMES STRONGER THAN THE PISTON, MOVEMENT INDUCED NEGATIVE PRESSURE (VACUUM) IN THE INTAKE RUNNERS
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Id also point out that cam timing matched to the exhaust scavenging has a huge effect on potential intake flow rates
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YES I KNOW YOU DON,T WANT TO READ THRU SUB LINKS,
BUT THEY CONTAIN A HUGE AMOUNT OF INFO

SO YOUR ONLY HURTING YOURSELF BY IGNORING THE RESOURCE


http://www.aa1car.com/library/exhaust_backpressure.htm

http://victorylibrary.com/mopar/header-tech-c.htm

http://www.popularhotrodding.com/engine ... 505em_exh/

viewtopic.php?f=56&t=1503&p=3419#p3419

http://www.thrashercharged.com/tech_htm/exhaust.shtm

http://www.pontiacstreetperformance.com ... haust.html

http://www.mk5cortinaestate.co.uk/calculator5.php

http://www.tuneruniversity.com/blog/201 ... -upgrades/

http://www.superchevy.com/technical/eng ... 05phr_exh/

http://www.nsxprime.com/FAQ/Miscellaneo ... theory.htm

viewtopic.php?f=56&t=3049&p=8053&hilit=oxygen+sensors#p8053

http://www.veryuseful.com/mustang/tech/ ... enging.pdf

viewtopic.php?f=52&t=10947&p=48126#p48126

http://www.boyleworks.com/ta400/psp/exhaust.html

http://www.aa1car.com/library/exhaust_backpressure.htm

http://www.superchevy.com/technical/eng ... index.html

viewtopic.php?f=56&t=260

viewtopic.php?f=56&t=2916&p=10987&hilit=mufflers#p10987

viewtopic.php?f=56&t=1303

viewtopic.php?f=56&t=45&hilit=coke+can

viewtopic.php?f=56&t=260&p=315#p315

http://www.tuneruniversity.com/blog/201 ... -upgrades/

http://www.pontiacstreetperformance.com ... haust.html

http://www.underhoodservice.com/Article ... nosis.aspx

http://www.centuryperformance.com/exhau ... g-137.html

http://www.asia.donaldson.com/en/exhaus ... 053747.pdf

viewtopic.php?f=56&t=496

viewtopic.php?f=56&t=496&p=3300&hilit=cutouts#p3300

http://www.burnsstainless.com/TechArticles/Theory/theory.html

http://grapeaperacing.com/GrapeApeRacing/tech/exhaustheaders.cfm
the short answer is ANY BACK PRESSURE IS BAD!
what your trying to do with a correctly designed exhaust is to have the majority of the exhaust system act like extended collectors on the headers, in effect using the inertia of the hot exhaust pulses from each cylinder to cause a cyclic low pressure wave to be timed to assist the next firing cylinders to scavenge the cylinders


keep in mind installing an (X) almost increases the effective cross sectional area,of the dual exhaust ,to double what it had been behind a single header collector, by doubling the area that the exhaust flow sees, dropping the restriction to flow almost in half

http://www.antechlabs.com/K0GFM/pipechart.gif
heres the info you need,
length

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html
diam.

well worth reading

http://victorylibrary.com/mopar/header-tech-c.htm

http://home.flash.net/~ivc1/cobra/xpipes.html

http://www.superchevy.com/technical/eng ... index.html

http://www.s262612653.websitehome.co.uk ... /heads.htm

viewtopic.php?f=55&t=1509

to adjust for your altitude
http://rshelq.home.sprynet.com/density_altitude.htm

and if you want to get it perfect you should use your DYNAMIC COMPRESSION RATIO VOLUME in the formula at the rpms for peak hp that your engine makes

http://cochise.uia.net/pkelley2/DynamicCR.html
most of the time that comes out close to the same as the static cylinder volume at the peak torque f.y.i.

LET ME ASK THIS QUESTION?
WHATS THE FIRST THING YOU DO AT THE TRACK TO YOUR EXHAUST SYSTEM?
you open the headers to have less back pressure correct!!
BACK PRESSURE IS BAD, BAD,BAD, the idea has gotten around that you need back pressure to build torque, FALSE!!!!!! that IDEA was prevalent because if you stick too large of an exhaust pipe on a low rpm engine it loses the ability to scavage the cylinders with the exhaust gas pulse in the exhaust with stock exhaust manifolds, some how the old wife's tale got around that you needed back pressure...FALSE.. what you need was EFFECTIVE CYLINDER SCAVENGING which the smaller tail pipe diam. was providing by acting like the collector on a set of headers!if you have headers , especially full length headers with a merge type collector you can,t make the exhaust too large,THINK ABOUT IT! the first thing you do at the track is un-cork your headers....why? because YEARS OF TESTING PROVES, less BACK PRESSURE MAKES MORE HP WITH HEADERS!!!! now some one is saying yeah but that's at the track and your running the engine at 5000rpm-6500rpm , well true and a good point because we typically only run 1000rpm-3000rpm on the street but look here,your stock exhaust if fine for the original intended 1000rpm-5000rpm band in most cases , but increase engine displacement,change the cam timing , on that added displacement,increase cylinder head and intake flow rates etc. and buzz that engine to 6000rpm and you'll soon find the stock exhaust systems pathetic at the upper rpm ranges, causing a very noticeable restriction to exhaust flow.
guys think simply adding headers cures this, but headers can,t function at near full potential efficiency unless the exhaust system mounted behind the collectors offers only a very minimal restriction. gaining 40hp or more is very common when adding headers and a low restriction exhaust system.

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

what it shows clearly is the collector on a low rpm engine needs to be about 10ft long for max effect so you need an exhaust that works with a collector that's about 20" long at 6000rpm and one 10 feet long at idle, THAT'S WHERE THE IDEA OF TOO LARGE A EXHAUST comes from but as long as you follow these rules you'll be fine, READ THIS,

http://victorylibrary.com/mopar/header-tech-c.htm


btw Im writing this like this to get you guys to think about it then we will look into the answers of how to get both ends of the rpm range covered and yes Im leaving out some info on purpose

http://autolounge.net/calculators/exhau ... izing.html

most exhaust pipe is 16 gauge, or .065 wall thickness

http://www.engineersedge.com/gauge.htm

How do you judge the i.d of a pipe?

3" o.d = 2.87" diam. inside
2.75 o.d = 2.62"diam. inside
2.5 o.d = 2.37" diam. inside
2.25 o.d = 2.12" diam. inside



is exhaust back pressure killing performance ?
its a fast easy test that needs to be done and more than a few cars running older cats are having a restricted exhaust that are noticeably hurting your performance , and because the problem tends to build up slowly many people don,t notice the gradual power loss
first thing you'll need to realize is that the TEST POINT needs to be after the header collector and in front of the CAT AND MUFFLERS. next thing you need to know is that to get a valid answer you'll need to have a long connector hose, and a reliable pressure gauge, so someone in the pass seat can CLOSELY WATCH the gauge as you accelerate under load (FLOOR THE CAR THRU THE FIRST 1-2 GEARS and watch the pressure surge. back pressure readings above 1psi mean somethings partly restricted, readings above 3psi are hurting your performance noticeably.

this gauge will work
http://www.harborfreight.com/cpi/ctaf/d ... mber=93547

you drill a 1/8" hole in the collector to insert a temporary test probe made from 1/8" copper tubing attached to a section of vinyl hose back to the test gauge, angle or point the end of the test probe tube,toward the exhaust exit not the engine, that hole you drill for test access can easily be brazed closed after the test, OR in an ideal world you weld a 1/8" Nation Pipe Thread bung on the collector, so you can test with a removable screw threaded adapter and close the threaded bung with a brass plug when your done, a buddy reads the gauge with the line run into the passenger seat and the gauge or in an ideal world you mount the gauge in the car as a reference for tuning


the test kit is about $50.00
http://www.jdsdiagnostic.com/eptspage.htm

ITS NOT RARE TO LOOSE 20% or more of your hp to a restricted cat
your unlikely to lose much if any hp especially if you install an (H) pipe just before the reduction in tail pipe dia. and use a gradual reducer to change from 3' to 2.5" mostly because the exhaust gases have cooled a great deal by the time they reach that point in your exhaust system and with that heat loss a reduction in volume and potential back pressure

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http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=1313&prmenbr=361

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http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=1306&prmenbr=361

http://store.yahoo.com/exhaustpros/hustxpi.html
http://store5.yimg.com/I/exhaustpros_1775_13682224

http://www.summitracing.com/search/?keyword=exhaust%20cones&dds=1

IF your trying to reduce noise levels, you can buy or fabricate these cones that tend to break up and reduce exhaust noise, once installed in the exhaust, without producing much of a restriction since the total surface area of the mini holes is greater the the pipes cross sectional area
the tabs can be tack welded or sheet metal screw attached, and adding an (X) pipe near the collectors and an additional (H) near the muffler entrance point tends to mellow the tone

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being a bit pragmatic here, Id say that its rather rare for a hot street/strip car combo with 400 cubic inches or less and under 500 hp that probably 75% of us drive to need an exhaust larger than dual 3" pipes with an (X) and that's usually what Ive found works on most street strip cars.
now obviously if your building something significantly larger or more powerful you'll want to do some calculation before selecting an exhaust design, but what always amazes me is how few people bother to buy and use a simple vacume/pressure gauge to actually test and measure back pressure.
without, testing and having the facts,and knowing what your dealing with it seems foolish to rush out and spend hundreds of dollars on a new exhaust that may or may not solve your problems or to ignore a problem that you can easily test for and verify before spending any money on a problem you can prove is caused by a restrictive exhaust system, basing your parts selection on what your trying to accomplish, and whats required to reach your goal, rather than random guess work seems to be the cheaper route in the long run in my opinion.
drilling a small easily re-welded test hole in the header collector , or an adapter on an oxygen sensor bung gives you access for testing back-pressure
you might be surprised at what a few tests show you and how much time and money you can avoid wasting

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http://www.harborfreight.com/fuel-pump- ... 93547.html

http://www.harborfreight.com/fuel-pump- ... 93547.html

a simple vacuum gauge CONNECTED TO MANIFOLD VACUUM will be a surprisingly accurate tool to diagnose problems,providing you know how to use it.
you don,t need an expensive tool, while your testing you may as well connect it to your carbs intake plenum or throttle body plenum as a next test, because if your pulling over about 1.5" of vacuum at wide open throttle the carb or throttle body your using is a bit restrictive, and if you restrict the exhaust or intake flow it hurts power[
/color]





BTW if youve got one of these nearly worthless (for a true high hp application)fuel filters, that can break and cause a fire hazard,
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they can be quite useful inserted in the vacume line between the exhaust or intake your testing and the gauge your testing with as they make a rather effective pulse damper or reducer that limits the gauge needle rapidly twitching making it harder in some applications to get a clear reading
A CORRECTLY TUNED SET OF HEADERS , MATCHED TO A CORRECTLY DESIGNED CAM TIMING HAS A SIGNIFICANT EFFECT ON INTAKE FLOW AND CYLINDER SCAVENGING EFFICIENCY, EXHAUST SCAVENGING CAN BE 5 TIMES STRONGER THAN THE PISTON, MOVEMENT INDUCED NEGATIVE PRESSURE (VACUUM) IN THE INTAKE RUNNERS
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[color=#BF0040]these threads may also help


viewtopic.php?f=50&t=383&p=1637&hilit=+infrared+tuning#p1637

viewtopic.php?f=50&t=383&hilit=+infrared+tuning

viewtopic.php?f=44&t=579&p=743&hilit=+infrared+tuning#p743

UNLESS you match ANY header with a TRUE low restriction exhaust its can NOT possibly work to NEAR its FULL potential, headers are designed to SCAVENGE the cylinders of oil burnt exhaust gases and INCREASE the percentage of fresh fuel air mix, if theres any significant back pressure beyond the header collectors that cuts the headers efficiency way down



http://forum.grumpysperformance.com/viewtopic.php?f=56&t=1730

http://forum.grumpysperformance.com/viewtopic.php?f=56&t=3155

http://forum.grumpysperformance.com/viewtopic.php?f=56&t=1503

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adding an (X) pipe effectively doubles the exhaust cross sectional area cutting the resistance to exhaust flow restriction nearly in half
mike

 
Posts: 7
Joined: October 14th, 2008, 9:07 am
Location: florida

Re: is backpressure hurting your combo?

Postby grumpyvette » October 22nd, 2008, 6:07 pm

IS it possible to have too little back pressure in your exhaust?
NO!, not if maximizing the engines power curve is the goal!
but its common to have the wrong size headers, exhaust system , or header collectors, to match your engines displacement, cam timing and compression, or for your engine tune to not take full advantage of the cylinder scavenging or ignition curve,
increased back pressure in the exhaust is ALWAYS going to restrict flow, and its almost always going to hurt power, some engines have the cam timing or headers designed to compensate for back pressure, but in a correctly designed system with decent headers the power tends to go up as the back pressures REDUCED provided the cars FUEL/AIR mix and timing are adjusted to maximize the potential the lower restriction to flow provides.

a poorly tuned engine will frequently get leaner as back pressure reduced,and that may hurt the engine torque curve because max torque is normally found in the fuel/air range of about 12.7:! while most modern cars computers tend to be tuned for a fuel air ratio of between 14.7:1-15:1 for decent mileage and low emissions, and if your headers or exhaust is to large it will tend to reduce the cylinder scavenging, but its NOT back pressure but controlling exhaust gas velocity , or the lack of it thats the cause.

1)General exhaust pipe air-flow is about 115 CFM per square Inch.

2)Muffler flow rate should be about twice of what the carburetor flow rate is to keep back pressure low. IE: 600 CFM Carb, means 2 x 600 CFM Mufflers

3)6 PSI Max exhaust Back pressure, 2 – 3 PSI is acceptable in performance cars exhaust, but less is much better.

4)Tuned length or equal length headers is not extremely important, a 6" inch difference in primary tube runners in the same header has little effect on performance, if the headers are mated to a slightly restrictive exhaust and mufflers

5)The smaller the header tube, the more acceleration (filling ability) for intake gases into the compression chamber, unless the pipe is unreasonably too small….

6)Exhaust airspeed is the primary combustion chamber charging component in Naturally aspirated engines.

7)Large tube headers do not assist in pre-charge of the combustion chamber they hurt it.

Collector length, the longer the better, and wider is bad.

9)Glass pack mufflers are seen as collector length for tuning purposes

10)Loud mufflers don’t mean better airflow or performance

11)Flowmaster STYLE mufflers are a very good design, have no reflections back into the headers, where glass pack mufflers do.

12)Once gasses enter a muffler, any length of pipe or bending of the pipe, after the muffler are no longer of concern to the engine or performance.
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Destroying a myth. (that your engine runs better with back pressure in the exhaust)

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

Some say that "an engine needs exhaust back pressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs back pressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate back pressure with torque, and others fear that too little back pressure will lead to valve burning.

The first reason why people say "back pressure is good" is because they believe that increased back pressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that back pressure produces more torque. The increase in torque is not due to back pressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "back pressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of back pressure, but to the changes in the a/f ratio.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in back pressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetors often could not adjust because of the way that back pressure caused air to flow backwards through the carburetor (reversion pulse) after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced back pressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if back pressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing back pressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust back pressure reduction, these limits won't be reached.

- Adapted from Thomas V.

viewtopic.php?f=56&t=1730

viewtopic.php?f=56&t=185

whats the first thing most guys do at the track after installing their SLICKS, thats right! they open their headers, why? because open collectors on headers usually result in more horsepower than leaving the stock exhaust connected, look, if your headers are designed correctly the combo of the primaries and collectors are the correct length to efficiently scavenge the cylinders and help draw in the following intake charge as the low pressure wave reflects back to the exhaust port., this only works over a limited rpm band but it helps fill the cylinders, by tending to allow the fast exiting exhaust to drag in the following intake runner volume PROVIDED the cam timing matches the header design.
ANY BACK PRESSURE tends to reduce cylinder scavenging


catalytic converters can be tested on car, with the engine running, at operating temperature.
its a good idea to test with a vacuum gauge as described earlier, but a quick check can be done with an I.R.TEMP GUN,
Using an infrared temperature tester, like I link to below, the "gun" style with the red laser pointer, measure the inlet and outlet temps of the converter.

The temperature difference from one end to the other, should be approximately 100 deg/F.

The INLET should be 100 deg/F or so COLDER than the OUTLET end.

If the outlet is significantly colder than the inlet, converter is usually plugged.

heres the one I use and recommend
Image
http://www.professionalequipment.com/ex ... ermometer/
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » November 5th, 2008, 1:05 pm

While your checking for a restriction in the exhaust, you look for the resulting back pressure that tends to hurt flow rates and horse power, the "bounce" you see on the gauge at idle speeds is the result of the exhaust PULSES ,readings at idle are basically meaningless unless the cats are clogged significantly , your looking to read less than 1.5 PSI in positive back pressure at both in front of and behind the cats at the upper rpm ranges, now it should be obvious that behind the cats it will never read higher than in front of the cats because any restriction behind the cats will only increase the restriction reading in front of the cats,so only the bung in-front (between the engine and the cats) only is really necessary,remember the VOLUME of exhaust gases increases with the engine rpms, Ive generally welded a bung into the exhaust and put a screw plug into that bung when its not in use.

Image
if the cats are plugged or partly plugged you'll see the back-pressure start a steady increase as the rpms build over about 3000rpm, and you can almost always have a rather obvious increase in hp if you remove a clogged cat, if the readings are above that 1.5 psi .
if the back pressure reading went over 1.5 psi during the testing the cats or the exhaust is restricting your power potential at least to some degree, at any point where the back-pressure readings fell above that level, naturally the higher the reading the more likely its causing you a problem

BTW an IR temp gun can sometimes indicate a partly clogged , or non-functioning cat

viewtopic.php?f=50&t=578

a few tests to determine the cause of the problem before you start throwing parts at the problem, is a smart route.
but a simple vacuum gauge CONNECTED TO the exhaust pipe thru a small tube inserted thru a hole you drill and weld up later, in the exhaust pipe between the engine and catalytic converter will be a surprisingly accurate tool providing you know how to use it.
you don,t need an expensive tool
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93547


Image

if you have over 3 psi of back pressure at w.o.t. the cats clogged
Item#%20http://www.motorsportsinnovations.com/boss-stainless-te-301.gif

TE-301
Stainless weld boss WEB Price
$3.00 1/8 pipe weld boss - stainless steel

http://www.motorsportsinnovations.com/WB_Pressure.htm

you test by installing a test barb
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and a 7 ft long hose and having a buddy watch the gauge as you run thru the gears, at the upper rpm ranges the positive reading should ideally stay under 1.5 psi
Image
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knowing a few constants in engine pressure and flow helps


an engine usually requires approximately 2.257 cubic feet per minute per horsepower to maximise intake flow and exhaust flow at about 115 cfm per square inch

so assuming your building a 500 hp engine / 2 (divided by 2 as there's normally two header collectors on a v8) we have 250hp per header collector, (open header collectors) multiply that by 2.257 cfm and you see you need 565 cfm and divide that by 115/square inches and we see we need a 4.9 square inch minimum exhaust collector pipe, per side (open header collectors).
its a rule of thumb or rough guide on the expected combined potential max exhaust port flow rates of the exhaust primarys, taken from an observed average of hundreds of recorded dyno results, and not taking into account any flow loss or restriction,and calculating some heat expansion ,its basically worthless except as a way to figuring what size exhaust pipe size thats likely to work, or what size will be restrictive to flow.

you can also use intake port flow as a rule of thumb, ie number of cylinders x max port flow art max cam lift x .257 = max theoretical hp an engines likely to produce, before the heads and intake become a restriction

IE if a vortec head 350 flows 229 cfm at .600 lift and your spinning the engine fast enough too maximize the port flow rates,so in an ideal world you can expect 229 cfm x.257 x 8 cylinder =470hp before the heads become a restriction if everything else is perfectly tuned

viewtopic.php?f=52&t=322

as a cross check 500hp /8=1129/8=142 hp per header primary , 2.257 x 142/115=2.76 sq inches 0r a header primary a bit larger than 1 3/4 and smaller than 2" or a 1 7/8 to maximize peak hp, per header primary, but keep in mind you'll spend most of your time below peak rpms so a slightly smaller 1 3/4" primary on a street strip engine that sacrifices a bit of peak hp for better mid rpm torque makes sense, and once you install longer exhaust pipes and mufflers you'll need to steep up the exhaust pipe size cross section past the header collectors or they will tend to be restrictive at the minimum size the formula predicts

http://www.harborfreight.com/cpi/ctaf/d ... mber=93547

btw A LOT of guys that have an engine combo that seems to nose over or loose power rapidly at 4500rpm -5000rpm, and blame it on the cam, eventually find out its never been the cam, or bad valve springs or the carb, but, its a restricted exhaust system that's caused the power to fall off as the rpms build up, that's one reason Ive said many times you need to go back to basics and tune up your current combo and find out what the current restrictions are before throwing parts and cash at a car hoping to improve performance!, you can,t hope to make things better until you've removed the weak links from the chain, as they say
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » November 6th, 2008, 3:49 am

yes its NORMAL for a sudden increase in rpms to show a spike in the back-pressure but under hard accelleration the pressure should not keep climbing,rapidly well past the 1.5 psi range, it should ideally peak/jump durring hard shifts and bleed off even durring accelleration with a low restriction exhaust, in use.if it hits over 4 psi Id say your running into a restriction, and yeah! im fully aware its a censored to get accurate info from a buddy holding a gauge in the pass seat as the car rapidly accellerates thru the gears.
btw
if you build a small, sealed vacuum resevior can, (about a pint) with the tube from the test bung/barb going into it and a second tube going out to the gauge the needle tends to be far more consistant and stable

Image
Image
or you can get one dirt cheap at salvage yards
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby paul » November 27th, 2008, 5:11 pm

MBZ06 wrote:Headers are the wrong length? Can you explain what would be the correct length headers ?


Read :
http://teamzr1.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=503#Post503

and

http://teamzr1.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=491#Post491

and

http://teamzr1.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=455#Post455

and finally this

http://teamzr1.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=440#Post440

When done then you see most of the headers sold for the Corvette are "canned" and crap for best performance, add their way over bloated pricing and you see why some vendors love to sell OEM'd header systems.
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Re: is backpressure hurting your combo?

Postby bob » December 1st, 2008, 9:38 am

I drove a FORD F350 that got 9 MPG for about a year, I started noticing the truck was sluggish at over 55mph, I did that back pressure test (with grumpys help) and found I had 7 psi resistance...mileage jumped to 12 MPG and the truck cruised effortlessly at 70 mph after the clogged cats were replaced
bob

 

Re: is backpressure hurting your combo?

Postby grumpyvette » January 19th, 2009, 4:20 pm

HEY GRUMPYVETTE?
does a dual-outlet muffler have less back pressure than a similar single-outlet muffler? all other things being the same, 3" outlet vs a pair of 2.5" outlets.. ??

3" equals approx 7 sq inches of area
2.5" x 2 equals approx 9.8 sq inches of area, so in theory that about a 35% deduction in the restriction, but because the pipe feeding the muffler and the baffles in the muffler tend to be more restrictive it may provide no extra flow, exhaust flow pulses its not constant and if youve got an (H) or (X) pipe installed the flow is split at least partly so there's time for the flow rates to peak, bleed off and level off before the next peak in pressure to at least some degree.

viewtopic.php?f=79&t=497

EXHAUST, HEAT, PRESSURE & FLOW RESTRICTION


"So what are you saying grumpy? I am only reporting what I have read on this forum (posted by a couple of engineers). Are they mistaken?

I posted that I had read that the flow is controlled by the smallest dimension in your exhaust system. I read this in relation to mandrel bends vs regular bends that crimp the pipe. It said that if you have a 2.5" pipe with crimp bends, it will flow like a 2" pipe if that is the dimension of the pipe at the bend.

I was corrected immediately by some who explained the fact that they are running 3" pipe with 2.5" tail pipes and it doesn't effect flow because the cooler exhaust doesn't have as much volume.

Who is right?"


SHORT ANSWER, AND DEPENDING ON SEVERAL FACTORS,AT LOW RPMS YOUR CORRECT, AT HIGH RPMS YOUR PROBABLY NOT NEARLY CLOSE, YES THERES a good deal of heat lost and a reduction as the exhaust pulse travels away from the exhaust port but as the rpms increase the time drops between pulses and the heat increases dramatically, keep in mind that "The flow is controlled by the smallest dimension in your exhaust system. I read this in relation to mandrel bends vs regular bends that crimp the pipe. It said that if you have a 2.5" pipe with crimp bends, it will flow like a 2" pipe if that is the dimension of the pipe at the bend." does not apply if you feed two 3" pipes into four 2.5" pipes because, "3" exhaust from the engine to the muffler equals approx 7 sq inches of area
2.5" x 2 equals approx 9.8 sq inches of area, so in theory that about a 35% deduction in the restriction,dumping a 3" into dual 2.5" pipes


OK think it thru without getting on either side of the discussion UNTIL you’ve got some FACTS vs OPINIONS.

First it should be rather obvious that exhaust cools as it moves from its high heat point (THE EXHAUST EXITING THE HEADS) to its low point, (the outside air temp as it exits the car)
BUT were not really concerned, much with the heat levels in the exhaust unless that cooling affects the gas volume enough to measurably affect the resulting pressure that can restrict the following flow rates, of exhaust exiting the car.
Now lets look at FACTS
exhaust temp commonly runs between about 700F-800f but can get to -1300F depending on cam timing, compression ratio, ignition timing and the fuel air ratio, octane, ETC. Exhaust flow rates can easily exceed 200ft per second and its usually higher, naturally the engines displacement, gearing and RPM effect the resulting VOLUME AND RESULTING PRESSURE the exhaust needs to deal with and as the rpms go up the time allowed for heat to transfer out of the gas flow tends to be drastically reduced.
FACT as the rpm rate goes up both the volume of exhaust and the temp. of that exhaust tends to go up rapidly., as the volume goes up and the temp. goes up but the interior of the exhaust system stays constant the pressure tends to go up also.
Fact the EXHAUST GAS VOLUME is about 8-12 times larger that the intake charge that entered the cylinder, that was compressed and burned, that almost instant expansion is what forced the piston away from the heads and provides the engines power,and at peak pressure might hit 600 psi as it forces the piston down and drops rapidly as it exits the engine

FACT at only 200ft per second it takes far less than a second for the exhaust gasses to exit an exhaust, and each previous exhaust pulse has bled of or transferred some heat , so each later pulse has a less effective surface to release its heat content into.
HOW much do you think the exhaust flow cools during the trip thru the exhaust and how long does it take to bring the exhaust pipes up to a temp range where heat transfer out of the flow slows noticeably?
REMEMBER exhaust headers frequently glow cherry red in well under a minute on a dyno. And that’s not in a cramped engine compartment and usually with lots of fans blowing..
Flow rates in the exhaust are limited by the smallest cross sectional area, the temperatures and the resulting pressures, resulting from the engines displacement, compression ratio, f/a ratio, and the rpms its running at.
An exhaust that functions perfectly at 3000rpm may easily be very restrictive at 6000rpm., and as the time and temperature increases the exhaust temp tends to go up slightly as does the resulting restriction to flow.BUT the volume of exhaust is mostly linked to the engines displacement, rpms and f/a ratio

Its more info like These links below that might interest you, skipping over it would be a mistake


viewtopic.php?f=79&t=497

http://www.blksmth.com/heat_colors.htm

http://www.tech.plym.ac.uk/sme/ther305-web/Combust1.PDF

viewtopic.php?f=4&t=1337&hilit=infrared

viewtopic.php?f=56&t=1303&p=6673#p6673

viewtopic.php?f=56&t=1503

viewtopic.php?f=56&t=260

viewtopic.php?f=56&t=45

http://en.wikipedia.org/wiki/Boyle's_law

viewtopic.php?f=56&t=495

viewtopic.php?f=56&t=352
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » February 5th, 2009, 1:10 pm

LOOK THRU THESE

http://www.exhaustsoundclips.com/cfm.pdf

http://autospeed.com/cms/article.html?&A=0381&P=1

http://autospeed.com/cms/A_0383/article ... larArticle

if space is a problem, there are semi small mufflers

http://www.jegs.com/i/Flowmaster/389/94 ... ProductId=

http://www.jegs.com/i/Flowmaster/389/94 ... 7AodtlIAtg

http://www.jegs.com/i/Dougs-Headers/776 ... Id=1282659

Image
OEM..................................... ......................... ...225
Gibson SuperFlow 788200.................................. ............311
TTS Bullet Cat Converter............................... ..............324
Flowmaster 40 series 42540................................... ........352
Flowmaster 50 Series.................................. ...............362
Flowmaster 40 series 43040................................... .'......392
Dynomax Super Turbo................................... ...............410
Flowmaster 62631................................... ..................435
Carsound Cat Converter............................... ................436
FLP Cat Converter............................... .....................440
Edelbrock Victor 5535.................................................562
Flowmaster 40 series 435409.................................. ........576
Edelbrock RPM series 5511.............................................579
Flowmaster Delta Force Racing 54040-10................................634
Edelbrock 304 series 5560.............................................640
Edelbrock Gen App 5505.................................... ...........708
Dynomax Race Magnum Welded 17216......................................711
Borla XR-1 40600......................................................836
Borla XR-1 40615......................................................854
Dynomax UltraFlow SS 17263................................... .......1000
Dynomax Race Magnum Welded 17218.....................................1000
Borla XR-1 40575.....................................................1100
Dynomax UltraFlow....................................................1133
Edelbrock Victor 5537................................................1400
Borla XR-1 40450.....................................................1400
Borla XR-1 40741.....................................................1450
Dynomax UltraFlow SS 17296, 17268....................................2200
Dynomax Race Magnum Welded 17220, 17224..............................2200
Dynomax Race Magnum Welded 17225.....................................2600



http://www.pontiacstreetperformance.com ... haust.html

http://www.popularhotrodding.com/engine ... ndex1.html

http://broaderperformance.com/muffler_flow_tests.htm
We are no longer testing mufflers and have no further information available

This is a chart of popular performance and a few stock type mufflers I have tested on our flow bench. These were all the most popular mufflers sold and installed at a local exhaust shop. Straight pipes have been tested and are used to compare these mufflers to "potential" flow of a muffler. When running a performance muffler on the street there are many more things to consider then just good flow. For instance pleasant tone and enough sound reduction to avoid any "unwanted" attention from local authorities. One thing to keep in mind is how restrictive tailpipes are. You can have a great flowing muffler but if your tailpipes are restrictive you may not be getting any gain from loud, high flow, and not to mention sometimes "expensive" mufflers.


Originally Posted by FAST LS1
The results in HP order:
1.Flowtech Warlock (open) 374.2 hp/ 333.1 tq
2.Bassani Real Street 373.7 hp/ 333.8 tq
3.Hooker Maxflow 373.5 hp/ 333 tq
4.Borla XS 373.3 hp/ 332.6 tq
5.Magnaflow 372.8 / 332.5 tq
6.MAC 372.3 hp / 331.5 tq
7.Flowtech Afterburner 372.3 hp/ 330.1 tq
8.Hooker Aerochamber 372.1 hp/ 330.4 tq
9.Bassani 372 hp/ 333.5 tq
10.Spintech 371.6 hp/ 332.2 tq
11.Edelbrock Performer RPM 370.9 hp / 331.3 tq
12.Borla XR1 370 hp/ 334 tq
13.Flowtech Terminator 369.5 hp / 331.3 tq
14.Dynomax Ultra Flo 369.4 / 333.2 tq
15.Flowmaster 369.4 hp / 331.8 tq
16.Flowtech Warlock 366.3 hp / 325.3 tq
17.No muffs 365.2 hp / 330.1 tq

Sound Off in Quietest to Loudest (Decibels):

1.Borla XS, 80db at idle, 90db at 2,000 rpm, 110db at WOT.
2.Hooker Maxflow, 80db at idle, 90db at 2,000 rpm, 120db at WOT.
3.Magnaflow, 82db at idle, 91 at 2,000 rpm, 114db at WOT.
4.Flowmaster, 82db at idle, 93 at 2,000 rpm, 115db at WOT.
5.Flowtech Warlock, 83db at idle, 92 at 2,000 rpm, 112db at WOT.
6.Dynomax UltraFlo, 83db at idle, 94 at 2,000 rpm, 113db at WOT.
7.Borla XR1, 83db at idle, 96db at 2,000 rpm, 118db at WOT.
8.Bassani Street, 83db at idle, 96db at 2,000 rpm, 120db at WOT.
9.Edelbrock RPM, 84db at idle, 93db at 2,000 rpm, 118db at WOT.
10.Bassani Real Street, 84db at idle, 96db at 2,000 rpm, MAX. (120+db).
11.Flowtech Afterburner, 86db at idle, 94db at 2,000 rpm, 115db at WOT.
12.Flowtech Terminator, 86db at idle, 94db at 2,000 rpm, 119db at WOT.
13.Hooker Aerochamber, 87db at idle, 94db at 2,000 rpm, 114db at WOT.
14.SpinTech, 87db at idle, 97db at 2,000 rpm, 116db at WOT.
15.MAC, 87db at idle, 98db at 2,000 rpm, 119db at WOT.
16.No Muffler, 91db at idle, 103db at 2,000 rpm, MAX. (120+db).

Muffler Flow Testing

All tests via an independent lab
All tests @ 15” wc

2 “ Straight Pipe 283 CFM
2 ¼ “ Straight Pipe 365 CFM
2 ½ “ Straight Pipe 521 CFM

2 ¼” Typical Bent tailpipe 268 CFM
2 ½” Typical Bent Tailpipe 417 CFM

2 ¼” Inlet/Outlet_ Glass Pack Tips- No Louvers- Smooth 274 CFM
2 ¼” Inlet/Outlet- Glass Pack Tips-Louvered 133 CFM
Same as above set for reverse flow 141 CFM
2 ¼” Cherry Bomb 239 CFM
2 ½” Cherry Bomb 294 CFM

2 ½” Inlet/Outlet Dynomax Super Turbo 278 CFM
2 ½” Inlet/Outlet Ultraflow Bullet 512 CFM
2 ½” Inlet/Outlet Gibson Superflow 267 CFM
2 ½” Inlet/Outlet Flowmaster ( 2 Chamber) 249 CFM
2 ½” Inlet Outlet Flowmaster ( 3 Chamber) 229 CFM
2 ¼” Inlet/Outlet Thrush CVX 260 CFM
2 ½” Inlet/Outlet Maremount Cherry Bomb 298 CFM
2 ½” Inlet/Outlet Hooker Aero Chamber 324 CFM
2 ½” Inlet/Outlet Hooker Max Flow 521 CFM
2 ½” Inlet/Outlet Borla Turbo 373 CFM
2 1/2" Inlet/Outlet Magnaflow 284 CFM

Standard OEM 2 ¼” Inlet/Outlet 138-152 CFM
Standard OEM 2 ½” Inlet/Outlet 161-197 CFM
a well thought thru design on cut outs
keep in mind headers loose a great deal of their potential horsepower gains if there matched up to a restrictive exhaust

Image
300bhp wrote:I've been reading the sticky about designing a zero loss exhaust. It all sounds good and feasable.
However one of the key factors was muffler flow capacities. The equation used in the article is this:
MINIMUM OF 2.2cfm for every 1hp
So a 500bhp engine would need 1100cfm muffler /exhaust combined flow capacity.
On a dual setup you could divide this by two. So all you'd need is 2 mufflers that flow 550cfm, easy isn't it. (For a complete zero loss system there's a bit more to it, but this would be a good start. Try reading the sticky if you want more info).
The biggest problem I have come across is finding flow rates for the different mufflers out there.
So if you have flow data for any mufflers please post it up. Where possible state:
-Make of muffler
-Series or name of muffler
-Length
-inlet/outlet diameter
-and of course flow rate
Failing that any links to other sites with this information will be most helpful.

THESE WERE THE MINIMUM SIZE EXHAUST FOUND TO ALLOW ADEQUATE FLOW WITH OUT RESTRICTING POWER NOTICEABLY MORE THAN A LESS RESTRICTIVE EXHAUST
A 2.75” (stock) single system is good for a 310hp engine with zero loss…
A 3” Single system is good for a 370hp engine with zero loss…
A 3.5” Single system is good for a 503hp engine with zero loss…
A 4” Single system is good for a 657hp engine with zero loss…
A 2.25” dual system is good for a 457hp engine with zero loss…
A 2.5” dual system is good for a 513hp engine with zero loss…
A 3” dual system is good for a 812hp engine with zero loss…
Muffler Flow Testing

All tests via an independent lab
All tests @ 15” wc

2 “ Straight Pipe 283 CFM
2 ¼ “ Straight Pipe 365 CFM
2 ½ “ Straight Pipe 521 CFM

2 ¼” Typical Bent tailpipe 268 CFM
2 ½” Typical Bent Tailpipe 417 CFM

2 ¼” Inlet/Outlet_ Glass Pack Tips- No Louvers- Smooth 274 CFM
2 ¼” Inlet/Outlet- Glass Pack Tips-Louvered 133 CFM
Same as above set for reverse flow 141 CFM
2 ¼” Cherry Bomb 239 CFM
2 ½” Cherry Bomb 294 CFM

2 ½” Inlet/Outlet Dynomax Super Turbo 278 CFM
2 ½” Inlet/Outlet Ultraflow Bullet 512 CFM
2 ½” Inlet/Outlet Gibson Superflow 267 CFM
2 ½” Inlet/Outlet Flowmaster ( 2 Chamber) 249 CFM
2 ½” Inlet Outlet Flowmaster ( 3 Chamber) 229 CFM
2 ¼” Inlet/Outlet Thrush CVX 260 CFM
2 ½” Inlet/Outlet Maremount Cherry Bomb 298 CFM
2 ½” Inlet/Outlet Hooker Aero Chamber 324 CFM
2 ½” Inlet/Outlet Hooker Max Flow 521 CFM
2 ½” Inlet/Outlet Borla Turbo 373 CFM
2 1/2" Inlet/Outlet Magnaflow 284 CFM

Standard OEM 2 ¼” Inlet/Outlet 138-152 CFM
Standard OEM 2 ½” Inlet/Outlet 161-197 CFM

************************************************** ***************

This test was performed by 5.0 Mustang & Super Fords Magazine, December 2002 issue (page 55) on a '90 Mustang LX coupe with 370hp and 11 second potential. 66 pulls were made for this comparison:

The results in HP order:
1.Flowtech Warlock (open) 374.2 hp/ 333.1 tq
2.Bassani Real Street 373.7 hp/ 333.8 tq
3.Hooker Maxflow 373.5 hp/ 333 tq
4.Borla XS 373.3 hp/ 332.6 tq
5.Magnaflow 372.8 / 332.5 tq
6.MAC 372.3 hp / 331.5 tq
7.Flowtech Afterburner 372.3 hp/ 330.1 tq
8.Hooker Aerochamber 372.1 hp/ 330.4 tq
9.Bassani 372 hp/ 333.5 tq
10.Spintech 371.6 hp/ 332.2 tq
11.Edelbrock Performer RPM 370.9 hp / 331.3 tq
12.Borla XR1 370 hp/ 334 tq
13.Flowtech Terminator 369.5 hp / 331.3 tq
14.Dynomax Ultra Flo 369.4 / 333.2 tq
15.Flowmaster 369.4 hp / 331.8 tq
16.Flowtech Warlock 366.3 hp / 325.3 tq
17.No muffs 365.2 hp / 330.1 tq

Sound Off in Quietest to Loudest (Decibels):
1.Borla XS, 80db at idle, 90db at 2,000 rpm, 110db at WOT.
2.Hooker Maxflow, 80db at idle, 90db at 2,000 rpm, 120db at WOT.
3.Magnaflow, 82db at idle, 91 at 2,000 rpm, 114db at WOT.
4.Flowmaster, 82db at idle, 93 at 2,000 rpm, 115db at WOT.
5.Flowtech Warlock, 83db at idle, 92 at 2,000 rpm, 112db at WOT.
6.Dynomax UltraFlo, 83db at idle, 94 at 2,000 rpm, 113db at WOT.
7.Borla XR1, 83db at idle, 96db at 2,000 rpm, 118db at WOT.
8.Bassani Street, 83db at idle, 96db at 2,000 rpm, 120db at WOT.
9.Edelbrock RPM, 84db at idle, 93db at 2,000 rpm, 118db at WOT.
10.Bassani Real Street, 84db at idle, 96db at 2,000 rpm, MAX. (120+db).
11.Flowtech Afterburner, 86db at idle, 94db at 2,000 rpm, 115db at WOT.
12.Flowtech Terminator, 86db at idle, 94db at 2,000 rpm, 119db at WOT.
13.Hooker Aerochamber, 87db at idle, 94db at 2,000 rpm, 114db at WOT.
14.SpinTech, 87db at idle, 97db at 2,000 rpm, 116db at WOT.
15.MAC, 87db at idle, 98db at 2,000 rpm, 119db at WOT.
16.No Muffler, 91db at idle, 103db at 2,000 rpm, MAX. (120+db).
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » March 1st, 2010, 8:47 pm

Ford Mustang Muffler Comparison Test - American Rumble

http://www.mustang50magazine.com/techar ... index.html

On the Dyno

Open Exhaust Bassani Bassani Real Street Borla XS Borla XR1 DynoMax Edelbrock Ultra Flo
RPM PWR TRQ PWR TRQ PWR TRQ PWR TRQ PWR TRQ PWR TRQ
2,750 103 196.7 87.1 182.9 89.9 188.8 111.7 213.4 102.1 195 105.1 200.7
3,000 138.9 243.1 109.1 208.4 107.2 204.7 148.3 259.6 141.1 247 140.9 246.7
3,250 168.6 272.4 146.2 255.9 148.6 260.2 176.6 285.4 168.9 273 168.4 272.2
3,500 189.6 284.5 173.2 279.8 177.1 286.2 195.4 293.2 189.1 283.7 189.3 284.1
3,750 202.3 283.3 192.3 288.5 194.8 292.3 207.9 291.1 203.7 285.3 204.5 286.4
4,000 214.6 281.8 204.8 286.8 208.3 291.8 219.2 287.8 215.6 283.1 214.9 282.2
4,250 239.6 296.1 216.6 284.3 220.7 289.8 244.3 301.9 243.2 300.5 243.1 300.4
4,500 269.5 314.5 241.3 298.2 244.7 302.4 274.1 319.9 270.9 316.2 271.9 317.3
4,750 291.6 322.4 272.3 317.8 274.1 319.9 294.6 325.7 295 326.2 295 326.1
5,000 308.7 324.2 293.7 324.7 295.5 326.8 310.1 325.7 311.7 327.4 310.9 326.5
5,250 326.3 326.4 308.7 324.3 310.9 326.6 329.4 329.6 330 330.2 329.2 329.3
5,500 345.7 330.1 329 329.1 330.8 330.9 348.3 332.6 349.8 334 349 333.2
5,750 359.2 328.1 349.3 333.5 349.6 333.8 363.8 332.3 362.4 331 362.8 331.4
6,000 365 319.5 361.4 330.1 364.1 332.6 370.9 324.6 370 323.9 368.7 322.8
6,250 365.2 306.9 369.3 323.3 371.2 324.9 373.3 313.7 369.6 310.6 369.4 310.4
6,500 358.8 289.9 372 312.6 373.7 314.1 369.2 298.4 369.8 298.8 367.4 296.9

Performer RPM Flowmaster Flowtech Afterburner Flowtech Terminator Flowtech Warlock FlowtechWarlock
(open)
RPM PWR TRQ PWR TRQ PWR TRQ PWR TRQ PWR TRQ PWR TRQ
2,750 105.3 201.2 102.8 196.3 103.9 198.4 102 194.8 102.6 195.9 109.5 209.1
3,000 143.9 251.9 140 245.1 140.9 246.7 138.9 243.2 142 248.6 151.5 265.1
3,250 170.6 275.7 168.9 273 168.4 272.2 170.3 275.1 169.4 273.7 178.9 289.1
3,500 190.2 285.5 189.4 284.1 189.8 284.9 189.1 283.8 185.6 278.5 193.3 290.1
3,750 203 284.4 202.8 284 202.9 284.1 202.4 283.4 199.7 279.7 207.1 290
4,000 215.1 282.4 213.2 279.9 212.9 279.5 213.2 279.9 211.6 277.8 220.8 290
4,250 242.6 299.8 242.2 299.3 240.5 297.2 241.6 298.6 237.8 293.9 248.1 306.6
4,500 272.2 317.7 271.7 317.1 270 315.1 270.7 316 264.4 308.6 276.9 323.2
4,750 294.6 325.7 291.2 322 292.7 323.6 294.1 325.2 281.9 311.8 292.9 323.9
5,000 310.3 326 306.6 322 306.8 322.3 307.4 322.9 295.7 310.6 306.2 321.7
5,250 328.2 328.4 327.6 327.7 325.9 326 326.8 327 318.3 318.4 328.5 328.7
5,500 347 331.3 347.4 331.8 345.7 330.1 346.9 331.3 340.7 325.3 348.8 333.1
5,750 362.2 330.9 361 329.7 359.7 328.6 361.3 330 356 325.2 362.3 331
6,000 368.7 322.7 369.4 323.3 368.4 322.5 368.4 322.5 363.1 317.8 370.7 324.5
6,250 370.9 311.7 368.3 309.5 372.3 312.9 369.5 310.5 366.3 307.8 374.2 314.4
6,500 366.1 295.8 363.7 293.9 366.3 296 366.2 295.9 363.7 293.9 369.7 298.8

Hooker Aerochamber Hooker MAXflow MAC MagnaFlow SpinTech
RPM PWR TRQ PWR TRQ PWR TRQ PWR TRQ PWR TRQ
2,750 103.4 197.4 102.1 195 104.6 199.8 105.3 201.2 105.2 201
3,000 140.5 246 142 248.6 142.3 249.1 143.8 251.8 142.2 249
3,250 168.3 272 172.5 278.8 166.3 268.7 171.3 276.8 167.3 270.3
3,500 190.7 286.2 189 283.6 188.6 283 189.4 284.3 190 285.1
3,750 203.2 284.7 204 285.7 202.9 284.2 204.3 286.1 203.6 285.1
4,000 213.6 280.4 214.7 282 213.1 279.9 215.8 283.4 213.3 280
4,250 240.2 296.9 244.8 302.6 240.2 296.8 244.5 302.1 240.7 297.5
4,500 270.6 315.9 274.8 320.8 268.5 313.4 274.2 320.1 269.5 314.6
4,750 293.5 324.5 296.1 327.4 292.7 323.6 294.1 325.2 293 324
5,000 308.3 323.9 309.9 325.5 310.3 325.9 308.8 324.4 310.8 326.5
5,250 327.1 327.3 329 329.2 328.2 328.4 328.9 329 329 329.1
5,500 346 330.4 348.7 333 347.1 331.5 348.2 332.5 347.9 332.2
5,750 360.4 329.2 362.7 331.3 362.7 331.3 363.6 332.1 362.2 330.8
6,000 368.8 322.9 370.3 324.2 368.7 322.7 370.6 324.4 369.6 323.5
6,250 372.1 312.7 373.5 313.9 372.3 312.9 372.8 313.2 371.6 312.3
6,500 368.8 298 371.8 300.4 363 293.3 369.1 298.3 369.5 298.6

There's so much data here, we simply don't have room to dissect it all, but you have it now, so you can choose the mufflers you are interested in and run the numbers through Excel if you are really serious about eking out every last horsepower. If so, Rick Anderson says to pay particular attention to the average horsepower between 4,700 and 6,600 rpm. This range is critical for drag racing this combo. Rick says the best muffler in that range is the Bassani Real Street, but it's probably too loud for the street (and that's saying something coming from Rick). The Hooker MAXflow, both Borlas, and the MagnaFlow closely followed the Bassani RS with good average horsepower in that range. In the end, we say find one that sounds good to you and your wallet and go have fun.

Sounding Off
Muffler Decibels At Idle Decibels At 2,000 RPM Decibels At WOT
No Muffler 91 103 Max.
Bassani Street 83 96 120
Bassani Real Street 84 96 Max.
Borla XS 80 90 110
Borla XR1 83 96 118
DynoMax Ultra Flo 83 94 113
Edelbrock RPM 84 93 118
Flowmaster 82 93 115
Flowtech Afterburner 86 94 115
Flowtech Terminator 86 94 119
Flowtech Warlock 83 92 112
Hooker Aerochamber 87 94 114
Hooker MAXflow 80 90 120
MAC 87 98 119
MagnaFlow 82 91 114
SpinTech 87 97 116
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » May 21st, 2010, 8:21 am

if you do a back pressure test and find the catalytic converters are plugged, its going to be hurting your engines power without any doubt.
if your in an area that requires you maintain your catalytic converters they will need to be replaced with newer high flow versions to regain performance legally your supposed to have them , but I can tell you from experience that the car runs much better in most cases with headers and a full 3" exhaust, now theres a dozen company's out there selling c4 exhaust systems , but something like this lay out produces good results, don,t forget to buy headers with o2 bungs or weld in your own as the computer needs the info to run correctly, and the less back pressure you have the better your car will run, once its correctly tuned.
check those trouble codes, because if the cats plugged up its frequently the result of a overly rich f/a mix, and a bad o2 sensor or injector or fuel pressure regulator, are suspect.

and yes reading the links below will provide a good deal more useful info

http://www.melroseheaders.com/products/smooth_flow_exhaust_systems/c-4.php

Image

viewtopic.php?f=56&t=1503

viewtopic.php?f=56&t=91&p=115#p115

viewtopic.php?f=56&t=3113

viewtopic.php?f=56&t=1303

viewtopic.php?f=56&t=1166
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » February 8th, 2011, 8:22 am

http://www.chevyhiperformance.com/techa ... index.html

Image


These three circles illustrate the difference in internal square-inch area between 2.25-, 2.50-, and 3-inch exhaust pipes. Weâve calculated the area based on a wall thickness of 0.065. The 2.25-inch pipe has a flow area of 3.80 square inches, a 2.5-inch system increases the area 25 percent to 4.7 square inches, and a 3-inch pipe pumps the area up to 6.8 square inches.

http://www.pypesexhaust.com/xpipes.html

Image

heres a good VISUAL shot to show why an (X) pipe is far less restrictive to balanced/ blended flow
Image
one question that comes up frequently is "why does my engine tend to run cooler after I removed the cats or hotter now that I replaced the cats?"
think it thru, any reduction in exhaust flow velocity means theres slightly more time for engine generated exhaust heat to dissipate,into locations closer to the engine, before the cats rather than past the cats like a faster flow rate provides as it carry's some engine generated exhaust heat past the cats.
purely as a hypothetical example
think of it this way ,if your cats are operating correctly they provide a MINIMAL restriction to flow, but if they have partly melted or clogged up they provide a significant restriction to exhaust flow. cats provide a slight restriction to flow and can even add heat to the exhaust as unburnt fuel is oxidized,lets say each BTU of heat was a marble that you were pouring down thru a 2.5" pipe thats 12 feet long, and it takes 3 seconds for the heat to dissipate from 450f exhaust to drop to the surrounding temps, along the trip at a rate of 50 degs f per 1/3 second, if the cats delay flow of marbles flowing thru the exhaust for only a 1/12 of a second, as the marbles wait to pass thru the slight restriction, your engines going to run about 12 degrees f hotter minimum, even if the cats added no heat themselves , add to that the fact that any back pressure tends to reduce header scavenging, effectively reducing the volume of cooler fuel/air mix entering the cylinders and absorbing heat, before it ignites, thus less heat absorbed and a slower movement of the burnt fuel thru the exhaust.
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » June 29th, 2011, 9:37 am

Techno’s

http://www.miata.net/garage/KnowYourCar/S4_Back.html
“Know your car” Series #4



Back pressure, Exhaust velocity and scavenging



As an avid reader of Miatanet.com’s Forum section, it is quite intriguing to see just how misunderstood the need, or otherwise, is for backpressure in the exhaust system. There are comments that MX-5s need backpressure and those who see it as a bad thing. Often there is no real understanding of what backpressure is or of its consequences.



OK, so here is Backpressure 101.



The purpose of the car’s exhaust system is to evacuate gases from the combustion chamber quickly and efficiently. The exhaust gasses do not flow in a smooth stream. Because the gasses are vented at each opening of the exhaust valves there is a pulse of gasses from each cylinder. Just put you hand near the exhaust tip and you will feel the pulses. In a MX-5 engine there are four pulses per cycle (except if it’s John Pitt’s supercharged V8 then there are eight really big pulses per cycle).



The exhaust gasses produce a positive flow in the exhaust pipe. Backpressure can be likened to resistance to the positive flow of the exhaust stream. Taken to its extreme backpressure can lead to a reversal (albeit momentarily) of the exhaust stream.



Is Bigger Better or is Faster Best?



When contemplating a modified exhaust system there are those who want the biggest diameter pipe that can be had. Their idea must be that fatter pipes are more effective at venting than narrower pipes. This sounds reasonable but it is not quite correct. Sure wider pipes have greater volume and higher flow capacity, but that is just half of the story. Capacity is one consideration but gas velocity is the other factor.

An experienced exhaust designer knows that the best exhaust is one that balances flow capacity with velocity. A given volume/time of gasses will travel faster through a 2" pipe than the same volume of gas passing through a 3" pipe. So when taken to its extremes we can see that a too narrow pipe will create backpressure (restrictions to positive flow) problems and a too wide pipe will cause a very slow flow with no backpressure.

The optimum is where the fastest velocity is achieved with the least constriction possible.



This situation will arise when the pipe is wide enough so that there is the least level of positive back pressure possible whilst achieving the highest exhaust gas velocity.

The faster the exhaust gas pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The scavenge effect can be visualised by imagining the high-pressure pulse with a trailing low-pressure area behind. The faster the high-pressure pulse moves the stronger the draw on the low-pressure gasses and the gasses behind that. The scavenge action is like (but not exactly) suction on the gasses behind.



The greater the clearance burned fuel from the combustion chamber the less diluted the incoming air/fuel mix is. Scavenging can also aid intake on overlapping valves (where the exhaust and inlet valves are open at the same time) by drawing in the intake. These are good things to happen.



So instead of going for the widest pipe possible we should be looking for the combination of the narrowest pipe that produces the least backpressure possible. In this scenario we achieve the least restriction on positive flow and the highest gas travel speed.



Exhaust pipe diameters are best suited to a particular RPM range. If we used a constant RPM engine this would be easy to specify. But a variable RPM engine will mean that not one size suits all. It is possible to vary the size of exhaust volumes according to rpm but it is very expensive (Ferrari has done it). The optimum gas flows (volume and speed) are required at the RPM range that you want your power band to be located. For a given engine configuration a small pipe diameter will produce higher exhaust velocities at a low RPM (good) but create unacceptably high amounts (bad) of back pressure at high rpm. If you had a car with a low RPM power band (2,000-3,000 RPM) you would want a narrower pipe than if your power band is located at 5,000-7,000 RPM.


Urban Myth Number 42:you need back pressure"



It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realises that he has lost power right through the power band. The connection is made in his throbbing brain…. put on 3" pipe = loss of back pressure = loss of power. Max erroneously concludes that you need back pressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.



If Max had chosen a 2 1/4" pipe he would have achieved better performance in the mid- to high-RPM power band. You need the combination of the least positive (close to zero) backpressure possible with the highest gas velocity achievable to create performance. The diameter of the pipe (and smoothness of internal finish and bends) will strongly influence if your exhaust change is going to create performance or lose power.

As a general rule, a normally aspirated MX-5 will get better high RPM performance with a 2 1/4" exhaust system (2 1/2" or above is just too wide to retain exhaust gas velocity for street driving). The general consensus is that a 2 1/4" system is for mid to high RPM petrol heads. Your mechanic should be able to advise you what exhaust system will best suit you driving style and needs.



Forced induction (turbo or supercharged) MX-5s perform better with the high volume pipes (2 1/2" to 3"), but that’s another story. The choice of a 4 into 2 into1 or a 4 into 1 header to exhaust set is yet another story



Rob (Techno) Spargo
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » June 29th, 2011, 9:46 am

http://www.quickhonda.net/exhaust.htm#R ... Air%20Flow

http://www.gomog.com/allmorgan/exhaustbackpressure.html

http://www.custom-car.us/exhaust/turbo.aspx

http://autolounge.net/tech/exhaust.html

Exhaust Performance and Scavenging

The Basics

As are most things automotive, the exhaust system is a compromise. It must balance noise and performance. In addition, the exhaust system can be designed to increase performance in the low RPM range or at the top end. All of these aspects must be taken into account when designing the best possible setup for a given application. A larger pipe diameter typically will reduce backpressure allowing the engine to expend less energy pushing out exaust gas. If the system was being restricted, this will result in more power at a certain range of RPMs. The downside to larger pipes is a loss of power in other areas of the power band and increased noise.

Scavenging
Image

At first glance one might think that a maximum pipe size would result in the best performance. It should provide the most flow. This is true for a steady stream of gas. An engine ejects exhaust gas out of the ports in pulses though. This results in a stop-go-stop-go situation as the exhaust valve opens and closes. A properly sized pipe will help to suck the gases out of the cylinder at certain RPMs. Since air has mass and velocity, it has momentum. When in the scavenging RPM, the exhaust charge exiting actually creates a vaccum for part of the exhaust stroke. This provides for a nice boost in power and torque. For ideal scavenging an exhaust speed of 240-260 feet per second is desired. Thus as engine speed increases for a given displacement so must the pipe diameter.

Below is a typical graph of exhaust port pressure vs. piston position:

NOTE: The exhaust pressure during scavenging nets out to be positive. Thus there is not an all around vaccum. Energy is still being wasted pushing exhaust gas out. The scavenging is not "free". Work is still done. Scavenging merely helps to aliviate what would be even more work being done by the engine. Also note that the psi values will vary from engine to engine, but the curve remains approximately the same. Values over 6 psi are generally considered excessive backpressure.

The graph above is a typical representation of what exhaust port pressure looks like as the engine rotates through almost a full revolution during the scavenging RPMs. Exhaust pressure builds as soon as the exhaust valve opens. This occurs before bottom dead center. Most of the torque the engine generates is produced by 90 degrees, thus it is not necessary to keep the exhaust valve closed much past 90. By opening the exhaust valve early, the high cylinder pressure helps to accelerate the charge in the exhaust manifolds. Pressure peaks before bottom dead center and then rapidly falls as the accelerated charge pulls the exhaust out of the engine. As the piston approaches top dead center, the intake valve opens. Pressure drops again and the exhaust gas actually helps to suck the intake charge into the cylinders.

These ideal conditions can only be met at certain RPMs. To do this one can vary the size of their piping to chose what will provide them the best performance. Smaller pipes will move an equal volume of gas at a higher speed. This typically comes at the expense of increased backpressure (resistance to flow) which inturn leads to poorer performance. A smaller pipe will help to maintain the ideal exhaust gas velocity when the engine is running in the lower RPMs. This provides for a torque boost off the line with improved streetability and driveability. For this reason many manufacturers size their pipes on the more conservative side. As the RPMs increase though, the backpressure builds due to the excessively small pipe size. This pushes the velocity of the exhaust gas out of the scavenging range and saps power on the top end, reducing performance in that spectrum.

A larger pipe will maintain ideal exhaust gas velocity on the top end. This will increase peak power. The downside comes with a slow exhaust charge on the low end. Their will not be enough suction to aid in the expulsion of exhaust gas. Torque will suffer in the low ranges for this reason. Too large or too small of a pipe will push the scavenging bonuses out of the useable RPM range and will result in diminished overall performance. It is important to size the piping correctly for the desired application: low end or top end. One should also consider how this will effect the rest of the car. For instance, if the car has an automatic with a stall speed of 1,500 RPMs it won't do any good to have all the power on the top end when off the line performance will suffer terribly. Likewise if the car is mainly driven in the city or through heavy traffic, a lack of low end grunt might make driving more of a chore.

On valve overlap scavenging also helps to draw in the fresh intake charge. The pressure in the exhaust port drops again and the exhaust flows more easily. It should be noted though that too much exhaust velocity on overlap can cause "over scavenging". This occurs when the exhaust vaccum becomes so great, due to overly small tubing, that it actually sucks the intake charge right through. This leaves spent charge in the cylinders and causes a loss in performance.

Common Views

A couple of commonly recommended modifications are increasing backpressure for lowend performance, and the use of equal length headers. Backpressure is almost always a bad thing. The higher velocity of a smaller pipe is what one wants on the low end. Unfortunately this ultimately chokes off the top end to an extent. Since these two go hand in hand (small pipes and top end back pressure), people often consider them the same. They are not. Putting a sharp bend in a pipe will cause high backpressure, but it will have little benefit for performance and scavenging. Backpressure is very useful for quieting an exhaust though and muffling the sound waves. As for equal length headers, companies often claim that they help with scavenging. They are used to line the exhaust pulses up. This can help during scavenging as the exiting charges will not try to occupy the same collector at the same time. Instead, the exhaust pulses will alternate and "zipper" there way into the collector. This results in smooth flow and optimal performance. Equal length headers, or headers designed to take advantage of scavenging in general are a plus. It is important to buy from a reputable company though as equal lengths will not guarentee optimal performance. Dynometer testing is the best way. Most reputable exhaust companies do make use of a dyno which gives some assurance as to what one is getting.

Summation

1. Choose a smaller pipe for better low RPM performance, a larger one for high RPM performance.

2. Remeber that peaks are not everything, as seen in Understanding Performance, thus a proper sized pipe will deliver better results than one that may give the highest peak low end or top end performance.

3. Back pressure is almost always a bad thing.

For approximating appropriate pipe sizes visit the exhaust sizing or header sizing calculators.
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » August 15th, 2011, 1:55 pm

"hey grumpyvette?
I swapped to a different mufflers and now the car sounds a bit different, but it seems to have lost some low rpm torque, any ideas??"



As I stated several times, you can get a very good idea, as to your exhaust systems back pressure levels, and cylinder scavenging if you do a few tests, you need to know how to tune an engine obviously, and if you don,t know how to do it, id suggest you have a muffler shop in your area that will test the back pressure in your car for you with a pressure gauge and a DB meter for noise levels do so.., or if your really into hot rods, and engine tuning, it would sure be worth the minimal cost to get the required tools to do it yourself.
you can gain a great deal of info reading spark plugs and you can,t do much without knowing how the engines currently running before making changes
you can,t make intelligent decisions without facts, and you get the facts by testing!
and it should be obvious that testing needs to be done under the current operational conditions at the rpm range that your concerned with.

Image
Image
http://www.harborfreight.com/fuel-pump- ... 93547.html

a simple vacuum gauge CONNECTED TO MANIFOLD VACUUM will be a surprisingly accurate tool to diagnose problems,providing you know how to use it.
you don,t need an expensive tool, while your testing you may as well connect it to your carbs intake plenum or throttle body plenum as a next test, because if your pulling over about 1.5" of vacuum at wide open throttle the carb or throttle body your using is a bit restrictive, and if you restrict the exhaust or intake flow it hurts power

Image
check plenum/manifold vacuum
Image
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IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Re: is backpressure hurting your combo?

Postby grumpyvette » April 30th, 2012, 7:28 pm

I recently had a guy in the shop that was having engine tuning issues , with his c4 corvette, one bank of cylinders consistently ran a bit cooler than the other bank , when we checked with an IR temp gun , but the injectors seemed fine and there were no vacuum leaks found..
most tuning issues are easily located with a step by step isolate and test procedure, this took a bit of extra effort because it had a rather rare cause that we over looked for 20 plus minutes until its symptoms lead us to the cause, it acted like one header or exhaust pipe was partly crushed restricting exhaust flow but up on the lift there was no obvious damage..
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now most guys would start looking at the injectors and use a multi meter to check resistance, which was inconclusive, many guys would over look the simple test of putting a hand behind the tail pipe exit to feel pressure and flow differences between the sides , when this was done there was a noticeable difference in the exhaust pulse strength between the two sides?
and use of a infrared temp gun and vacuum gauge to test exhaust temps along the exhaust pipes confirmed that most of the heat and pressure went to only one side, and yes we looked for clogged or kinked pipes but after checking carefully we eventually found several chunks of melted catalytic converter core honey comb that resembled charcoal brickettes inside the exhaust pipe on one side just past the (X) PIPE junction in the exhaust pipes, that were restricting exhaust flow above moderate rpm levels , chunks that had obviously worked loose and traveled to the rear from a previously replaced converter that had melted internally but no one thought to look previously for crud that had become caught where the exhaust dropped from the 3" header collector and previous catalytic converter pipe diam, to just past the (X) PIPE where the exhaust reduced to 2.5" pipe back to the mufflers
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IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Re: is backpressure hurting your combo?

Postby Indycars » May 1st, 2012, 7:49 am

Did the temperatures from the IR Gun give any clues as to where in the exhaust pipe the blockage was located ???
Rick
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