basics on exhaust



basics on exhaust

Postby grumpyvette » September 17th, 2008, 8:10 am

theres plenty of fluid dynamics math and research out there to show that the distances the exhaust travels between exhaust pulses and the diam. and length are easily calculated, and past that length the second previous pulse has little effect compared to the current and previous pulse energy and reflective wave
and lets not forget the cam timing displacement and intake port all effect the cylinder scavenging the headers can effectively provide also
heres some tools to help, if your building your own custom exhaust
http://www.metalgeek.com/static/cope.pcgi
http://pipemastertools.com/store/page1.html

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

http://www.rbracing-rsr.com/runnertorquecalc.html

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

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

http://www.headerdesign.com/

http://www.pontiacracing.net/js_header_length1.htm

http://www.slowgt.com/Calc2.htm#Header

viewtopic.php?f=56&t=961

example , my 383 vette has a cam with exhaust cam timing that opens at 83degs bbdc, thats 97 degs atdc, http://www.cranecams.com/?show=browseParts&action=partSpec&partNumber=119661&lvl=2&prt=5
Bore: (Inches) 4.03"<BR>Exhaust Valve Opening Point: (Degrees ATDC) 97 degs
Peak Power RPM: 5500rpm Calculated information appears below
Header Pipe Diameter: (Inches) 1.84"<
Header Pipe Length: (Inches) 37.65
Collector Diameter: (Inches) 3.5
Collector Length: (Inches) 18.82


so ideally the (X) is placed at that point to maximize scavenging but that's not always possible due to clearance, and if you choose to place an (H) just before the mufflers its mostly to reduce resonance or noise not increase scavenging but it tends to reduce the restriction to flow



http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=50

Image

"I could put BOTH and X and an H- pipe in the system.

Would this be a waste of effort???

Does the H-pipe equalize the pulses to the extent that a downstream X-pipe no longer functions as it should?"


if you place TWO it tends to mellow the exhaust tone and reduce resonance in the car and depending on the location and pipe diam. and length it usually does help the cylinder scavenging a bit more than one alone, but again the closer to the exhaust headers the better off you'll be and ideally the (X) should be closer to the engine than the (H) but that being said up front, I installed an 3"(H) just behind/under the bell housing rear , where the headers ended,and a second 3"(H) just in front of the mufflers on a friends GTO 3" full exhaust and a very nice rumble/lopey idle and a good wide power curve for his 455 pontiac was the result, were both pleased

you may want to keep in mind the HEADERS and the primairy diam. amd length and the collector design are what effects the cylinder scavaging , the exhaust past that is basically designed to supply a level of noise reduction and a low resistance to flow path for the exhaust to safely exit the car, if the headers are designed correctly the engines exhaust system past the collectors just needs to be designed to reduce noise and provide that low resistance exit path.
the (H) or(X) reduces the restriction to flow and blends the exhaust pulses to reduce the noise, if the exhaust past the collector does act as a collector extension on the headers is usually a bit smaller and restrictive than ideal.
in an ideal world the restriction to flow past the header collector would be very similar to running open headers and the main function of the exhaust would be only noise reduction, and exiting the exhaust where it would not re-enter the crew compartment. but the truth is most exhaust system pipes are too small in dia. to allow that so the (h) or (X) provides a way to reduce flow restriction by increasing the cross sectional area of the exhaust path,and blend and cancel out the pulses that make the noise
the IDEA of the (H) pipe is to allow some of the flow mass in the orrignating flow route to exit the original exhaust thus lowering the resistance,to flow the restriction presented by the single pipes cross sectional area provided,and pressure dropping off,and due to a significant percentage of that mass exiting and taking the lower resistance secondary route lowers pressure and the disruption of the sound waves tends to lower the sound of the exhaust also.
keep in mind properly designed headers and collectors provide the cylinder scavenging and there length and dimensions are calculated to maximize that scavenging effect the exhaust past the collectors is basically designed for noise abatement and safely exiting the exhaust gases.
you get very little scavenging effect from anything past the collectors if the systems designed correctly.
you might also consider the fact that flow restriction tends to increase with the rpm band, your stock exhaust is probably fine at the 1500rpm-4000rpm band it was designed for but when your expecting to zing the rpms up in the 4500rpm-6500rpm or above range the headers and collectors, and exhaust behind them,that are used effect a much bigger part of the potential power curve
__________

________most guys I talk and deal with,seem to think that slapping a set of headers on their car is all that's required to boost performance....ANY HEADERS! ...and they fail to realize that both the header design and the exhaust system behind them require a good deal of thought if you want the maximum benefits, and that simply hooking a restrictive exhaust system to the collectors on the best headers will negate most of their potential benefits.
most commercial headers are designed mostly for ease of manufacturing and ease of installation,little thought goes into maximizing the cylinder scavenging which is their main function.
your fooling yourself if you think headers will provide a big boost in hp/tq without the low restriction exhaust behind them,and in most cases that requires a larger diam. exhaust system and adding an (X) to the system and extending the header collectors at their full diam. up till at least the exit of the (X) so the twin exhaust pipes cross sectional area can provide that reduced restriction to flow, rather than the reducers many guys install to the collector exits to adapt them to the stock exhaust system

If your wondering why exhaust valves are smaller and exhaust ports normally flow less, its because they are flow rated with the same flow bench and at the same pressure drop as the intake ports (normally at 28" of water) your not alone in that question.
but EXHAUST PORTS operate in the real world at far higher pressure than intake ports do which alters the resulting flow significantly
the reason is that unlike the intake charge thats forced in by outside air pressure when the valves open on a cylinder with a lower than the outside air pressure, because the the pistons descending, and rapidly creating a partial vacuum which basically is limited to 14.7 psi at sea level change in internal vs external pressure to fill the cylinders the exhaust has THREE separate processes helping the exhaust exit
ONE
the burnt cylinder gases are under far greater pressure, easily 300psi-600psiwhen the valve opens
TWO
the piston on the exhaust stroke physically forces out the gases as it rotates to TDC, sweeping the cylinder volume out the exhaust
three
the headers if tuned correctly have a low pressure wave pulse bouncing back to reach the exhaust valve as it opens to help scavenge, and drag out the exhaust as it try's to follow the inertia of the previous exhaust pulse.
Image
http://www.auto-ware.com/combust_bytes/pv.htm
http://performancetrends.com/blog/?p=53



read these links

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

http://autoclub.rso.siuc.edu/frange.html

viewtopic.php?f=56&t=1303&hilit=coke

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

viewtopic.php?f=56&t=260

viewtopic.php?f=56&t=789

example of a well laid out exhaust (MELROSE)
Image

but even that system could be hurt if you slap restrictive mufflers on it

a well thought thru design on an (X) pipe and cut outs
Image

Image
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

User avatar
Site Admin
Site Admin
 
Posts: 14105
Joined: September 14th, 2008, 1:40 pm
Location: florida

Re: basics on exhaust

Postby grumpyvette » September 17th, 2008, 8:12 am

Grumpy,
If you get time and/or want to, could you talk a little more about scavenging and how the exhaust pulses <sp?> bounce back to create back pressure? I think a lot of us don't understand the fundementals of improving performance with exhuast modifications.

As a young man (a long time ago) I thought bigger and/or opening everything up would be less "restrictive" and therefore make more power. It was a real revelation to me years later, i.e. putting in an "x" pipe in an effort to use these pulses to help pull the exhuast out rather than bouncing back against each other creating back pressure.

How close am I? Do I have it all wrong? Some exhuast 101 would be appreciated to correct me where ever I am messed up.

Foy
Las Vegas

you seem to grasp the ideas,... BIGGER is NOT always BETTER, and since both the header primairy dia. and length and the collectors , which have a huge effect on the resulting scavaging can,t be changed as we change engine rpms, we need to maximize the cylinder scavaging charicteristics so as to maximize the cylinder filling and extend the rpm band of the torque curve but once your have the collectors and headers primairy designed to maximixe the scavaging in your chosen and intended rpm range and run the collectors to a (X) to induce both increased scavaging and lower restriction to flow theres not much that a larger exhaust past that point can do badly but increase the noise levels while it should be rather obvious that a smaller than ideal exhaust will hurt the upper rpm band as it tends to be a restriction
yes if you have a smaller exhaust dia. it tends to act like an extended collector and increase low rpm torque at the cost OF being A restriction ONCE THE RPMS BUILD PAST A CERTAIN POINT.
having both collectors empty into an (X) pipe EFFECTIVELY instantly doubles the cross sectional area of the exhaust pulse and significantly reduces the return reflected pressure wave, almost making the collectors act as if its running without any restiction compared to a true dual exhaust IF the exhaust pipes are large enough to provide a very low restriction at that point

as I POINTED OUT ABOVE...
theres plenty of fluid dynamics math and research out there to show that the distances the exhaust travels between exhaust pulses and the diam. and length are easily calculated, and past that length the second previous pulse has little effect compared to the current and previous pulse energy and reflective wave
and lets not forget the cam timing displacement and intake port all effect the cylinder scavaging the headers can effectively provide also

http://www.engr.colostate.edu/~allan...ngth/pipe.html

http://www.rbracing-rsr.com/runnertorquecalc.html

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

http://www.headerdesign.com/

http://www.pontiacracing.net/js_header_length1.htm

http://www.slowgt.com/Calc2.htm#Header



LETS ASSUME I WANT MY 383 TO MAKE MAX POWER IN THE 5000RPM-6300RPM BAND (mostly so I can run street gears and pump high test gas and a low maintinance hydrolic roller cam, and IM willing to sacrifice a good deal of street driveability to maximize my corvettes track potential)

useing the above calculators we quickly find I should have about a 3 sq inch intake port cross sectional area, the exhaust should be about 39" long in the primairy 1.825 dia,and about 18" -20" long in the collector, about 3"-3.5" dia.
a matching compression of about 10.5:1-11:1 and a cam in the 230-235 durration range at .050 lift, heads that have the same 3 sq inch port and flow about 280cfm this will tend to maximize the power at THAT rpm band, and ideally a 3.90:1-4.11:1 rear gear ratio and a 3000rpm-3500rpm stall converter
but that above will NOT work nearly as well as a smaller and less radical combo in the 1500rpm-4500rpm most cars spend 90% of thier time in
its all a compromise and most people don,t realize how miserable that combo will make the daily driven car that rarely get above 4500rpm, where a smaller and longer exhaust would scavage more effectively but give up some of the potential for max power when the cars raced
bjhines posted these and thier good examples
looking thru an (X) pipe

Image

mocking it up
Image

Image
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

User avatar
Site Admin
Site Admin
 
Posts: 14105
Joined: September 14th, 2008, 1:40 pm
Location: florida

Re: basics on exhaust

Postby grumpyvette » January 20th, 2009, 7:39 pm

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

OK think it thru without getting on either side of the discussion UNTILL 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, as it forces the piston down and 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

This might interest you[/color]

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

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

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

viewtopic.php?f=56&t=495
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

User avatar
Site Admin
Site Admin
 
Posts: 14105
Joined: September 14th, 2008, 1:40 pm
Location: florida


Return to Exhaust and Mufflers

Who is online

Users browsing this forum: No registered users and 1 guest

cron