is backpressure hurting your combo?



Re: is backpressure hurting your combo?

Postby grumpyvette » May 1st, 2012, 8:56 am

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



OH! YEAH! you could follow the heat transfer with the IR temp gun,along the exhaust pipe its was very very obvious that the difference in flow of exhaust varied enough that past the (X) pipe there was a 25F-to-30F degree difference in the pipe surface temps due to about 70% of the exhaust exiting the pass side because the rear drivers side past the (X) pipe was restricted, this was strongly suspected because the pass side exhaust pulse strength was noticeably stronger especially if the cars engine was revved
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if you have an older CATALYTIC CONVERTER, its rather common for the core to break up and partly clog the exhaust.
while it may not be your issue its smart to be aware if the potential problem and check it out.
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viewtopic.php?f=56&t=8401&p=29318&hilit=clogged+cats#p29318
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I got these pictures off the internet but its a fairly common issue too have several chunks of melted CAT come loose ,we found several chunks restricting the exhaust catalytic converter core and exhaust pipes in several corvettes where it transitioned from 3" down to 2.5" diam.
as past the (X) he used reducer adapters

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to allow use of standard 2.5" corvette 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!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » August 31st, 2012, 10:36 am

HEY GRUMPY??
HSR 406 down on power or at least peak RPM power?
I have a 406 that I just built for my 1988 Firebird. The car appears to make peak power at a lower RPM than what I thought it would and I would like your opinion of the combination and where the bottle neck could be. This is the combination:
6sp 3.91 gears, some where between 10:1-10.6 cr, SLP cold air kit, MAS gutted, TPIS air foil, TPIS AFPR, Superram, MAC headers, Mac under drive pulleys, MAC cat back, Relocated MAT, E-Tec 200 heads, SDPC Vortec manifold, ZZ-409 cam.
Best ET 12.17
Best 60 FT 1.754
Best MPH 113.1
RWHP 369
RWTQ 399
-406 SBC
-10:1-10.6 to 1 compression
-ETec 200 heads
-Ported HSR that matches the heads
-ZZ-409 HR cam 226/226 at @.050, .550 lift with 1.6 rockers, 112 LCA installed at 108 ICL
-1-5/8 Hedman long tube headers
-2-1/2 exhaust per side that joins into a 3" single exhaust
-52mm throttle body
-SLP cold air high flow intake
-Gutted MAS
-Tuned by me on a Mustang dyno AFR is 12.7 to 1 and flat
-Timing at WOT 32 degrees which neted best power on the dyno

During the dyno runs my WOT intake vacuum climbed to 2-1/2" of water, the exhaust backpressure was 2-1/2 PSI measured at the 3" single exhaust part of the pipe. The car peaked at 404 RWTQ at 4100 RPM and 360 RWHP at 5400 RPM and then slowly dropped to 345 RWHP by 6000 RPM. I thought the car should have peaked around 5800 RPM to maybe 6000 RPM and have been giving this some thought and want some opinions on the low RPM peak. Maybe the heads are too small for a 406, Intake restriction too high, exhaust too restrictive, cam too small? I know the CR is slightly low but that should not effect peak RPM.

The car runs excellent but I am looking for more power, I would like to get 400 RWHP with this car. I took it to the strip last Friday and on a 90 degree night it went 12.13 at 113 MPH at a race weight of 3600 LBS but that is not that much better then the previous 355 SR combination.

Any thoughts on the combination and low peak RPM?





My first impression is that your 2.5" into a 3" exhaust with 2.5 psi of back pressure certainly doesn,t help, Id suggest getting exhaust dumps for use when racing

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http://www.jegs.com/i/Doug%26%23039%3Bs ... K/10002/-1
http://www.summitracing.com/parts/QTP-Q ... /?rtype=10
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but the cams duration is too small,to allow that set up too breath efficiently near peak rpm, Id swap to a crower 00471 and youll get a noticeable improvement, keep in mind the holley stealth ram is a fuel injection tunnel ram and its designed to run efficiently in the 3500rpm-6500rpm power band
why not call crane,
1-866-388-5120
crower,1-619.661.6477
erson,1-800-641-7920
lunati 1-662-892-1500
and
isky 323.770.0930
and talk to the tec support guys about what they suggest but DON,T mention anything any other cam manufacturer suggests during the conversations, just present the info on the parts used in that combo, rear gear ratio, compression, etc and see what they suggest, then make up your mind.

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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!!
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Re: is backpressure hurting your combo?

Postby grumpyvette » August 31st, 2012, 6:37 pm

HEY GRUMPYVETTE?].... I do have a ZZX cam (240/240 @ .050 .595/.595 lift with 1.6 rockers, 112 LCA) laying around that I could try. I decided to try the smaller ZZ-409 cam since I only have between 10:1 and 10.6:1 CR and was concerned that the ZZX would not perform well with the 10:1 CR. What do you guys think of trying the ZZX cam?"

viewtopic.php?f=55&t=1509&p=3459&hilit=base+runner+plenum#p3459


its probably going to be an improvement over the current cam, Id sure swap, don,t get to worried about the port size the extra cam duration will help that restriction once the exhaust is less restrictive as scavenging will improve
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 » October 6th, 2012, 11:55 am

I recently had a guy I know complain that his car seemed to be really restricted or reduced in the power it made, a few checks with an IR heat gauge and a vacuum gauge ,showed a restricted catalytic converter with the core, honeycomb basically 70% clogged, we replaced a defective 02 sensor that was making the engine run excessively rich, which caused the problem then replaced the catalytic converters on both sides with new low restriction higher performance versions and he swears it responded with a huge gain in the "seat of the pants dyno" feel.

http://www.youtube.com/watch?v=9TlygJMx ... re=related

http://www.youtube.com/watch?v=YPQBFvPT ... re=related

http://www.youtube.com/watch?v=Lafv2c4s ... re=related
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 » December 29th, 2012, 10:38 am

" GRUMPY? I recently swapped from 1 .625" shorty headers and a 2.75" dual exhaust to 1.75" full length headers and 3" exhaust with an (X) pipe to reduce the cars effective back pressure, the car runs noticeably better once the rpms exceed about 5000rpm but my 60 foot times got slower and I seem to have lost some lower rpm torque, do I need to make a change to increase my exhaust back pressure?"


first lets point out a few things, headers are designed to increase cylinder scavenging, increasing cylinder scavenging in theory reduces the level of previously burnt exhaust gases , and increases the percentage of fresh fuel/air entering the combustion chamber and being compressed , thus increasing the potential power the engine can make. but in most cases this also means some of the new fuel/air charger is drawn out thru the exhaust before the valves seat on the compression stroke. the diameter of the primary pipes and the length of those pipes plus the collector they feed all effect the exhaust gas velocity as does the cars displacement, compression ratio and cam timing, its also common that changes to the cylinder scavenging will require changes to the jetting, or intake design to maximize the cylinder fill efficiency. and the intake design will also effect how efficiently the cylinders get fed and scavenged by the exhaust gas inertial drawing in the next intake runner charge of fuel/air mix.
back pressure is always bad, but if you increased the header size too much you reduced the exhaust gas velocity thus reducing the cylinder scavenging, this has zero to do with back pressure, but a good deal to do with effectively using the velocity of the gases flowing out of the cylinder to draw in the next fuel fuel air charge.
in many cases adding an extended collector to the headers will get you back the previous headers efficiency, in many case shorty headers use the exhaust system they are matched with as an extended header collector, swapping to a full length header and a larger exhaust effectively reduces the working length of the exhaust.
BACK PRESSURE will ALWAYS restrict the engines power levels IF the engines properly tuned, the cam timings correct and youve got the car geared correctly, if you do some careful investigation Im reasonably sure youll find the increased exhaust header primaries and lower restriction exhaust slowed the exhaust gasses and may have EITHER reduced or increased cylinder scavenging , as youve changed the pressure levels in the cylinder and/or the change in scavenging changed the effective fuel air ratio, in the engine, this is VERY COMMON, and in many cases just using a vacuum/pressure gauge on the intake and header collectors and reading the plugs condition or ideally using a fuel/air ratio meter and an infrared temp gun on the headers where they exit the cylinder heads,will show you that change. in many cases a tighter LSA cam will help increase the power levels as the slightly change overlap tends to offset the increased scavenging and increase torque due to the valves closing a bit earlier.
in some cases retarding the cam will allow the engine to breath at the slightly increased rpm levels the better scavenging allows.
in most cases youll need to make changes to the tuning to try to get as close to 12.5:1 -13:1 on the fuel air ratio as you can, as thats where most engines make the best power levels, in some cases a cam change to more closely match the ideal cylinder scavenging will be required or a rear gear change to match a change in the effective rpm range where your now making power.
Ive seen several cars where cheaper "LONG TUBE HEADERS" like these in this picture, failed to provide the intended power until an extended section like the one posted just below was added, which markedly increased the effectiveness of the header scavenging.
keep in mind most of the less expensive headers are designed with low cost and ease of installation as far more important goals in the fabrication process than maximizing power levels, plus the header manufacturer has zero idea as to the cam timing compression, displacement or other factors in the engine they will be matched with.
you might also consider that longer and larger diameter headers will more effectively handle increased exhaust gas flow, if you feed the flow into a common (X) pipe that smooths and blends the exhaust flow from both sides of the engine

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READ THE THREAD THRU TO THE END AND BE AWARE IT, AND ALL OTHER THREADS ARE CONSTANTLY UPDATED WITH NEW LINKS AND INFO

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http://www.summitracing.com/parts/FLO-C ... /?rtype=10
This is not, or at least should not be, random guess work, its a process of, testing, observing and correcting how the engine runs from observed test results. while most guys rely on a system of trail and error, the results can be calculated, the physics involved are well known, so you,ll have too spend far less time swapping random parts to find the ideal component config. if you read the links and do some calculations and testing
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related info, the factors effecting your engines current power level can be tested, observed and if necessary corrected,

http://maxracesoftware.com/pipemax36xp2.htm

viewtopic.php?f=56&t=1503&p=3419&hilit=coke+tape#p3419

viewtopic.php?f=56&t=7017

viewtopic.php?f=52&t=1070

viewtopic.php?f=56&t=1503

viewtopic.php?f=56&t=1303

http://www.circletrack.com/enginetech/c ... ewall.html

http://www.bing.com/search?q=how%20head ... owAppsUI=1

http://perfweldheaders.com/headerscavengetech.html

http://www.youtube.com/watch?v=pDQXsfeZGwk

http://blog.racingarticles.com/2008/02/ ... aders.html

http://www.hondatuningmagazine.com/tech ... ewall.html

viewtopic.php?f=56&t=185

viewtopic.php?f=56&t=2537

http://www.carcraft.com/techarticles/he ... ewall.html
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 6th, 2013, 8:11 pm

http://www.popularhotrodding.com/engine ... ewall.html
quote
"For me the first really serious look at how to muffle a high-performance race engine without loosing a significant amount of power started in 1980 when I built a 400lb-ft, 404hp 350 to replace the very lame 158hp 305 in my California-spec Pontiac Trans Am. Having worked very hard to build a pump gas fueled engine (gas was really bad in those days), that would cross the 400 hp barrier, I was very disappointed to find that, regardless of what mufflers were used, the output dropped by some 20 lb-ft and 25 hp. Having had some experience designing a no-loss system for the original style British Mini Coopers, I felt confident I could pull off the same stunt for significantly bigger V-8 engines. The result, aided by an acoustics expert friend, was the Sonic Turbo. This design went on to be manufactured by Cyclone (now a division of Walker/ Dynomax). After the smoke cleared from a big muffler shootout (done at Gale Banks facility and published by Hot Rod magazine), a pair of 2.25-inch Sonic Turbos (the 2.5-inch ones were still a couple of months off) sunk everybody else's 2.5-inch items. This, it seemed, was just what the hot rod fraternity wanted and they sold by the hundreds of thousandths. That was good, but more importantly, it appeared to spark the industry into aggressively pursuing significantly more functional mufflers and exhaust systems. The result is that 20-some-years later, all the necessary components to build a highly effective, no-loss system are at hand, and not necessarily that much money either. All that appears to be lacking is widespread know-how as to what is needed to achieve this happy state of affairs. As of now, we are going to make a start on putting that right.

Simple Steps to Success
Although the mode of function of an exhaust system is complex, it is not (as so often is believed, even by many pro engine builders) a black art. To help appreciate the way to get the job done I will go through the process of selecting exhaust system components for a typical high-performance V-8 in a logical manner from header to tail pipe. Although the entire exhaust functions as a system, we can, for all practical purposes, break down many of the requirements that need to be met into single entities. Fig. 1 details the order of business. But before making a start, it is a good idea to establish just why getting the exhaust correctly spec'd out is so important. This will allow realistic goals, improved component choice, and a more functional installation.

The V-8 engines we typically modify for increased output are normally categorized as four-cycle units. Although pretty much the case for a regular street machine, this is far from being the case for a high-performance race engine. If we consider a well-developed race engine, the usual induction, compression, expansion (power stroke) and exhaust cycles have a fifth element added (Fig. 2). With a race cam and a tuned-length exhaust system, negative pressure waves traveling back from the collector will scavenge the combustion chamber during the exhaust/intake valve overlap period (angle 5 in Fig. 2). To understand the extent to which this can increase an engine's ability to breathe, let's consider the cylinder and chamber volumes of a typical high-performance 350 cubic-inch V-8.

Assuming for a moment no flow losses, the piston traveling down the bore will pull in one-eighth of 350 cubic inches. That's 43.75 cubic-inch, or in metric, 717cc. If the compression ratio is say 11:1, the total combustion chamber volume above this 717cc will be 71.7cc. If a negative pressure wave sucks out the residual exhaust gases remaining in the combustion chamber at TDC, then the cylinder, when the piston reached BDC, will contain not just 717 cc but 717 + 71.7 cc = 788.7 cc. The result is that this engine now runs like a 385 cubic-inch motor instead of a 350. That scavenging process is, in effect, a fifth cycle contributing to total output.

But there are more exhaust-derived benefits than just chamber scavenging. Just as fish don't feel the weight of water, we don't readily appreciate the weight of air. Just to set the record straight, a cube of air 100 feet square will weigh 38 tons! If enough port velocity is put into the incoming charge by the exhaust scavenging action, it becomes possible to build a higher velocity throughout the rest of the piston-initiated induction cycle. The increased port velocity then drives the cylinder filling above atmospheric pressure just prior to the point of intake valve closure. Compared with intake, exhaust tuning is far more potent and can operate over ten times as wide an rpm band. When it comes to our discussion of exhaust pipe lengths it will be important to remember this.

At this time a few numbers will put the value of exhaust pressure wave tuning into perspective. Air flows from point A to point B by virtue of the pressure difference between those two points. The piston traveling down the bore on the intake stroke causes the pressure difference we normally associate with induction. The better the head flows the less suction it takes to fill (or nearly fill) the cylinder. For a highly developed two-valve race engine the pressure difference between the intake port and the cylinder caused by the piston motion down the bore, should not exceed about 10-12 inches of water (about 0.5 psi). Anything much higher than this indicates inadequate flowing heads. For more cost-conscious motors, such as most of us would be building, about 20-25 inches of water (about 1 psi) is about the limit if decent power (relative to the budget available) is to be achieved. From this we can say that, at most, the piston traveling down the bore exerts a suction of 1 psi on the intake port Fig. 3.

The exhaust system on a well-tuned race engine can exert a partial vacuum as high as 6-7 psi at the exhaust valve at and around TDC. Because this occurs during the overlap period, as much as 4-5 psi of this partial vacuum is communicated via the open intake valve to the intake port. Given these numbers you can see the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore. The only conclusion we can draw from this is that the exhaust is the principal means of induction, not the piston moving down the bore. The result of these exhaust-induced pressure differences are that the intake port velocity can be as much as 100 ft./sec. (almost 70 mph) even though the piston is parked at TDC! In practice then, you can see the exhaust phenomena makes a race engine a five-cycle unit with two consecutive induction events.

With the exhaust system's vital role toward power production established, it will be easy to see that understanding how to select and position the right combination of headers, resonators, routing pipes, crossovers and mufflers will be a winning factor. This will be especially so if mufflers are involved in the equation. I first started putting out the word on how to build no-loss systems as much as 20 years ago and I am somewhat surprised that it is still commonly believed that building power and reducing noise are mutually exclusive. Historically, this has largely been so, but building a quiet system that allows the engine to develop within 1 percent of its open exhaust power is entirely practical. Be aware that knowing what it takes in this department can easily deliver a 40-plus hp advantage over your less-informed competition.

Headers -- Primary Pipe Diameters
Big pipes flow more, so is bigger better? Answer: absolutely not. Primary pipes that are too big defeat our quest for the all-important velocity-enhanced scavenging effect. Without knowledge to the contrary, the biggest fear is that the selected tube diameters could be too small, thereby constricting flow and dropping power. Sure, if they are way under what is needed, lack of flow will cause power to suffer. In practice though it is better, especially for a street-driven machine, to have pipes a little too small rather than a little too big. If the pipes are too large a fair chunk of torque can be lost without actually gaining much in the way of top-end power.

At this point determining primary tube diameters is starting to look like a tight wire act only avoidable by trial and error on the dyno. Fortunately, a little insight into what it is we are attempting to achieve brings about some big-time simplification. Our goal is to size the primary pipes to produce optimum output over the rpm range of most interest. The rate exhaust is dispensed with, and consequently, the primary pipe velocity, is strongly influenced by the port's flow capability at the peak valve lift used. From this premise it has been possible to develop a simple correlation between exhaust port-flow bench tests and dyno tests involving pipe diameter changes. This has brought about the curves shown in the graph Fig. 4 which allow primary sizing close enough to almost eliminate the need for trial-and-error dyno testing.

Primaries For Nitrous UseSince nitrous injection is so popular, it's worth throwing in the changes needed to optimize with the nitrous on. For a typical race V-8 the area of the primary pipe needs to increase about 6-7 percent for every 50hp worth of nitrous injected. For street applications, where mileage and performance when the nitrous is not in use is the most important, pipe size should not be changed to suit the nitrous.

Headers -- Primary Pipe Lengths
Misconceptions concerning exhaust pipe lengths are widespread. Take for instance the much-overworked phrase "equal-length headers." More than the odd engine builder/racer, or two, have made a big deal about headers with the primary pipes uniform within 0.5 inch. The first point this raises is whether or not what was needed was known within 0.5 inch! If not, the system could have all the pipes equally wrong within 0.5 inch! Trying to build a race header for a two-planed crank V-8 with lengths to such precision is close to a waste of valuable time. Under ideal conditions it is entirely practical for an exhaust system to scavenge at or near maximum intensity over a 4,000 rpm bandwidth. Most race engines use an rpm bandwidth of 3,000 or less rpm. If the primary pipe scavenging effect overlaps by 3,000 rpm then it matters little that one pipe tunes as much as 1,000 rpm different to another. Since this is the case, then all other things being equal, pipe lengths varying by as much as 9 inches have little effect on performance. A positive power-increasing attribute of differing primary lengths is that it allows larger-radius, higher-flowing bends and more convenient pipe routing to the collector in often confined engine bays.

Apart from the reasons just mentioned, there is also another sound reason why we should not unduly concern ourselves about equal primary lengths. In practice, the two-plane cranks that typically equip V-8 race engines render the exhaust insensitive to quite substantial primary length changes. Experience indicates inline four-cylinder engines are more sensitive to primary pipe length, but a two-plane cranked V-8 is not two inline fours lumped together. It is two V-4s and, as such, does not have even exhaust pulses along each bank. With a conventional, as opposed to a 180-degree header, exhaust pulses are spaced 90, 180, 270, 180, 90 and so on. The two cylinders discharging only 90 degrees apart are seen, by the collector, as one larger cylinder and accounts for the typical rumble a V-8 is known for. This means the primaries act like they do on a four-cylinder engine, but the collector acts as if it were on a 3-cylinder engine having different sized cylinders turning at less revs. (Doesn't life get complicated?) This, plus the varied spacing between the pulses appears to be the cause of the system's reduced sensitivity to primary length.

These uneven firing pulses on each bank seem to work in our favor. Evidence to date suggests that single-plane cranked V-8s, which have the same exhaust discharge pattern as an in-line four-cylinder engine, make less horsepower and are more length sensitive. Dyno tests with headers having primary lengths adjustable in three-inch increments show that lengths between 24 and 36 inches have only a minor effect on the power curve of V-8s that you and I can typically afford, although the longer pipes do marginally favor the low end.

Secondaries -- Diameters and Lengths
Well, so much for primary pipe dimensions and their effect on output. Let us now consider the collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively easily and it is often best optimized at the track rather than the dyno.

As for the secondary length-that is from about the middle of the collector to the end of the secondary (or the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice, collector length and diameter can have more effect on the power curve than the primary length. A basic rule on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end. Except for the most highly developed engines, many collectors I see at the track are too large in diameter and either too short, or of excessive length. For a motor peaking at around 6,000-8,500 rpm, a collector length of 10-20 inches is effective.

Getting secondary lengths nearer optimal can be worth a sizable amount of extra power as Fig. 5 shows. If you want to bump up torque at the point a stock converter starts to hook up the engine, you may want a secondary as long as 50 inches but something between about 10 and 24 is more normal. The shorter of these two lengths would be appropriate for an engine peaking at about 8,500 rpm whereas the longer length would be best for an engine that peaked at about 4,800-5,000 rpm.

Mufflers -- Two Golden Rules To Avoid Power Loss
Inappropriate muffler selection and installation (which appears so for better than 90 percent of cases) will, in a very effective manner, negate most of the advantages of system length/diameter tuning. The question at this point is what does it take to get it right and how much power are we likely to loose if the system is optimal? The quick and dirty answers to these questions are "not much" and "zero." This next sentence is the key to the whole issue here, so pay attention. To achieve a zero-loss muffled high-performance race system we need to work with the two key exhaust system factors in total isolation from each other. These two factors are: the pressure wave tuning from length/diameter selection, and minimizing backpressure by selecting mufflers of suitable flow capacity for the application. If we do this then a quiet (street-legal noise levels) zero-loss system on a race car is totally achievable without a great deal of effort on anybody's part. Ultimately, it boils down to nothing more than knowledgeable component selection and installation, so let's look at what it takes in detail.

Muffler Flow Basics
We select carbs based on flow capacity rather than size because engines are flow sensitive, not size sensitive. This being so, why should the same not apply to the selection of mufflers? The answer (and here I'd like muffler manufactures to please note) is that it should, as the engine's output is influenced minimally by size but dramatically by flow capability. Buying a muffler based on pipe diameter has no performance merit. The only reason you need to know the muffler pipe size is for fitment purposes. The engine cares little what size the muffler pipe diameters are but it certainly does care what the muffler flows and muffler flow is largely dictated by the design of the innards. What this means is that the informed hot rodder/engine builder should select mufflers based on flow, not pipe size.

A study of Fig. 6 will help to give a better understanding as to how the design of the muffler's core, not the pipe size, dictates flow.

Let's start by viewing a muffler installation as three distinct parts. In Fig. 6, drawing number 1, these are the in-going pipe, the muffler core and the exit pipe. Drawing number 2 shows a typical muffler which has, due to a design process apparently unaided by a flow bench, core flow significantly less than an equivalent length of pipe the size of the entry and exit pipe. Because the core flow is less than the entry and exit pipe then the engine "sees" the muffler as if it were a smaller and consequently more restrictive pipe as per drawing number 4. If the core has more flow than the equivalent pipe size, as in drawing number 5, it appears larger than the entry and exit pipe. Result: the muffler is seen by the engine as a near zero restriction. A section of straight pipe the length of a typical muffler, rated at the same test pressure as a carb (10.5 inches of mercury), flows about 115 cfm per square inch. Given this flow rating, we will see about 560 cfm from a 2.5-inch pipe. If we have a 2.5-inch muffler that flows 400 cfm, the engine reacts to this just the same as it would a piece of straight pipe flowing 400 cfm.

At 115 cfm per square inch, that's the equivalent to a pipe only 2.1 inches in diameter. This is an important concept to appreciate. Why? Because so many racers worry about having a large-diameter pipe in and out of the muffler. This concern is totally misplaced, as in almost all but a few cases, the muffler is the point of restriction, not the pipe. The fact that muffler core flow is normally lower than the connecting pipe can be off set by installing something with higher flow, such as a 4-inch muffler into an otherwise 2.75-inch system"
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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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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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