Exhaust Back Pressure- Good or Bad?

[JOEX]

The reference to needing back pressure for a motor to run properly has been mentioned many times with some debate.

I did some quick looking around and it seems exhaust back pressure isn't, for the most part, a good thing.

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

Destroying a myth.



Some say that "an engine needs backpressure 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 backpressure 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 backpressure with torque, and others fear that too little backpressure will lead to valve burning.

The first reason why people say "backpressure is good" is because they believe that increased backpressure 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 backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure 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 backpressure.

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 backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor 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 backpressure, 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 backpressure 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 backpressure 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 backpressure reduction, these limits won't be reached.

- Adapted from Thomas V.

[JOEX]

http://auto.howstuffworks.com/question172.htm

How do exhaust headers work to improve engine performance?


Headers are one of the easiest bolt-on accessories you can use to improve an engine's performance. The goal of headers is to make it easier for the engine to push exhaust gases out of the cylinders.

When you look at the four-stroke cycle in How Car Engines Work, you can see that the engine produces all of its power during the power stroke. The gasoline in the cylinder burns and expands during this stroke, generating power. The other three strokes are necessary evils required to make the power stroke possible. If these three strokes consume power, they are a drain on the engine.

During the exhaust stroke, a good way for an engine to lose power is through back pressure. The exhaust valve opens at the beginning of the exhaust stroke, and then the piston pushes the exhaust gases out of the cylinder. If there is any amount of resistance that the piston has to push against to force the exhaust gases out, power is wasted. Using two exhaust valves rather than one improves the flow by making the hole that the exhaust gases travel through larger.

In a normal engine, once the exhaust gases exit the cylinder they end up in the exhaust manifold. In a four-cylinder or eight-cylinder engine, there are four cylinders using the same manifold. From the manifold, the exhaust gases flow into one pipe toward the catalytic converter and the muffler. It turns out that the manifold can be an important source of back pressure because exhaust gases from one cylinder build up pressure in the manifold that affects the next cylinder that uses the manifold.

The idea behind an exhaust header is to eliminate the manifold's back pressure. Instead of a common manifold that all of the cylinders share, each cylinder gets its own exhaust pipe. These pipes come together in a larger pipe called the collector. The individual pipes are cut and bent so that each one is the same length as the others. By making them the same length, it guarantees that each cylinder's exhaust gases arrive in the collector spaced out equally so there is no back pressure generated by the cylinders sharing the collector.

[JOEX]

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

Exhaust Backpressure Study

Replacing the stock production exhaust system with a low-restriction, free-flow one is usually one of the first modifications made to any vehicle in the name of performance. We all know they're louder, but how much performance do they really add? We've all seen supposed dyno tests, usually run by the exhaust manufacturer's themselves on their own dyno, indicating vast power gains, and psychologically, we always equate a healthy exhaust rumble with increased power in the seat of the pants, but how much power are we really gaining? To find out, we're running a simple backpressure study, and our results will be posted here as they come. Admittedly this study is not totally scientific as there are many uncontrolled variables, but it should be sufficient to provide a rough estimate.
It is generally accepted by automotive engineers that for every inch of Hg of backpressure (that's Mercury - inches of Hg is a unit for measuring pressure) approximately 1-2 HP is lost depending on the displacement and efficiency of the engine, the combustion chamber design, etc. Our sources indicated that in the case of the L67 3800SC, 1HP per inch of Hg is reasonable.

1 inch Hg backpressure = 1 HP lost

For reference, we have the following conversions factors:

1 ATM = 14.7 PSI = 76 cm of Hg = 29.921 inches of Hg = 1.013 bar

[JOEX]

And a couple more....

http://www.proficientperformance.com/tech_back_pressure.php

http://www.team-integra.net/sections/articles/showarticle.asp?ArticleID=355

Happy Reading:)

[RiderTRX400ex]

wow.. i have questioned this in the past too.. as i always hear backpressure this and backpressure that.. great research man

STICKY:cool:

[2004exrider]

PLEASE make this a sticky, great look up!

Jimmy

[JDRider]

Ok, I realize this is all a theoretical conversation but I have pondered this whole backpressure issue also.

If no backpressure produces the most power, wouldn't it be easier to just run a large header and a straight pipe or a straight through glasspack type muffler, then install a huge air filter with no airbox.

Then start jetting to match the airflow and be done with it??

Of course you can get into lager carbs, and porting, ect.

I've experimented with different backpressures and found I got a stronger power increase at a lower RPM with backpressure variences.

Now of course, all this is measured by my Buttdyno. I once believed in the zero backpressure idea also, but now I have felt a difference and I have changed my oppinion slightly.

Once again, just peoples theories.

JDRider

[JOEX]

From the reading I did it appears some back pressure helps in the lower rpm range which might be noticeable on the butt-dyno.

This may be a poor comparison but look at pro drag cars, they all run very short headers. They also run at a fairly high rpm. I would think since they are trying to maximize horsepower with little regard to longevity they will be doing everything they can to get the maximum horsepower.

Just my opinion of course:)

[GPracer2500]

Originally posted by JDRider
....If no backpressure produces the most power, wouldn't it be easier to just run a large header and a straight pipe or a straight through glasspack type muffler, then install a huge air filter with no airbox.

Then start jetting to match the airflow and be done with it??

Of course you can get into lager carbs, and porting, ect....

Not really. Or at least, not in many cases. Big, large diameters that can flow huge amounts of air are not always best.

One thing I didn't see emphasized in the posted links was the importance of air velocity (both for intake and exhaust). It's a very important point that's sometimes missed.

To the best of my knowledge backpressure is always bad. It never helps, it only hurts. So you might think that a huge header, intake, carb, etc. would be the best. The problem is that if you have a huge header that produces no backpressure the velocity of the gases is reduced. Imagine bolting up a 4" header to a 450cc engine. There will be little to no backpressure. But the exhaust gases will also be flowing through it at a comparatively low velocity. High velocity is good because it helps to scavenge gases into & out of the cylinder.

As I understand it, the goal should be an exhaust with the highest gas velocity possible with the least backpressure possible. Changing one can affect the other and therein lies some confusion. But they are separate factors.

I believe the "backpressure helps torque" idea stems from what backpressure will sometimes do to gas velocity. If adding some backpressure through a smaller diameter or longer length header or smaller exhaust outlet increases gas velocity, the benefit from the increased velocity may overshadow the detriment of the backpressure. The net result is an increase in power and it looks like the backpressure was the reason. Really it was the increase in velocity that is providing the improvement.

I believe increased backpressure and increased low end torque are erroneously correlated because it's at lower rpm that gas velocities are lowest and exhaust restrictions have the least effect. Hence, at low rpm the positive effect of increased gas velocity is more likely to overshadow the negative effect of increased backpressure.

The principle behind stepped headers centers around increasing gas velocity. Too large of a carb hurts low end power power because of their lower gas velocities at lower rpm. Gas velocity crops up again with porting. Hogging out ports with no consideration for gas velocity can take you backwards. Check out some recent porting changes to OEM Honda and Yamaha heads....

http://i34.photobucket.com/albums/d133/gpracer2500/42.jpg

http://i34.photobucket.com/albums/d133/gpracer2500/0405CRF250R.jpg



It's all about increasing gas velocity. It's not about increasing pressure with restrictions.

JOEX those are some good reading articles. Nice find. I agree make this one a sticky

[JOEX]

Did a little more looking around after reading GPracer's post:D

I like to learn new things;) Hopefully I can retain some of it:p

Most of these articles are about car motors but i'm sure alot of it applies to any four stroke motor.

http://www.miata.net/garage/KnowYourCar/S4_Back.html

Techno’s
“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 backpressure 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 backpressure 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: "MX-5s need backpressure"


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 backpressure = loss of power. Max erroneously concludes that you need backpressure 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.



Safe journey



Rob (Techno) Spargo

Mazda MKX-5 Club Victoria

[JOEX]

This is a long one cut into two parts....


http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/

Exhaust Science Demystified
The fact is most cars are leaving horsepower on the table. We show you how to get it back.
all contributors: David Vizard
photographer: Various Manufacturers

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.

[JOEX]

Part two.....

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 syste

[JOEX]

http://headerdesign.com/extras/design.asp


HOW HEADERS WORK
Headers are much more than just suitably sized exhaust pipes attached to your engine. Headers are an integral part of your engine. The significant energy remaining in the exhaust gas in the cylinder after the power stroke can be used to increase engine power and efficiency by:
Minimizing the contamination of the intake charge with exhaust gases.
Starting the flow of induction during the valve overlap period.
Minimizing pumping losses on the exhaust stroke.

The header performs its function by:
Utilizing the compression wave that is created when exhaust pressure in the cylinder is blown down after the exhaust valve opens. This compression wave travels through the exhaust gas remaining in the primary header pipe, and accelerates the exhaust gas toward the collector.
Passing this strong compression wave through an expansion chamber (collector) located at the end of the primary header pipe. This produces a strong suction wave. This suction wave travels back up the header pipe toward the open exhaust valve, accelerating the fresh exhaust gas in the pipe toward the collector.
Timing this reflected suction wave (scavenging wave) to arrive back at the exhaust valve during the valve overlap period, when both the exhaust and intake valves are off their seats.

For the header to function properly it must have the following characteristics:
Primary header pipes must be small enough to maintain the strength of the compression waves and suction waves in the pipes.
Primary header pipes must be large enough to allow optimal flow of gases past the exhaust valve, and down the header pipes.
Primary header pipes must be of proper length to ensure correct arrival time of the scavenging wave that will be reflected back to the exhaust valve.
Collector must be of proper diameter and length to provide the correct duration and intensity of the scavenging wave.
Primary header pipes and collectors must produce a scavenging wave that is present at the exhaust valve during the valve overlap period over an established operating RPM range for the engine.
The basic design philosophy at HeaderDesign.com is to create a collected header that produces the best torque and horsepower from your engine over its operating RPM range. The primary header pipes and collector dimensions can be fine-tuned to optimize the portion of the operating RPM range that is most important to you. The following paragraphs will help you understand how headers influence engine performance at various RPM levels.

At very low RPM the first returning scavenging wave in the header pipe will arrive far too early to provide scavenging during valve overlap. In fact the first scavenging wave will be followed by a compression wave, and possibly another scavenging wave and compression wave before the valve overlap period. Since low RPM engine operation tends to be dominated by very light throttle application, these waves in the exhaust are weak and have only minor influence on engine operation. Low RPM performance is influenced primarily by intake valve closing time, compression ratio, valve overlap duration, and to a lesser extent exhaust valve opening time.

A performance engine’s operating range begins when the engine first starts to make good torque under full-throttle application. This is usually at an RPM that is about half of your peak horsepower RPM, and marks the beginning of the midrange torque band. At this RPM the scavenging waves in the header pipes may still be arriving a little prematurely. This will cause the piston to be literally sucked up the bore on the exhaust stroke, and will reduce the engine’s pumping losses to essentially zero. But the compression wave that follows the first scavenging wave in the header pipe may arrive at the exhaust valve during valve overlap. This compression wave has the potential to spoil the intake charge by injecting exhaust gas during the valve overlap period.

Our Header Design Program, provided with your membership, can be set (using a Performance Factor of 1 or 2) to make the header perform well at this relatively low RPM level. The combination of properly sized primary header pipes and collector work together to bring the torque curve in as early as possible, dampen the strength of the following compression wave, and keep exhaust velocity high to eliminate reverse flow.

As your engine is accelerated through the middle of its midrange torque band, the header will fully “tune-in” and provide strong scavenging before and during the valve overlap period. The scavenging wave from the header may actually drop the cylinder pressure down to half of atmospheric pressure. Intake charge can be drawn past the intake valve during valve overlap even though the piston is still moving up the bore on the exhaust stroke. The combustion chamber is thoroughly scavenged of exhaust, which increases engine efficiency and power. Flow of induction down the intake manifold runner is maintained by the powerful scavenging wave from the header during the valve overlap period. As the exhaust valve closes, the piston begins to speed down the bore on the intake stroke, continuing the induction process. Use a Performance Factor of 4 through 7 in our program to optimize the tune of the header in your engine’s midrange.

Torque will decrease as the engine reaches its peak horsepower RPM. The scavenging wave in the header will also arrive back at the exhaust valve with little time to spare before intake valve opening. The strength of the scavenging wave will be at its maximum just before the exhaust valve closes. The scavenging wave is very strong at high RPM, and helps draw the intake charge across the intake valve. When using the Header Design Program, a Performance Factor of 9 or 10 can be selected to allow the engine to make good horsepower well past the peak horsepower RPM. These settings are for all-out drag racing, and can be used to intentionally kill-off mid-range torque to a small degree.

Regardless of the performance level of your engine, you can use our Header Design Program to calculate the size and length of both your primary header pipes and collector. You should never guess at header sizes. Your engine will only operate well when it is equipped with a properly sized header and collector combination. The design produced by our program is a non-tapered design. Tapered and stepped designs, usually used in high RPM applications, can be done on an individual basis.

[chad502ex]

:cool: ;)

[wilkin250r]

Joe needs a girlfriend...

:blah:

[chad502ex]

Originally posted by wilkin250r
Joe needs a girlfriend...

:blah:

LOL!! sounds like your offering.... LOL!:devil: just kidd'n:D

[JOEX]

Originally posted by wilkin250r
Joe needs a girlfriend...

:blah:
I'd rather have a housekeeper with benefits:blah:

[JDRider]

Joex, I say thank you. There is alot of information to be absorbed in your posts.

When it described the fifth cycle ect. it really hit home.

I can see in the header design the size of the pipe can make a difference. On the other hand I can see where an exhaust manifold could destroy the whole idea of scavenging, where multiple cylinders are pumping into the same manifold.

Back to the ATV world. We kinda have the perfect situation with one cylinder, one pipe and one muffler. We can control the individual cylinder with more precision.

Question..... In controlling the velocity (speed) of the exhaust, would it be better to contol the velocity with the diameter of the header pipe, or, use a larger diameter header pipe and use the muffler/baffle to control the velocity?

By the way, I'm not sure a Housekeeper would know any more about exhaust backpressure than a girlfriend.

:eek: :D

[wilkin250r]

Originally posted by JOEX
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.

I don't doubt that scavenging has a significant impact, but I didn't think it was this high. I would like to see how they came up with those numbers.

[Toadz400]

I haven't read through it completely yet, but I just had to say this....Great, now all the ricers are going to think their cars really are fast!

[MarkyNark]

And now the short version ....

Generally speaking, the more back pressure you have the more torque you have. There is a point, where thats not true, its a matter of tuning the engine for your particuar needs.

This is also true with Disced Spark Arrestors, like the Supertrapp system. The more discs you add, generally speaking, the more flow, the more top end you have, but you also sacrifice low end torque.

Ok, Im done ... :rolleyes:

[cataway]

Originally posted by MarkyNark
And now the short version ....

Generally speaking, the more back pressure you have the more torque you have. There is a point, where thats not true, its a matter of tuning the engine for your particuar needs.

This is also true with Disced Spark Arrestors, like the Supertrapp system. The more discs you add, generally speaking, the more flow, the more top end you have, but you also sacrifice low end torque.

Ok, Im done ... :rolleyes:

no i dont think so, maybe you could explane "Generally speaking, the more back pressure you have the more torque you have. "
:)

[Toadz400]

Originally posted by MarkyNark
And now the short version ....

Generally speaking, the more back pressure you have the more torque you have. There is a point, where thats not true, its a matter of tuning the engine for your particuar needs.

This is also true with Disced Spark Arrestors, like the Supertrapp system. The more discs you add, generally speaking, the more flow, the more top end you have, but you also sacrifice low end torque.

Ok, Im done ... :rolleyes:

If you would've read through all of it you probably would've seen that there is a point where flow is hurt because of the lack of backpressure completely. Just because you're adding more discs or putting on a bigger pipe doesn't mean you're getting more flow. The most important thing is obtaining maximum velocity with the least amount of backpressure to gain the best results. This can be done without sacrificing any torque. I think that would be the considered the short version.

[kraemer]

when your exhaust gasses exit through the exhaust stroke, their is an overlap between exhaust and intake strokes, which allows scavanging. scavanging is when exhaust gases that are still in the cylinder, draw in the colder intake air.

think about this: why are exhaust valves always smaller than intake valves? b/c exhaust gas is pressurized, it is trying to escape, it will go any which way it can expand... so as that out rush of high pressure is moving, it causes and in rush of low pressure.

If you have too little back pressure scavanging will not be as effiecient...

Go take you bone stock vahicle, unbolt the exhaust system from the header, or manifold. remove it and bolt on a performance header. now go run it like a raped date in red reeboks. now go home and bolt on the rest of your exhaust system, keeping the performance header. You will notice a extremely dramatic increase in bottom end; TORQUE. the vehicle will have balls with no exhaust, once you get into higher rpms, when their is very little engine vaccuum, and you have much greater volumes of fuel being consumed. but with the back pressure of the CAT and Muffler you will have much more noticeable bottom end.

I work on Diesel engines for a living. Most over the road Diesels turn about 2200 rpms. the peak torque is at about 1800 to 2200 for most of them. These engines rely highly on Scavanging for their Torque at a dead stop. once the engine moves into a greater fuel position (higher rpm) the turbo will spool and exhaust flow is a lot more prevalent to be able to keep that turbo spooled.

the greatest problem with people today is they don't understand every engien that comes of an assembly line has to meet some sort of emissions for federal compliance, whether that be for noise pollution or gasseous pollution. all manufacturers would love to give you the biggest baddest loudest bike they can make, but they can't. everyone knows dodge use to make the most powerful cars, first to break 11s, 10s and 9s at the strip, after emissions got tight they had nothing, so they made the minivan?

You have to build smart.


EDIT: HP is just a mathmatical equation from torque and rpm, as rpm increases torque decreases, by matching an exhaust system you are trying to find that happy medium that will keep your engine sucking air at low rpm as well as being able to flow that air at high rpm...

and JOEX when you find the housekeeper with benefits, send me her sisters number...