lww

I've spent a number of years arguing with people about the relationship between the volumetric efficiency of an engine and the affect different header designs have on the practical performance of a motor. I've spoken at length with several REAL experts in the field and argued with countless others that base their knowledge off of hearsay, conjecture, ignorance and superstition.

Much of this has been accumulated over the years from various sources that I can't even begin to remember or properly attribute. They know who they are and I thank them for their expertise and willingness to share their knowledge.

I finally decided to write it all down for your reading pleasure...

This explains what headers do and how they can make or lose power based on a specific engineered application.

I'll bet you didnt know headers can make you slower in the 1/4 mile. I'll bet you didn't know that with the right header set up you can give your car power right where it needs it at any RPM you want. Well read on and you will know all there is to know about headers.

Introduction

No header/exhaust system is ideal for all applications. Depending on their design and purpose, all headers compromise something to achieve something else. Before performing header or other exhaust modifications to increase performance, it is critical to determine what kind of performance you want.
- Do you want the best possible low-end power, the best mid-range power or maximum top-end power?
- Do you plan to use nitrous oxide or forced induction (supercharger or turbocharger)?
- Are you going to increase displacement?
- Will you be using an aftermarket cam with different lift, duration, timing and overlap?
- Do you understand the relationship between torque (force/work) and horsepower (amount of force/work over a period of time)?
- Can you distinguish between cosmetic headers and performance headers?
- Have you considered vehicle weight, transmission (stall speed, if applicable) and gear ratios?

Without careful thought about these variables, a header/exhaust system can yield very disappointing results. Conversely, a properly designed system that is well-matched to the engine can provide surprising power gains.

The distinction between "maximum power" and "maximum performance" is significant beyond conversational semantics. Realistically, one header may not produce both maximum power and maximum performance. For a vehicle to cover "X" distance as quickly as possible, it is not the highest peak power generated by the engine that is most critical. It is the highest average power generated across the distance that typically produces the quickest time. When comparing two horsepower curves on a dynamometer chart (assuming other factors remain constant), the curve containing the greatest average power is the one that will typically cover the distance in the least time and that curve may, or may not, contain the highest possible peak power.
In the strictest technical sense, an exhaust system cannot produce more power on its own. The potential power of an engine is determined by the amount of air and fuel available for combustion. More air and fuel must be introduced to increase potential power. However, the efficiency of combustion and engine pumping processes is profoundly influenced by the exhaust system. A properly designed exhaust system can reduce engine pumping losses. Therefore, the design objective for a high performance exhaust is (or should be) to reduce engine-pumping losses, and by so doing, increase volumetric efficiency. The net result of reduced pumping losses is more power available to move the vehicle. As volumetric efficiency increases, potential fuel mileage also increases because less throttle opening is required to move the vehicle at the same velocity.

Much controversy (and apparent confusion) surround the issue of exhaust "back-pressure". Many performance-minded people who are otherwise well-enlightened still cling tenaciously to the old cliché.... "You need some back-pressure for best performance."

For virtually all high performance purposes, backpressure in an exhaust system increases engine-pumping losses and decreases available engine power. It is true that some engines are mechanically tuned to "X" amount of backpressure and can show a loss of low-end torque when that backpressure is reduced. It is also true that the same engine that lost low-end torque with reduced back-pressure can be mechanically re-tuned to show an increase of low-end torque with the same reduction of back-pressure. More importantly, maximum mid-to-high RPM power will be achieved with the lowest possible backpressure. Period!

The objective of most engine modifications is to maximize air and fuel flow into, and exhaust flow, out of the engine. The inflow of an air/fuel mixture is a separate issue, but it is directly influenced by exhaust flow, particularly during valve overlap (when both valves are open for "X" degrees of crankshaft rotation). Gasoline requires oxygen to burn. By volume, dry, ambient air at sea level contains about 21% oxygen, 78% Nitrogen and trace amounts of Argon, carbon dioxide (CO2) and other gases. Since oxygen is only about 1/5 of air’s volume, an engine must intake 5 times more air than oxygen to get the oxygen it needs to support the combustion of fuel. If we introduce an oxygen-bearing additive such as nitrous oxide, or use an oxygen-bearing fuel such as nitromethane, we can make much more power from the same displacement because both additives bring more oxygen to the combustion chamber to support the combustion of more fuel. If we add a supercharger or turbocharger, we get more power for the same reason…. more oxygen is forced into the combustion chamber.

Theoretically, in a normally aspirated state of tune without fuel or oxygen-rich additives, an engine’s maximum power potential is directly proportional with the volume of air it flows. This means that an engine of 2.5 L has the same maximum power potential as an engine of 3.5 L, if they both flow the same volume of air. In this example, the powerband characteristics of the two engines will be quite different but the peak attainable power is essentially the same.

Flow Volume & Flow Velocity:
One of the biggest issues with exhaust systems, especially headers, is the relationship between gas flow volume and gas flow velocity, which also applies to the intake track. An engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the powerband for maximum performance. This is where a fundamental conflict arises. For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter header tube will provide higher flow velocity than a larger diameter tube. Unfortunately, the laws of physics will not allow that same small diameter tube to flow sufficient volume to realize maximum possible power at higher RPM. If we install a larger diameter tube, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, torque and horsepower across the entire powerband.

A very common mistake made by some performance people is the selection of exhaust headers with primary tubes that are too large in diameter for their engine's state of tune. Bigger is not necessarily better and is often worse.

Equal Length Primary Tubes:
The effectiveness of equal length header tubes is widely debated.
Assuming that a header is otherwise properly designed (and many headers are not), equal length primary tubes offer some benefits that are not present with unequal length tubes. The benefits are smoother engine operation, tuning simplicity and increased low-to-mid range torque.

If the header tubes are not equal length (most commercial headers are not equal length), both inertial scavenging and wave scavenging will vary among engine cylinders, often dramatically. This, in turn, causes different tuning requirements for different cylinders. These variations affect air/fuel mixtures and timing requirements, and can make it very difficult to achieve optimal tuning. Equal length header tubes eliminate these exhaust-induced difficulties. "Tuning", in the context used here, does not mean installing new sparkplugs and an air filter. It means configuring a combination of mechanical components to maximum efficiency for a specific purpose and it can not be overemphasized that such tuning is the path to superior performance with a complex system of parts that must work together in a complimentary manner.

If a header is otherwise properly designed for it’s application, equal length header tubes are, of necessity, longer than unequal length tubes. The lengths of both primary and collector tubes strongly influence the location of the torque peak(s) within the powerband.

lww

In street and track performance engines, longer header tubes typically produce more low-to-mid range torque than shorter tubes and it is torque that moves a vehicle. This begs the question... Where in the powerband do you want to maximize torque?
- Longer header tubes tend to increase power below the engine’s torque peak and shorter header tubes tend to increase power above the torque peak.
- Large diameter headers and collectors tend to limit low-range power and increase high range power.
- Small diameter headers and collectors tend to increase low-range power and limit high-range power.
- "Balance" or "equalizer" tubes between the collectors tend to flatten the torque peak(s) and widen the powerband.

There is limited space in most engine compartments for header tubes and equal length tubes complicate the design process and are more costly to build than "convenient" length or cosmetic headers. Exhaust header designers are severely compromised by these limitations. Among the more astute (and responsible) professional header builders, it is more-or-less understood that header tube length variations should not exceed 1" to be considered equal. Even this standard can result in a 2" difference if one tube is an inch short and another tube is an inch long. By this definition, equal length headers are quite rare. By absolute measurement, it is probably impossible to find equal length headers from a commercial manufacturer. Because of this, it is no surprise that many people have little knowledge of the benefits of equal length headers since the average user is unlikely to have experience with them. If you have headers that are supposedly equal length, carefully measure each tube and you will know the truth.

Exhaust Scavenging and Energy Waves:
Inertial scavenging and wave scavenging are different phenomena but both impact exhaust system efficiency and affect one another. Scavenging is simply gas extraction. These two scavenging effects are directly influenced by tube diameter, length, shape and the thermal properties of the tube material (stainless, mild steel, cast iron, etc.). When the exhaust valve opens, two things immediately happen. An energy wave, or pulse, is created from the rapidly expanding combustion gases. The wave enters the header tube (or manifold) traveling outward at a nominal speed of 1,300 - 1,700 feet per second (this speed varies depending on engine design, modifications, etc., and is therefore stated as a "nominal" velocity). This wave is pure energy, similar to a shock wave from an explosion. Simultaneous with the energy wave, the spent combustion gases also enter the head tube and travel outward more slowly at 150 - 300 feet per second nominal (maximum power is usually made with gas velocities between 240 and 300 feet per second). Since the energy wave is moving about 5 times faster than the exhaust gases, it will get where it is going faster than the gases. When the outbound energy wave encounters a lower pressure area such as a larger collector pipe, muffler or the ambient atmosphere, a reversion wave (a reversed or mirrored wave) is reflected back toward the exhaust valve without significant loss of velocity.

The reversion wave moves back toward the exhaust valve on a collision course with the exiting gases whereupon they pass through one another, with some energy loss and turbulence, and continue in their respective directions. What happens when that reversion wave arrives at the exhaust valve depends on whether the valve is still open or closed. This is a critical moment in the exhaust cycle because the reversion wave can be beneficial or detrimental to exhaust flow, depending upon its arrival time at the exhaust valve. If the exhaust valve is closed when the reversion wave arrives, the wave is again reflected toward the exhaust outlet and eventually dissipates its energy in this back and forth motion. If the exhaust valve is open when the wave arrives, its effect upon exhaust gas flow depends on which part of the wave is hitting the open exhaust valve.

A wave is comprised of two alternating and opposing pressures. In one part of the wave cycle, the gas molecules are compressed. In the other part of the wave, the gas molecules are rarefied. Therefore, each wave contains a compression area (node) of higher pressure and a rarefaction area (anti-node) of lower pressure. An exhaust tube of the proper length (for a specific RPM range) will place the wave’s anti-node at the exhaust valve at the proper time for it’s lower pressure to help fill the combustion chamber with fresh incoming charge and to extract spent gases from the chamber. This is wave scavenging or "wave tuning".

From these cyclical engine events, one can deduce that the beneficial part of a rapidly traveling reversion wave can only be present at an exhaust port during portions of the powerband since it's relative arrival time changes with RPM. This makes it difficult to tune an exhaust system to take advantage of reversion waves which is why there are various anti-reversion schemes designed into some header systems and exhaust ports. These anti-reversion devices are designed to weaken and disrupt the detrimental reversion waves (when the wave's higher-pressure node impedes scavenging and intake draw-through). Anti-reversion schemes include merge collectors, truncated cones/rings built into the primary tube entrance and exhaust port ledges.

Unlike reversion waves that have no mass, exhaust gases do have mass. And since they are in motion, they also have inertia (or "momentum") as they travel outward at their comparatively slow velocity of 150 - 300 fps. When the gases move outward as a gas column through the header tube, a decreasing pressure area is created in the pipe behind them. It may help to think of this lower pressure area as a partial vacuum and one can visualize the vacuous lower pressure "pulling" residual exhaust gases from the combustion chamber and exhaust port. It can also help pull fresh air/fuel charge into the combustion chamber. This is inertial scavenging and it has a major effect upon engine power at low-to-mid range RPM.

If properly timed with RPM and firing order, the low pressure that results from gas inertia can spill-over into other primary tubes, via the collector, and aid the scavenging of other cylinders in that bank.

There are other factors that further complicate the behavior of exhaust gases. Wave harmonics, wave amplification and wave cancellation effects also play into the scheme of exhaust events. The interaction of all these variables is so abstractly complex that it is difficult to fully grasp. There are no absolute formulas/algorithms that will produce a perfect exhaust design.

Even factory super-computer exhaust designs must undergo dynamometer and track testing to determine the necessary adjustments for the desired results. Although there are some exhaust design software packages available, I have found none that embrace all aspects of exhaust physics.

A well designed header and exhaust system will provide minimum loss of low-end torque while improving mid to high-end power. This usually requires changes to the rest of the system in order to take advantage of the new potential or the end result can be a substantial decrease in actual average power production.

Now, I hope you all have a better understanding of what it takes to build a truly advantageous exhaust system. Unfortunately, you're most likely not going to get it from a store. Although, they do make the exhaust sound 'cool' they will have limited affect on your ultimate power production.

Unfortunately, the headers available for our cars were designed nearly 30 years ago and even the S130 L6 headers were generally only minor adaptations of pre-existing S30 L6 headers.

The moral of this story is, be skeptical when you see power improvment claims attached to headers designed for ANY vehicle.

hoov100

damn that took along time to read, so basicly you would have to build your own, to match your engine, or pay up for a set that actually work to your needs?

lww

For the most part yes. One thing you can do with a good quality over the counter header is tune your motor to take advantage of any improvment potential that may exist. That takes a lot of time, through trial and error, and dyno dollars. The best thing to do is find someone with a reputable history of building L6 race engines. They will have the experience and tools (dyno) necessary to tune the car properly. Unfortunately, in the L6 arena, there are only five people on the planet that, in my opinion, meet the criteria.

duowing

That was a big read. It was pretty interesting. A little of it is confusing to me, since I don't know much about headers. Everyone tends to say to go ahead and put headers on our cars. So do they really affect the performance of our S130s that much? It's quite interesting to see that you can actually lose power by installing headers. Also with what you said about the exhaust headers and their "equal lenghts" not being so equal. It makes for a quicker/easier production, and a much smaller margin of error. As opposed to getting the tubes withing say 1/4in of each other.

SHADY280

so has anybody ever dynoed our cars with the stock manifold on a modified car, 6-3-2-1 header and msa's 6-1 header, which is best? i got the 3-2-1 from msa, then i made the lower part 2.5 inch and felt a torque gain, and it didnt stop reving at my redline. i did it because the pacesetter one is like that. i know lww is gonna call me stupid for messing with a header, but its r+d

lww

Not at all Shady. The whole crux of that post was to help people understand, that if they want maximum performance out of a set of headers, you have to design them to work with your specific application.

Now, making modifications without any empiracle data is mostly a shot in the dark, but modifying one for reasons other than performance, ie. fit, is a completely legitimate modification as long as you understand the compromise you're making.

Again, our headers were designed nearly 30 years ago, so we're so far behind the 'R&D' curve as to make it a moot point anyway. If you want headers for look, sound and cool factor, go for it, if you get a slight improvement in performance, rejoice! If nothing else, it'll shave 20lbs off the weight of your front end by getting rid of the heavy stock exhaust manifold.

When buying headers, set your expectations very low and be happy if they exceed your expectations.

SHADY280

O.k Mr Rocket Scientist, What Set Of Headers Could I Build? What Are Your Calculations For A 3000 To 7000 Rpm Street Motor?

jfairladyz

RPM range isn't nearly enough information to determine the best possible header configuration. There is so much that must be taken into consideration. For example: Whats the lift and duration on your cam? How much power are you looking to make and at what RPM's as well as how much air flow your engine going to require to make that power? Carb or FI? The list goes on and on and on and on.

Headers by Ed for example has a 100 question questionaire that you fill out and send to them to help determine the best header design. How's that for how much information you need to provide

lww

http://www.headersbyed.com/question.htm

Enjoy!

SHADY280

im all over it, ill let you know.

jfairladyz

So we'll hear from ya in, what? 2 weeks Shouldn't take any longer than that to get that sucker filled out

EatmyDust

IWW you'r information is all correct, I am amazed on your compelation of information broght together in such a form. I have had the same battle telling people around me about vollumetric efficancy. I brake it down to them by telling them like chemistry. what is the point where gassoline burns most efficent? thats like 14.7:1 right, 14.7 parts gas to 1 part air.

if you can't **** are you going to eat? I don't think so. if you have a serious case of the ***** and you don't eat you die.

a well balance in eating and shiting will produc a well dieted well fitt person good health and peak performance.

jfairladyz

14.7:1 isn't the most efficient. It's the ratio at which combustion will be the most complete. I guess if emissions is your concern then 14.7:1 is efficient. But if anything other than emissions is your concern then 14.7:1 is far from the most efficient ratio.

SHADY280

it also depends on your altitude, i was talking to friends at work who race, its makes a big diff. im at sea level here so 13.9 ish is good.

EatmyDust

Thats corect my bad thanks for the corection.
but what I was trying to hit at, was that it needs to be perportioned eating and craping. breathing and exausting. I guess if the mixture dosn't burn as good as it is suposed to well that would be a power los. unburned fuel to me is a waste of potential. seeing how the power comes from the expanding gases upon ignition. I am not saying that you are allways going to burn every bit of fuel dumped in, specially at w.o.t. but the more burned the better.

the volumetric efficency to me means, the ballance of things comming in to the things gowing out being equal, or well ballanced.
various things play part in it.
combution chamber size
port size
cam(scavaging)(lift)
exaust system
intake manifold(runers)(throtle valve)
Valve diameter(Int.)(Ext.)
injectors
and there may be a bit left out but all things meeterd at certain levels can change power out put, and range drasticly.

jfairladyz

And dont forget combustion chamber shape/design. That plays a huge roll in the combustion process.

nick's81240zx

WOW, thank you for taking the time to write that this is the first time all of my question were anwered about how flow works! I have taked to a lot of the scca race car drivers and there engine builders and the ones that know what they are taking about dont like to sit there and explain. So thank you very much!

CWHammer

So lose the stock manifold for wieght loss only when using a "store bought" header?

I was going to order a header and setup for a 2.5" exhaust but not if a significant/real gain isn't going to be made. It's too much work otherwise. I can't afford to have the inevitable problems with my current work schedule.

jfairladyz

A header is more of a "potential" kind of thing. The stock manifold doesnt have as much potential as a properly matched header. The more air your engine is capable of moving then the more gains a header will give you. Put a header on the stock engine and you're not likely to see more then a few horsepower. Do a little engine work such as some head porting and a hotter cam, maybe raise the compression ratio and THEN bolt on a header and it'll probably net you 4-5 times what it would've on a stock motor.

CWHammer

Tat's what I needed to hear. I have an extra L28E that I want to raise the compression, change the cam, larger injectors, etc.. I just need to find a source that can do it that has a good workingg knowledge of the L28 engine. Know of any in the Detroit Mi area?

Z ya L8R

You guys are trippin' me out here, but I am still going to install my MSA 6-2-1 header that is if I can get the stock manifold to break free! I have removed ten 12mm nuts, and it feels glued to the head. Any suggestions? By the way, I have a beautiful '79 n/a, of course.

NismoPick

Did you remove the nut that is only accessible from the top middle of the intake manifold? There's a cover plate that you remove.

ZYAL8R

Yeah! I was zyaL8r, but forgot my password, so now I'm laterz. Anyway, those are my liscence plate picks.
So, yes, removed the last nut under the heat sheild plate on top, and then the intake manifold. Then I could get that heavy clunky ex. manifold off. I put the headers on and removed the exhaust pipes, pre-muffler, and muffler. I insalled a thrush glasspack muffler. I am still looking to add some power. I am going to put in a cold air intake, and then maybe a cam. I'm new to this, but since '02 I've been dreaming of Z car heaven. Thanks again for your help with the header. Any suggestions for future upgrades?