John Shiels

If I may ask, thanks!

John Shiels

ttt

VetteDrmr

John,

Sorry, I don't have an answer for you. I've been trying to figure out how intake runner length, backpressure, etc. affect torque. I can understand the impacts on horsepower (more restriction costs horsepower), but the ralationship between those same things and torque just seems to be black magic to me.

Hopefully I'll learn something with this thread.

Have a good one,
Mike

Patches

kewlbrz recently did an excellent synopsis on this. Very nice primer on headers and their effects.

I don't remember the name of the thread but if you search for it iwas within the last 2 or 3 weeks.

Ed

ScreaminDemon

Engines do not require backpressure from the muffler to make power. If your cam is right, your exhaust is a major factor toward making all the head work pay off. On a race motor the power brought about by the exhaust is due to its action on the intake during the overlap period. Past a certain point the longer the duration of the exhaust cam the smaller the optimal pipe diameter becomes. Any pushrod 2-valve motor that is making 2 to 2.2 hp per cube is living on the exhaust.

MitchAlsup

Back pressure in the exhaust decreases HP because the upward stroke of the piston on the exhaust stroke has to push the exhaust gasses out of the cylinder. This push requires energy from the flywheel.

Conversely: headers make power because (over certain RPM ranges) they provide a lower than atmospheric pressure to the exhaust valve, thereby, pulling the exhaust gasses out of the cylinder and saving the flywheel from supplying the power necessary to push those gasses out. The energy not used to push the exhaust gasses out of the cylinder is available at the flywheel for other purposes (like propelling the car).

Sam Lin

Back Pressure doesn't help anything. The saying that bp helps torque is because a smaller exhaust tube usually has more back pressure, but actually the smaller tube lets velocity stay high with less throughput. If you can get a larger bore yet keep velocity up, then you don't take a HP hit at high flow, and you keep your velocity at low rpms. That's part of why a long tube really works, the large tube doesn't take high rpm hits, but at low rpm, the long tube tuning waves keep velocity up, so tq doesn't take a hit.

Sam

no cure

John,
I've heard (and vaguely rememb seeing) on other cars, what you say, ie open exhaust looses some low end tq and gain some high end hp...
I can not recall why that is, nor the relationship (non of the above replies addresses this aspect of your topic, I believe your intended quesion)

anyway, a coupla months ago I was going thru a process trying to get the PCM programming in my car straightened out, and for grins I dynoed with the exhaust closed. normally I always dyno with open exhaust cause I can't get the A/F ratio with the supertrapp end caps on, and cause I care about the curves/etc in track mode, but I was curious what I was loosing in street mode vs track mode. there was no low end tq loss with the open exhaust, totally refuting the 'accepted wisdom'..

below is a scan of 2 pulls. open & closed exh overlayed. the dates are diff [I did the closed pull on a whim to start a session, I was going back n forth after mailing the PCM for changes.. and the dynoshop printed me out the overlay vs my last pull from the prior session (exh open) and I didn think to get a diff overlay from later that day, but the CF's were the same, if my burnt out mem is any use, an in any case these are corrected numbs, so it's a valid comparison]

note - my baby was not running well at the time of both pulls, I was trying to get some PCM programming probs addressed, which were not fixed then, so these numbs are interestin for comparing the effect of mufflers vs no mufflers, but (imho) don't represent my car in peak condition, but I'm not posting this graph to try to say "oh look how much power I'm making etc" and for some reason my tq was down about 5 lbs rwtq on the open pull selected for the overlay, I've done so many pulls there, my folder is quite large..



btw - there definitely is the 'loose low end tq gain high end hp' thang with going to more aggressive cams, that I have seen..

cheers,
David

yellow01

Goto http://www.n2performance.com/index.html and click on Lecture Hall.

Lecture 5 is on exhaust systems, but all the lectures are really good.

All you wanted to know about hte dynamics of IC engines.

:cheers:

HP-GURU

Back pressure affects torque because it gives us an idea of velocity in the exhaust tubing.

Basically a higher velocity (to a point) will have a higher momentum out of the exhaust system.

At the head port this high velocity will help pull fresh intake charge in during the overlap period. Through proper exhaust design you can overfill the intake cyllinders.

There is not a right tube diameter or length. It all depends on the application.

VetteDrmr

Aaahhh, my head hurts! :eek:

Now, don't beat me up, because I haven't gone to the lectures link yet, but the gist of the above posts is "keep the gas velocity up to help torque."

Ok, that's what I've heard before, applied both to intake and exhaust systems. But, why does an increase in gas velocity improve torque? I can understand it from a horsepower standpoint, of scavenging exhaust gases out of the cylinder. And, most aggressive cams lose torque at lower rpms. But, if they can breathe better, why do they lose out?

Thanks for the discussion, maybe I'm going to learn something yet.

Have a good one,
Mike

WALLstAL

A previous post about Header, exhaust and back pressure!

Short tube headers are simply more efficient at getting exhaust out of the engine than the production manifold. These 'headers' are too short in length to perform the taks that long tube headers perform.
Long tube headers are not just longer versions of short tube headers. Their length has been determined by the timing of the exhaust and intake periods on the camshaft and the RPM band where the engine make max TQ.
Basically, a well desinged header connects three CAMshaft intervals in time. When the exhaust valve opens, a high pressure wave starts dwon the header. When the HPwave reaches the collector, it encounters a low pressure region. This causes a low pressure wave to reflect back up the header.
We want this low pressure wave to reach the cylinder at the same time the intake valve opens. This low pressure wave can begin to draw fresh mixture into the cylinder even before the piston starts down on the intake stroke. Unfortunately, since the exhaust valve is open, it draws fresh mixture into the exhaust pipe. Fortunately, this mixture is pushed back into the cylinder by the HP wave which follows.
Meanwhile, back in the collector, the high pressure wave has reached the end of the collector, and encounters a restriction we normally call the exhaust pipe. This HP wave reflects off this restriction, and runs back up the header. We want this HP wave to arrive at the exhaust valve just before it closes so the HP wave can push the fresh mixture which entered the exhaust back into the cylinder. Thereby achieving greater than 100% volumetric efficiency.
Summary: short tube headers are just less restrictive than production
manifolds, while real headers actually extract energy from the pressure waves which exist from the 4-stroke nature of our engines.

HP-GURU

Nice post WallSt.

Now to add to the information given forth.

What about the overlapping exhaust pulses which are going to be present on any V8 motor?

Dennis

kewlbrz

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) and horsepower (amount of work within 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 fuel available for combustion. More 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, 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 350 cubic inches has the same maximum power potential as an engine of 454 cubic inches, 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. 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

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. The author is not aware of any 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, the author has found none that embrace all aspects of exhaust physics.

HP-GURU

Nice post. Now someone show me what happens during the exhaust pulses that "pile up" in a 90 degree crank V8.

On each cyllinder bank, there will be two cyllinders that inevitably fire one right after the other.

kewlbrz

Dennis, dont some race guys run 180 degree headers that use crossover tubes to either bank to maximize some of the things your eluding to?

clem zahrobsky

180 headers work in a very narrow RPM band. they leave big holes in the torque curve. using a venturi type collector on a header helps the scavageing effect a lot.


[Modified by clem zahrobsky, 2:21 AM 12/7/2001]

VetteDrmr

Kewlbrz,

Envision the following. The room is dim, lit by a single, small candle. Wait, it just got brighter. What could it be? Of course, that's IT, Mike just learned something!!! :D

I knew the theory of a tuned exhaust, but until your thesis finally got through to me why gas velocity affects torque (at least I think I do). Attempting to summarize what the teacher said, gas velocity is essential to maximizing torque for a given rpm or power setting. The problem is that parasitic losses go up with the (square, cube?) of the gas velocity. So, an engine designed with smaller intake runners and exhaust tubes will peak their torque curves at a lower rpm than would an engine designed with larger runners and exhaust. But, the engine with the larger runners may sacrifice lower rpm torque to increase the torque available at higher rpms.

But, (going out on a limb) this doesn't apply to mods made upstream of the throttle body and downstream of the header/manifold flange. Mods made in these areas just reduce losses that any engine design would have to deal with, so improvements made in these areas will benefit any engine design.

So, prof, did I pass?

Thanks,
Mike

kewlbrz

I cant take credit for that report on header design and theory, but glad I had it to offer. Yes, you are right. also remember however, that its better to develop your torque in higher RPMS so you can take advantage of gearing. ;)

VetteDrmr

Already got it saved on my system. Hopefully one day I'll have a Vette of my own to play with.

Have a good one,
Mike