Exhuast back pressure

[Z2TT]

Hello,

I recently read this article from Autospeed :

http://autospeed.com/cms/article.html?&A=0046

To quote the part I am interested in

Atmospherically inducted cars that use a tuned length system to improve cylinder scavenging (via extractors, for example) are sensitive to exhaust diameters within the tuned length part of the system. This means that the maximum effect of exhaust pulsing may come from an exhaust system that is small enough that some exhaust back-pressure is developed. However, that is a quite different concept to saying that engines "need" exhaust back-pressure! Turbocharged engines require as big an exhaust as possible, with the same applying for naturally aspirated cars once the tuned length part of the exhaust is passed.

Few tests have been done that clearly show the affect of changing back-pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back-pressure as a culprit difficult. However, Wollongong (Australia) mechanic Kevin Davis is one who has done very extensive testing of varying back-pressure on a number of performance engines. These range from turbocharged Subaru Liberty [Legacy] RS flat fours to full-house traditional pushrod V8's. In not one case has he found any improvement in any engine performance parameter by increasing exhaust back-pressure!

The tests came about because Kevin has developed a patented variable flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back-pressure at low loads "to help torque". However, he soon changed his mind when any increase in back-pressure proved to decrease torque (and therefore power at those revs) on a properly tuned engine! What increasing the back-pressure does do is dramatically quieten the exhaust.

One of the engine dyno tests carried out by Kevin was on warm 351 4V Cleveland V8. Following the extractors, he fitted a huge exhaust that gave a measured zero back-pressure. Torque peaked at 423 ft-lb at 4700 rpm, with power a rousing 441hp at 6300 rpm. He then dialled-in 1.5 psi back-pressure. Note that very few exhausts are capable of delivering such a low back-pressure on a road car. Even with this small amount of back-pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the butterfly position to give 2.5 psi back-pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back-pressure!

So from this I get that if you are after the most amount of power, get as large an exhaust pipe as possible with least back pressure as possible. I was always bought to believe that if you have a too large diameter pipe you will lose power because of too little back pressure, but that's not what this article says.

Can I get some of you guys thoughts on this?

Thanks.

[LeeRoy]

You need to use google.
http://www.aa1car.com/library/exhaust_backpressure.htm

...require as big an exhaust as possible, once the tuned length part of the exhaust is passed

Tuned length is critical, then anything after that is a pure restriction.

Go any shorter than the tuned length and power will be lost.

[Z2TT]

Yeah I did read that part but did not understand it.

[af300e]

The way I understand it, even the tuned section is not about any return pressure or "back pressure", but rather, the idea is to maintain gas velocity which then helps draw gases out of the chamber or, scavenging.
Any restriction in this tuned section is a side effect of having a pipe small enough to keep the velocities high.

A friend of mine had a 250hp NA 13b rotary built for his street car by a mate who had worked on the Mazda Le Mans cars in Japan. He mentioned that they way they tuned pipe lengths (after setting up diameters and basic runner lengths I suppose) was to run the engine's under race type conditions on a test bench and cut the pipe/s where it/they stopped glowing red. This was intended to keep the heat in the pipe high so there was less restriction from pushing higher density cold gases.

So, in effect the exhaust system finished at the tuned section which is what Leeroy has mentioned.

[TERRA Operative]

Back pressure being required in an exhaust is a fallacy. Back pressure is the restriction that the gas itself causes to the flow of more gas.

Flow velocity is what is desired.

High velocity gas means the exhaust is getting out quick.
A small pipe will have a high velocity at low RPM due to the smaller diameter pipe and low flow. This helpe torque etc at low rpm. However, at high RPM, it becomes a restriction due to backpressure as the gas cannot escape fast enough.
This does not mean you need back pressure for low end torque, it means you need a high gas velocity. The back pressure is simply an undesirable trade off of having a small pipe.

Stick a large pipe on and you now have high velocity at high RPM. As there is more capacity in the pipe for the increased gas.
However, at low RPM, there isn't enough gas to fill the pipe and keep the velocity up so the gas cools and slows down. (the gas still fills the pipe, but cools down as it does so) Now you have problems at low RPM.

Think of it like a river.
A set of rapids moves fast, but can only flow a small amount of water. A large river can flow huge amounts of water, but does so slowly.

It's all a trade off.

[abently]

The only negative trade off here is noise.

[ed]

its about harmonic tuning rather than velocity. pipe dia determines the resonance. velocity and getting the gas out quick is a bit crap

[ed]

cut the pipe/s where it/they stopped glowing red. This was intended to keep the heat in the pipe high so there was less restriction from pushing higher density cold gases.

that, id say, is a mis understanding of what they were doing... the harmonic wave form down the exhuast header will create areas of hot and cool(er) corrensponding to the harmonic nodes in the gas flow. cutting the pipes at a node will give you a correct harmonic length.

what youve said about cooler/denisty/restriction ect is all crap

[ddeane]

The current Street Machine has an article by an exhaust designer/constructor that is along the same lines of Autospeed.

Craig

[Kyosho]

I know Kevin personally, and he's a top bloke, the amount of exhaust design and construction that he does, is quite unbeleiveable, he used to build exhaust systems for one of the Fujitsu V8 teams.

He also designed, and built the twin mufflers sitting under my R33, and they're straight through mufflers, on a 3" pipe, and the thing idles pretty quietly, has a nice light note, and as the revs rise, all that you can hear in the car is the scream from the turbo and engine... Not a loud exhaust note... yet, I have no exhaust restriction...

His butterfly that he sells, is a very nice piece of gear also.

[TERRA Operative]

its about harmonic tuning rather than velocity. pipe dia determines the resonance. velocity and getting the gas out quick is a bit crap

Harmonics in the headers yes, but after the merge, it's all about flow. Harmonics plays no part.

[Kyosho]

Harmonics in the headers yes, but after the merge, it's all about flow. Harmonics plays no part.

It's actually after the cat convertor that harmonics are destroyed.
Before that, the pipe from collector to cat plays a role with harmonics as well, very minor, but it does.

[TERRA Operative]

Yea, technically it does. In a well designed set of headers, there is a chamber behind the merge with a restriction at the opposite end to the merge. This chamber allows the pulses to do their scavenging thing.
After the restriction it's all over for harmonics.

But that champer/pipe could be taken to be part of the merge.

[af300e]

that, id say, is a mis understanding of what they were doing... the harmonic wave form down the exhuast header will create areas of hot and cool(er) corrensponding to the harmonic nodes in the gas flow. cutting the pipes at a node will give you a correct harmonic length.

what youve said about cooler/denisty/restriction ect is all crap

I was told about the effects of exhaust gas temp (densities etc) further along the pipe (as in past the collector) by an ex-formula one team engineer during a physics lecture. Apparantly it is an issue, I wouldn't dismiss it as crap until I'd tested it.

Regarding the cutting where the pipe is cooling, I wasn't talking about the runner lengths.

I'm couldn't imagine that they had the luxury of cutting the runners like that and leaving them (top fuel style) as they had to get the gases further from the engine.
That said, I've had a look at some pics and they show only 1 exhaust outlet whilst the engine was a 4 rotor so I'm quite sure they were making said cuts well after the collector.

Harmonics therefore less likely to be the issue. It is third party info though, I've tried to put the info together.

My first post was probably a little basic in that I mentioned only gas velocity as it seemed to be what the intitial post was about.
But addition to harmonics, gas velocity/inertia in the headers is also a factor. Gas particle movement and sound/energy waves are two different things.
Generally it seems that pipe diameter is chosen to tune gas velocity, pipe length is altered in order to satisfy the sonic characteristics ie. it seems that the diameter of the pipe is chosen to satisfy flow/velocity/inertial scavenging effects and then that diameter pipe is length tuned to satisfy the exhaust's sonic characteristics. Naturally there is some overlap in diameter/length in that the lower gas pressures in a larger pipe affect the speed of sound due to temp changes and that the diameter affects the sonic characteristics of the pipe.

I'm not an expert but I have done a little research on this. Still learning though

[Sam_Q]

I also have varied the back pressure of two different 4age exhuasts that had a free flowing system and my findings although poorly done where the same, any increase in backpressure hurt power badly. I wish the rest of the world would stop believing myths and believing what you guys have said here.