Exhaust Theory

[BW Stein]

It was brought to my attention that design judges request the two tuned RPMs for a 4-2-1 exhaust configuration. I have only come across theory that deals with one RPM. Do you just use basic pipe theory? Do you know of any books that would help. I have read Exhaust and Intake Design and A.G. Bell's Four Stroke Tuning books without success.

Thanks,
Stein

[BW Stein]

It was brought to my attention that design judges request the two tuned RPMs for a 4-2-1 exhaust configuration. I have only come across theory that deals with one RPM. Do you just use basic pipe theory? Do you know of any books that would help. I have read Exhaust and Intake Design and A.G. Bell's Four Stroke Tuning books without success.

Thanks,
Stein

[KU_Racing]

Tuning for multiple rpm points is simple- provide a cross section change at the runner length that is required by the wave type and timing that you want. For a 2 into 1, you calculate the distance from each geometry change to 1 exhaust port and use that length in your dynamic calculations.

[MikeDutsa]

so do you all believe that a 4-2-1 is absolutly neccecary in these cars? or can you get away with something a little lass orthidox?

[Maverik]

4-1 has been working pretty well down here

[KU_Racing]

it is my opinion that a stepped runner 4 into 1 header design is the best solution for our package. Your requirement may differ.

[JuicedH22]

I agree with the stepped header design, it is relatively easy to calculate the wave pulses for the 'two step' rpm tune and IMO easier to manufacture than a 4-2-1 exhaust, the tq curve characteristics are unique and advantageous i think...

[Cody the Genius]

The main question that this makes me ask is why would you ever build a 4-2-11 header that is only tuned to 1 RPM. I understood that if only one rpm (or at least a narrow band) was being optimized than you would run a 4-1 so that there was a single, much more defined wave reflection as all of the runners are the same length and the 4-1 collector provides a greater step in crossectional area and therefore a stronger wave reflection. The main argument that I had understood for a 4-2-1 header was that effectively two resonance frequencies could be tuned (header flange to 1st y length and then header flange to second y length) so that the powerband could be braodened out by having one length tuned for say 7500 RPM and the other for 9000 RPM.


This correct, the basis of your question should have rather been "should I have an exhaust tuned for a narrow band (4-1) or a broadband (4-2-1)" as for any 4-2-1 exhaust should be tuned with two separate RPM in mind.


I guess the best way to see this is that in a 4-2-1 header the distance from the first y to the second y is proportional to the difference is tuning RPM chosen. A header tuned for 1500 RPM and 16,000 RPM would have a very large distance between the y's where as a 4-2-1 header tuned for 8500 RPM and 8500 RPM has zero distance between the y's and is then a 4-1.

This is also just assuming that all runners have Identical lengths as it is completely possible to build a 4-1 tuned for 4 different RPMs and a 4-2-1 to be tuned for 8 different frequencies; but that is another discussion entirely. All of these compromises are based on whichever design philosphy you choose and what best fits your application and aproach. There are always more than 1 right answers in life and in racing the stopwatch always wins.

[KU_Racing]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> I understood that if only one rpm (or at least a narrow band) was being optimized than you would run a 4-1 so that there was a single, much more defined wave reflection as all of the runners are the same length and the 4-1 collector provides a greater step in crossectional area and therefore a stronger wave reflection. </div></BLOCKQUOTE>

This is not entirely true. Think about the purpose of a 4-1 collector, or even the 3 2-1 collectors in a 4-2-1 header. There purpose is not to give the flow a large and immediate cross sectional area change- there purpose is to combine 4 (or 2) seperate flow streams into 1 flow stream of the same volume and rate through the same area. In other words, if you are using a very high quality merge collecter (Burns Stainless for example) then the wave reflection will be extremely small. The best way to provide a strong wave reflection at a specific RPM is to use and instantaneous rate of change- this means a stepped header design.

It is my opinion, (still an opinion, but backed up by pretty deep analysis of quite a few header designs) that with proper design and fabrication work, a 4-1 stepped header design will provide more benefit than a comparable 4-2-1, with the only penalty being a slight weight increase.

[VFR750R]

Cup cars use stepped 4-2-1 headers, and although it is a clearly different system, the 4-2-1 system is superior stepped primaries or not.

[Dan Nauts]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by KU_Racing:
it is my opinion that a stepped runner 4 into 1 header design is the best solution for our package. Your requirement may differ. </div></BLOCKQUOTE>

I went with a stepped primary design on a 4-1 header, but not for any reasons stated.

Acoustically my design is modeled from cylinder to tailpipe, with a rough estimate for the muffler damping. The reason the primary is stepped is threefold,... first, the additional "chamber" helps to accomplish my acoustic goals of the system. Second, the transition placement near the exhaust port reduces exhaust gas reversion (at low engine speed) back through the port, diluting the intake charge. And thirdly, the expansion provides a volume in the flow path for pressure recovery after the less than stellar flow across the ports. An expanding and reversing transition is also included after the shallow angle collector to accomplish the same thing.

IMO, I'm a little in disbelief with as many people, Formula and otherwise, that say their exhaust system has 1 (or 2) tuned RPM. I'd say that's an accurate statement if you run 4 pipes, each open to ambient, but when you consider the exhaust flow path, there are too many elements contributing to the acoustic "tune" of the system.

[Nima]

I think this subject (4-1 or 4-2-1) would be explained if you test an engine end tune it with the both piping and compare them. this could be done by dyno testing or CAE solutions. as I know the most of the fsae teams tune their engine by dyno tuning and by selecting one piping model.
In my opinion the 4-1 piping leads to bigger discharge coefficient and more power out put.

If I am wrong please correct me.