What's the real purpose of stepped headers?

[fueliefan]

I've read (mostly on enthusiast forums that I don't trust) that the steps are there for one of three reasons:

1: anti-reversion
2: velocity changes
3: Creation of a step change in area for pulse tuning

Is it one of those, a combination of those, or none of those? My intuition is that it's not for velocity, as most stepped headers increase in diameter, not decrease. I'm also not sure about the anti-reversion, as I've heard that the step change location is critical and that suggests wavelength importance. I'd like to hear what the real science is. Thanks!

 

[GregLocock]

The real purpose?

1 baloney
2 yes, but if you are trying to reduce backpressure then a 7 degree (I think it is) taper would be better

3 Yes. The strength of the reflected wave is a function of the ratio of the change in crossection. If the change were infinite then the reflected wave would be as strong as the incident wave. If the change is only 110% then the reflected wave will be very weak (sorry I can't remember the exact equation, you can work it out). Suffice to say you probably need a change of 300% or so (bigger or smaller it doesn't matter) to get a measurable difference in VE.







Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[murpia]

I thought stepped exhaust primaries were useful in engines that rev so high that the primary to collector distance that could be packaged for e.g. a 4 into 1 collector was longer than that required for best tuning?

However, that's anecdotal and I haven't analysed it...

Regards, Ian

 

[thundair]

My understanding (name dropping) from Jerry Branch @ Flowmetrics and Jim Feuling is the reversion wave is positive along the walls and negative in the center. The idea of preventing the positive wave from entering the chamber (anti-reversion) at valve over-lap is discussed much from Jim Feulings patents to the shade tree mechanic. Also IMO the narrow exit pipe is to keep velocities up to get some dynamic motion going in the right direction. This relative new concept has brought smaller exhaust valves and ports.
This has also affected cam profiles that see the need for asymmetrical profiles to favor exhaust exit speed.

Cheers

I don't know anything but the people that do.

 

[GregLocock]

That sounds like marketing speak.

Neither Heywood nor Taylor mention reversion or anti reversion in their indices.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tbuelna]

The purpose of the step (or cross sectional change) in a header primary is for acoustic tuning. But like everything else, it comes at a price. While it may help at some frequencies, it will surely be detrimental at other frequencies.

 

[BrianPetersen]

... and how long ago was the Heywood book written ? ? ?

The fundamentals in Heywood are still correct, but many tuning details have gone beyond what's in the textbook.

 

[GregLocock]

Well, OK, where are the documented results from an antireversion feature that cannot be explained by traditional acoustic/pulse/inertia tuning theory?

Why don't the expensive racing series use stepped headers?

Anti-reversion=marketing speak for conventional exhaust tuning, I suspect.

I just found the equation for the strength of the reflected wave

Preflected=-Pincident*sqrt((big-small)/big)

I was wrong about the ratio needed to see a measurable effect, a ratio of 2 will give a reflected wave -70% of the incident wave.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

http://www.badweatherbikers.com/buell/messages/43/16476.gif

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[oab]

This is a good read on the subject.

http://www.fh-zwickau.de/ftz/Konferenzen/muenchen_2003/htm/Papers/10_Costa.pdf

-
Harvey.

 

[GregLocock]

So, from what I can see is that if your exhaust system is so short that you can't benefit from the reflected wave, then it would be a good idea to suppress it.

That's not a case I've come across in practice, but it is a tenable position I guess, for low speed engines with short exhaust pipes.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[murpia]

"Why don't the expensive racing series use stepped headers?"

Greg, they do (it does?...):

http://www.usautoparts.net/bmw/pics/engine/p84.jpg

http://images.google.co.uk/images?q=f1+engine

http://images.google.co.uk/images?q=f1+exhaust

Hence my comment that I thought it was for acoustic tuning that could not be packaged properly otherwise.

Regards, Ian

 

[BrianPetersen]

"for low speed engines with short exhaust pipes"

... or a high speed engine with a normally tuned exhaust system at that high speed, to limit the "bad" effects when the engine is running slower than the speed for which the exhaust system is tuned. For a normal street vehicle, that's most of the time. The tech paper on the Ducati 999 exhaust system is interesting, although it's quiet about the actual geometry of the exhaust pipe that they designed.

 

[thundair]

My limited experience is; that we tune for third wave at 8400 RPM I think it was 31" for 1 5/8 dia pipe, but for the Land Speed Record we will try to eliminate the reversion effect. There are lot of good reasons to stay away from trying to dial it in
Although it feels good when it comes on strong as you go through the RPM.

Cheers

I don't know anything but the people that do.