What's the real purpose of stepped headers?
What's the real purpose of stepped headers?
(OP)
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!
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!





RE: What's the real purpose of stepped headers?
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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What's the real purpose of stepped headers?
However, that's anecdotal and I haven't analysed it...
Regards, Ian
RE: What's the real purpose of stepped headers?
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.
RE: What's the real purpose of stepped headers?
Neither Heywood nor Taylor mention reversion or anti reversion in their indices.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What's the real purpose of stepped headers?
RE: What's the real purpose of stepped headers?
The fundamentals in Heywood are still correct, but many tuning details have gone beyond what's in the textbook.
RE: What's the real purpose of stepped headers?
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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What's the real purpose of stepped headers?
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What's the real purpose of stepped headers?
w
Harvey.
RE: What's the real purpose of stepped headers?
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: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: What's the real purpose of 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
RE: What's the real purpose of stepped headers?
... 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.
RE: What's the real purpose of stepped headers?
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.