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posted on November 14th, 2005 at 03:28 PM
Question For Wayne Penrose
Wayne
I have just finished reading the article on the TIII in VWMA and am a little confused about the exhaust setup. If you are running a split pulse
housing and joining pipe 1 & 3 and 2 & 4, should the pipes not be equal length? From the setup you show in the mag the pipes are different lengths so
the exhaust pulses will not reach the turbine in sequence. Does this not defeat the purpose of the pipe matching and split pulse housing?? I am asking
because I am building a twin turbo engine and have been told it is better to have equal length pipes for equal distribution of the exhaust pulses.
Rob
2443TT
A.k.a.: Ian
Fahrvergnugen
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posted on November 15th, 2005 at 01:06 PM
I'll jump in here!
Yeah in an ideal world its great to have matched length exhaust pipes but more often than not it just works out to be what will physically fit.
Matched pipe lengths ensure the exhaust pulses arrive at the turbine at a consistant frequency, where mismatched pipe lengths will have pulses arrive
ar varying frequencies. The effects this has on the engine as a whole will vary at engine speed.
Now putting that aside, the reason naturally aspirated (NA) engines run extractors with matched port lengths is to maintain gas speed through a
collector. The momentum of the exhaust exiting the engine would then also help pull exhaust out of the combustion chamber and with cam over-lap pull
intake air/fuel into the combustion chamber. None of this is relivent with a turbo engine because.....
1. air/fuel is being forced into the engine so scavenging is irelivent.
2. cam overlap works to the detriment of a turbo engine. You want your exhaust event to be shorter than the intake event so that back pressure from
your exhaust doesn't interfere with exhaust flow. The exception to this rule would be supercharging, where scavenging is still relivent, and cam
overlap allows incoming cool air/fuel to help cool the exhaust valve.
3. exhaust back pressure in a turbo header is always higher than the incoming boost pressure. By how much is dirrectly related to the efficency of the
turbo. At low boost levels there will be less variation, but at higher boost the exhaust back pressure can double the intake air charge pressure.
In the end so long as there is boost, and the exhaust is getting out as quickly as possible and while maintaining as much heat as possible that is all
that matter.
shiftyvw
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posted on November 15th, 2005 at 09:53 PM
The pulses will still be grouped and fire in sequence, so the excact timing of them is unimportant. if they are grouped the way you say, and the
second firing cylinder in the group had the much shorter pipe then the pulses would be closer together? this may be beneficial? dunno just
brainstorming!
I thought one of big advantages of the split pulse as well was the fact that the closer you join your pipes to the turbine the better to impart
maximum energy on it and the split pulse housing keeps them separate right up to the turbine. ie the collector is right up inside the turbo so to
speak.
might be wrong there?
