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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Originally Posted by copbait73 View Post

...It goes like this: You blue print the combustion cycle components to make all cylinders operate under the same conditions, at all times. This doesn’t just mean the block related components but the air system also. When you’ve made every effort to achieve this you can reliably pull high HP. As it relates to exhaust, his comparative testing (yes he tested log manifolds that were the previous standard on turbo Indy motors) showed anything less than equal primary pipes (whatever length) gives you misfires on random cylinders at random RPM and misfires become more critical and damaging as power density increases. ...

This is gold! Especially on full race motors with high overlap cams.

This hints at it but no one has said anything about how equal length system helps to equalize temperatures at the exhaust ports.


As to keeping each bank split into a split scroll turbo. One resource noted that this works very well on a 4 cylinder motor and is a significant advantage that warrants the effort. It was noted that on 6 it may not be of significant value.

For us I would guess it is a solid way to mitigate running unequal length secondary tubes with equal primaries.


Glad to hear someone else say 'scavenging' dose not so much apply to turbo exhaust.

I suspect equal length is more about orderly exit of the exhaust pulses which reduces back pressure, increases VE, and thus power.

An extra 15hp, if that is the case, for equal lengths and more ballanced air flows / temps at each cylinder to me is worth the effort.

It might even me more if used in combination with the right turbo hot side. I suspect most 930 owners are suffocating the hot side in the name of less lag.

I suspect there could possibly be 40 +/- hp that could be freed up on a 425whp motor that is running a 7200 hot side based turbo with a change to equal length system and a larger turbine.


Yes, equal length is the ideal turbo exhaust for making power!

But, lower volume is strongly correlated with reduced spool time. (See graph at start of this thread.)

I still have not seen a credible source that says equal length reduces spool time over a well designed non-equal system.

This is why I would say making the assumption that any such system will do so is a 'leap of faith' until proven with repeatable tests that control for boost level and AFR.

It could very well be I just could not find any experts that confirm this and am mistaken.

What I have read is that unequal length has the potential to create higher pressure spikes at the turbine that can have the effect of accelerating a turbo quicker.


Comparing a K27-7006 turbo to a K27-7200 might not be that different than comparing a well designed small volume header to a well designed equal length 930 system.

One should spool faster, the other should makes for more hp.


For the track the 7006 & equal length's are going to be a better fit.

For commuting, the 7200 and a decent low volume header is probably a better fit.

One might mix the two for a better average.


Either turbo or header is probably an improvement over the stock units for the track or commuting.

Again, I am a big fan of what Brian is doing and it is what I would choose given my goals.

Last edited by 911st; 01-15-2010 at 07:16 AM..
Old 01-14-2010, 04:06 PM
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