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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
I have been wondering why reducing back pressure in the exhaust makes a differance.

If a smaller turbine is used to get full boost by 3000rpm it might be possable to see the pressure in the exhaust manifold go to over 3 times the boost pressure so lets say 3.2 times.

If we put a larger turbine on it might be possible to reduce that pressure to near 2.5 times I suspect.

If this is true, that would be exhaust pressure at one bar boost of about 3.2 bar compared to 2.5 bar. That is about a 28% difference.

I do not know if these numbers are accurate but they are probably not out of range of possibilities. We might benefit from testing for exhaust manifold pressure.

This might mean that there could be some 25% less resident exhaust gasses remaining in the cylinder after the exhaust valve closes.

Thus leaving less room for fresh air & fuel with the intake cycle.

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I am wondering if a larger turbine inducer with a smaller A/R housing might be better than a smaller turbine with a larger A/R housing? Anyone seen anything on this? I am concerned that when we increase the compressor Exducer diameter that we are throwing the leverage between the compressor Exducer and the Turbine Inducer out of wack. Not such a big deal on an EFI motor, but a bigger deal on a CIS with its built in intake restriction.
Old 01-04-2010, 08:58 AM
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