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      01-16-2012, 10:09 PM   #196

Drives: X1, 335i
Join Date: Sep 2006
Location: SF Bay, CA

iTrader: (0)

Originally Posted by ExpensiveTaste View Post
Ah, gotcha. I didn't think about the fact that it will take less exhaust pressure to push the same turbine at a lower speed that will produce the same amount of boost. I'm not sure how much this would increase VE, though. Does this engine have any overlap while in boost? I would think not...
The more exhaust back pressure there is between the exhaust port and the turbine inlet, the lower the engines effective VE. When this pressure gets excessive, you will get exhaust gas reversion (where the exhaust creeps back into the combustion chamber during even the smallest amount of overlap). On top of that, this pressure will have the same effect as a shoving a tennis ball into one of the tailpipes. Boost and IAT is only half the equation. Better way to look at the boost is the amount of air NOT getting into an engine. Instead it's just cramming up in the manifold because the engine isn't gulping it down quickly enough. What you want is the engine to gulp it down at a high rate (high HP!) and NOT have it accumulate/back-up in the manifold. The needle position on the boost gauges doesn't make the car go fast. Airflow rate through the engine does.