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      10-23-2012, 07:06 AM   #19
Brigadier General

Drives: 08 E90 M3, 99 E36 M3 Turbo
Join Date: Feb 2011
Location: Concord, NH

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Originally Posted by CarbonFoot View Post
Can you expand on this statement? I don't see a strong correlation between boost pressure and exhaust restriction. While, of course, a high flowing intake and exhaust should be well matched to produce good power, I see them as separate functions.

As I see it; when boost is created, it pressurizes against a closed exhaust valve during the intake stroke. During the exhaust stroke the intake valves remain closed until just past TDC. I can't imagine a lot of blow-by is occurring in the cylinders under boost, where the supercharger is pressurizing against a restrictive exhaust. Exhaust pressure is generated by the compressive force of the exhaust stroke and not the intake system.

That being said, I don't see how uncorking the exhaust will lower boost.

Please correct me if I'm wrong as I don't know much about actions of vanos and the resultant effects it may have on this.
The improved exhaust in effect increases engine VE. If the engine can pass more air, it passes more of what the blower feeds it. The blower drive is fixed by the pulley so there is zero compensation. The engine now can use more air but the blower won't provide it. Not as much air is backing up in the intake against the blower driving it, so boost drops.

There is overlap, especially on an 8000 rpm motor, even with dual vanos. But I am not sure that is the full explanation for the effect. Let's say you plop a fully ported big valve head on there that flows 15% more air, and the valves work to the theoretical perfection you propose and the exhaust or intake are either entirely open or entirely closed but never both open at the same time. The engine will process more of what the blower sends to it. To maintain the same boost, you would have to increase blower output by 15%. The boost is measured in the intake, and the engine just gulped up 15% more of what was in the intake so there is less pressure there.