12-16-2012, 07:25 PM
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bimmerfest Supporting Member
Location: Orlando, Florida
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Join Date: Nov 2007
Posts: 3,199
Mein Auto: '96 328iC, '04 325Ci
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Quote:
Originally Posted by JayMac
Dc you lost me. Wouldn't a more aggressive cam create some overlap as a result leave the cylinder open longer?
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The overlap when the engine is firing allows the escaping exhaust gases to help draw in additional air fuel charge. When you're doing a compression test the engine isn't firing and it's turning relatively slow. The cylinder still fills completely and the full volume for the compression stroke is still there so the overlap doesn't really have any noticeable affect on the compression reading.
I've done compression tests on engines with cams so aggressive the minimum idle speed was 1500 rpms and didn't see any difference from the basic atmospheric pressure x compression ratio value.
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 '96 328iC "Marie"
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