Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - compression ratio vs. pounds per square inch.
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Old 03-16-2013, 08:27 AM
454C10 454C10 is offline
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It isn't the overlap that effects cranking pressure (compression test). The engine isn't even trying to make compression at this point in the 4 stroke cycle. In fact, the engine is trying to make vacuum during the overlap period.

What effects cranking pressure is when the intake valve closes. Long duration cams tend to have late intake closing points which will reduce the effective length of the compression stroke as the valve is still open as the pistons is rising to compression the air/fuel charge.

Since big cams have lots of overlap, people mistakenly tend to blame the extra overlap for reduced pressure on a compression test. However, more overlap does lower the idle vacuum.
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