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Old 04-02-2008, 06:11 PM
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Camshaft overlap do not "bleed off compression".

The engines cranking cylinder pressure is effected by the intake valve closing point (start of compression), not the valve overlap. The cam overlap causes exhaust dilution of the intake charge at low engine rpm. Thats what give the rough racey idle sound.
The overlap induced exhaust dilution at idle and low rpm acts like EGR, diluting the air fuel mix with exhaust gas, This reduces peak cylinder pressure during combustion (at low rpm) as the exhaust will not burn twice. this tends to lower an engines tendency to knock at low rpm with a high compression ratio. Unless the motor is way over cammed, the EGR effect of the cam overlap is gone by the time the motor is near peak torque (high volumetric efficienty).
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