Originally Posted by 454C10
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.
Correct, and I agree and the reason for that is as follows.
Cylinder pressure is generated on the compression stroke and not during the valve overlap period. However,the reason valve overlap causes a loss of cylinder pressure is because long duration camshafts always have more valve overlap and later intake valve closing points and the intake valve closing point during the compression stroke determines the amount of cylinder pressure that is generated.
That is why the duration of 218* @ .050" with the CC-268H "RV" cam in my 455 Pontiac engine had about 25 PSI more cranking cylinder pressure than it did when it was equipped with a Ram Air IV camshaft that had valve duration of 231* / 240* @ .050". After I changed to the Ram Air IV camshaft, I lowered the rear gear ratio from 3.08:1 to 3.90:1 to compensate for the loss of low end torque.