If I remember correctly when I did a compression test on my old motor it yeilded roughly 200-210 in all 8 cylinders. I have built a new motor with slightly lower compression(11.5 vs 11.75) and a bigger cam(230/230 vs 232/238) Would my compression test yeild higher or lower numbers?
phillip

 

The more overlap the cam has the lower the compression will be during a compression test due to both valves being open at the same time but not a factor at higher rpms.

 

more overlap as in numerically higher number?

 

numerically higher number of the lsa (lobe seperation) means lower overlap of the valves. So for example, if you tested compression with the same duration cam and initial compression ratio, one with 115lsa and the other cam on a 110lsa, the 110lsa cam would show lower numbers because it leaks more air at low rpms than the 115lsa cam would.

Overlap at higher rpms is a good thing though because the exhaust exiting makes a vacuum in the cylinder. If the intake valve is open at this time for a little bit also, it pulls in intake air because of that vacuum.

 

thanks, i thought so. it just sounded like last one was saying the opposite.