cam query

[mayoman]

i just want to double check something. a cam with lots of duration
will cut down compression ratio.
is this correct??

[jveik]

well, not the compression ratio in terms of volumes, but in actual airflow, yes... although id say probably not after a certain rpm... at low rpm's a lot of power is lost because the air and fuel coming in the engine is simply pushed back out becasue it doesnt have enough momentum like at high rpm's when the mix can still enter the combustion chamber even while the piston is coming back up (to a point lol)

[MrPbody]

jvaik is on the right track. "Dynamic compression" is reduced at lower engine speed with a lot of duration. Static compression is not affected. What it's doing is "bleeding off" pressure at lower speed. As RPM gets into the cam's efficiency range, cylinder pressure begins to rise quickly.

This is a common misconception about dynamic versus static compression. It's assumed (erroneously) that detonation is eliminated by this. While it DOES help a little at lower speeds, at higher engine speeds, detonation WILL return, and cause more damage than at lower speeds. The ONLY "real" fix is to reduce the static compression, either by installing "dished" pistons or enlarging the size of the combustion chamber.

There's no "free lunch"...

Jim