Compression and cam

Bryan Wilson · #1 (**permalink**) 07-29-2007, 05:12 PM

Does the cam timing and the overlap (degree of lobe separation) affect compression ratio.
Cheers,
Bryan

wtm442 · #2 (**permalink**) 07-29-2007, 05:15 PM

YES. But only to a very little effect. The effect dissipates to almost no effect as the RPM's increase.

jwd · #3 (**permalink**) 07-29-2007, 05:43 PM

Very much so. You're talking Static vs. Dynamic compression. The street racers rule is to build it (compression ratio) with the pistons and take it out with the cam. It's all about intake valve closing.

Aussie Mike · #4 (**permalink**) 07-29-2007, 05:59 PM

Sure does make a difference Bryan. I've got a bigger cam for my motor but I'm looking at some heads with more comp to take real advantage of it. Bumping up the compression will give you a good boost in torque.

Cheers

KevinM · #5 (**permalink**) 07-29-2007, 06:04 PM

Bryan,

It doesn't effect static compression ratio, but it does effect dynamic compression.

It also drastically effects cylinder pressure, and the effect varies throughout the RPM range.

Dynamic compression doesn't take into account the ram effect from the moving column of air in the intake which causes cylinder pressure to vary by different amounts at different RPMs.

Have a look at these for a brief overview:

http://members.uia.net/pkelley2/DynamicCR.html

http://www.kb-silvolite.com/calc.php?action=comp

http://www.hcs.harvard.edu/~jus/0303/kuo.pdf

http://www.vehicular.isy.liu.se/~lar...d_2002_SAE.pdf

Kevin