At low RPM, where head airflow is not so much of a factor - a long duration cam will give you
less dynamic compression. Primarily, you want to look at the intake valve close (IVC) event. In a stock cam, it's a 10-20 degrees ABDC. In a really 'agressive' cam, it might be 40-50 degrees ABDC. That means that the piston is pushing a lot of air right back out the intake valve - so when it finally closes, there is less air in there to compress, so you get lower dynamic compression.
The reason a late IVC is useful is found at high RPMs. There, because the air is being rushed through the restrictive intake port, the piston has formed a little vacuum in the cylinder as it pulled down - so it's useful to wait a moment ABDC to let the air continue flowing in, even if it means the piston comes back up a bit.
Due to that same airflow restriction, dynamic compression is always highest at low RPM. Thus, the less agressive the cam, the higher the dynamic compression gets - all the way across the RPM range.
Make sense?
