Originally Posted by
miataspeed
The cam timing can effect the compression. As you probably alread know most cams are ground with +4 advance based on TDC. This is to make the cam more drivable, Idleable.... If you retard the cam to true zero, it can increase the "Static" compression and take some drivability out of it. It's all based on the intake valve closing position and how much intake charge is allowed to excape. It's call the Volume/pressure index and requires a large slide ruler to calculate. Good Luck , and thank you. zack
You're talking 1-2psi of compression... if that.
At cranking speeds, there's 100mS of time for the cylinder fill event. This is more than enough to ensure that cylinder pressure is equal to the manifold pressure.
The flowrate for that situation is just 5 Liters/sec. You're passing that through two holes that are an inch across... thats really not a whole lot of flow restriction.
comparatively, at 9,000RPM, you need a flowrate of 150 liters/sec. yikes.