Quote Originally Posted by DennyW
This is what Pat was talking about, at least that's what I thought he meant on the cam pick he mentioned.

Dynamic compression ratio should not to be confused with cylinder pressure. Cylinder pressures change almost continuously due to many factors including RPM, intake manifold design, head port volume and efficiency, overlap, exhaust design, valve timing, throttle position, and a number of other factors. DCR is derived from measured or calculated values that are the actual dimensions of the engine. Therefore, unless variable cam timing is used, just like the static compression ratio, the Dynamic Compression Ratio, is fixed when the engine is built and never changes during the operation of the engine.

Two important points to remember:

The DCR is always lower than the SCR
The DCR does not change at any time during the operation of the engine


Why it matters: A 355 engine with a 9:1 static CR using a 252 cam (110 LSA, 106 ICL) has an intake closing point of 52º ABDC and produces a running CR (DCR) of 7.93. The same 9:1 355 engine with a 292 cam (having an intake closing point of 72º ABDC) has a DCR of 6.87, over a full ratio lower. It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91 or better octane. The larger cam's DCR falls outside this range. It would have markedly less torque at lower RPM primarily due to low cylinder pressures, and a substantial amount of reversion back into the intake track. Higher RPM power would be down also since the engine would not be able to fully utilize the extra A/F mixture provided by the ramming effect of the late intake closing. To bring the 292 cam's DCR up to the 7.5 to 8.5:1 desirable for a street engine, the static CR needs to be raised to around 10:1 to 11.25:1. Race engines, using high octane race gas, can tolerate higher DCR's with 8.8:1 to 9:1 a good DCR to shoot for. The static CR needed to reach 9:1 DCR, for the 292 cam mentioned above, is around 12:1.
This lowering of the compression ratio, due to the late closing of the intake valve, is the primary reason cam manufactures specify a higher static compression ratio for their larger cams: to get the running or dynamic CR into the proper range.

ps: If you rotate the engine on a stand, if that's where it's at, you can rotate it enough, that the piston is on the upside of the engine tilt, and oil will not leak past the rings fast enough to matter.
Ok so basically I am good with 7.98 dynamic compression, since it is in the middle of the range of 7.5 to 8.5? That is what I was getting at, because I recall a post saying that you want to be as close to 8.5 to 1 as possible. But I wasn't sure if my motor would run like a dog with 7.98. That is what I don't want. So this means I will most likely be able to run on 91 pump gas? I might just take my heads off after break in and check the deck height anyways, because when I built the motor I did not know about squish clearances. If my engine has .020" deck and a .038" gasket= .058", that means I'm quite a bit out of the range of .033"-.045"? What kind of worried me was that I thought I was going to have around 10.5+ compression with this setup, and the cam requires at least 10.5. When I figured out I might have less than 10-1, it kind of worried me.