ptuomov wrote: ↑Sat Dec 02, 2017 1:56 pm Fair enough. Here’s what I think. If you increase compression, and hold everything else constant in the engine, the peak cylinder pressure always goes up. If you change other things in the engine at the same time, the peak cylinder pressure usually but not always goes up. Do you agree with that?
Not entirely. Too general of a statement. First off, the last part of your statement says "If you change other things in the engine at the same time, the peak cylinder pressure usually but not always goes up. My impression by that statement is you're assuming that cylinder pressure increases as a result of most changes, BUT..... You're trying to reword a blanket statement. I'll give you an example. My last engine in my dragster had a static compression ratio of 15.5:1 calculated. Want to try and guess what the cranking compression was? I'll help, it was 170 psi. Keep in mind, that my engine is more optimal than a street engine. On a stock 454 that I used to run in an old pickup I had, my cranking compression was 210 psi...and that was with 8.3:1 measured compression and a stock cam. What do you think accounts for that? The one thing to remember is static compression is nothing more than a difference in volume from TDC to BDC. That's it. DYNAMIC compression is what the engine actually sees, and is also partially responsible for cylinder pressure. The lower cranking compression was the result of camshaft events, not engine configuration. Change the timing of the cam, cranking compression changes. Change the cam itself, it will change again. Restrict the intake or exhaust enough to affect engine breathing, cranking compression likely will not change, but dynamic certainly will.