drumking15

Is this the right logic?

Ok so i was running some thoughts through my head, and I am not sure if this has been covered but maybe someone can shed some light. I’ve been thinking a lot about what compression ratio to run when i build my motor.
OK i know not all my numbers might be right or the calculations but bare with me you will get the idea
So say I did 8.5:1 and ran 11 psi of boost,

11 + 14.5(atmospheric pressure roughly) * 8.5 = cylinder pressure of 216.75

Now if i up the compression ratio to say 10 and try to get the same cylinder pressure

216.75( cylinder pressure) / 10 = 21.675 - 14.5 = 7.175 psi

so what my calculations say is that at 8.5:1 at 11 psi i get a cylinder pressure of 216.75 and if i up my compression ratio to 10 and try to reach the same cylinder pressure i would only have to run 7.2 psi

SO i guess what my question: Just because the calculations say that i will have equal cylinder pressure by adjusting the boost after i change the compression ratio, does this mean i will make more or less or the same power?


i know some of my cylinder pressure numbers might be off because im not sure how to take in to account for the dome of the head, but you get the idea of my question

boosted hybrid

Boost and compression go hand in hand in terms of overall cylinder pressure. In most cases its better to let the turbocharger make the power, in the case of lower compression and more boost pressure. You'll make equal amounts of power, but you'll make different levels of torque at the same power levels. This is due to the nature of higher compression and boost.

drumking15

So what would be a good balance? Im posting this for my freind and he evetaully wants to go w/ higjer boost levels in his h22...He plans on starting on rather low levels of 7-10psi but then pushing close to 20ish...I recomended him 9.0 or 9.5cr becuase when he is not making boost he still needs some "n/a power" and the lower cr he goes the less it has to get him going. Thanks any more input is helpful!