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I think i know what you're asking - compression is a big thing in our class, so here's what i can tell ya with a 49ci
I can get more exact usable number for you later. But off the bat, I know the "spec" for the stinger/formulaE combustion chamber is 27cc. I ran a stinger/formulaE head, and if you cc'd the head alone, it was ~25.3cc. The standard head is a fair bit deeper in pocket depth, but i still need to cc it.
Now here is my experience with compression - take it however you want to.
My engine started life with a stinger head at ~140psig. The competition was getting pretty fierce a few years ago so I put a milled down head on it (i believe it was 21cc on the head cc'd alone). The new head got me right to 160psi - great! Boat didn't feel any faster, and she blew up. Although, I was still using pump gas which was dumb - but I think she blew because the head gasket was found tore somehow.
We cleaned and honed the middle cylinder and put it back together with the stinger head. After running it few times on the lake, the bloody compression was 160psig!! With the stinger head! (25.3cc).
Today, a couple years later, it is 115psi. Nothing has been changed.
Conclusion: You can probably expect a pressure increase if you shrink your head, but the stinger head should keep you below 160psi. I would guess add ~15psi if you swap from stock to stinger head
Are you trying to do the 160psi thing that we are?
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Jake Elsey
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