Advice needed regarding compression ratio

[Ron]

A while back I bought a set of Jeff Morys GENII OEM Stage III cylinder heads for my 1996 GTS from Steve Viperair. They were used but refurbished by Jeff to be 'like new," which in fact they look to be. Believing in the Reagan doctrine, "trust by verify," I took them to a local shop (where they machine drag racing heads and dual quad tunnel ram intakes out of billet aluminum) to be checked, flow tested and cc'd. Turns out they are in fact in perfect shape, flow great and all Steve said they were but for one thing, chamber volume. The chambers are 77 cc's not the claimed .030 cut 75cc's.

My questions are;

What would my compression ratio be on a stock GENII engine with these 77cc chambered heads and/or what is the specific formula for calculation of same for a GENII?

and

Is it worth shaving them a bit more to up that CR number, and if so what would be the optimum compression ratio for a 98% street driven, normal pump fueled and occasionally tracked, stock 708 cammed GTS with no mods but Crower shaft mounted 1.7 rockers? Stock intake, exhaust, ECM, cats, etc. Know full well that this approach doesn't come close to taking full advantage of the head's capabilities, but it fits my wallet, reliability & drivability preferences, at least for the moment.

Thank you

 

[JohnnyBravo]

I think it depends on what you plan on doing in the future. I have a set of JM Stage III heads on my car, but we actually dropped the compression from 9.5:1 down to 9.0:1 because I put a Paxton on it and wanted to be able to run more boost on pump gas without worrying about detonation. I don't think the heads themselves are going to change your compression much. Could you shave them and bump the compression and get some more poower? Sure. But where you'd really see a gain from increased compression is with all of the additional supporting mods like headers, exhaust, cam, etc. A bone stock car with nothing but heads and more compression probably isn't going to do a lot for you. Just my $.02. Others with more experience in playing with compression numbers may have a different opinion.

 

[Camfab]

I can't comment on non Strikers, but assuming Jeff was trying to emulate a similar port and chamber design in his ported OEM heads (kind of a stretch). Based on those assumptions, compression will help you. Having said that, remember that stock castings aren't as robust and are known to have core shift issues. I'd call Steve and get Jeff"s number, ask him what he'd recommend. I can't find his number, I think it went bye bye with on of my long gone cell phones....

 

[Ron]

A bone stock car with nothing but heads and more compression probably isn't going to do a lot for you.

Don't disagree Johnny, in fact the more I research I do the more see the flaw in my plan. I just do want to fall into the endless abyss of just one more mod, while at the same time want to step things up a notch to keep it fun. A fine line.

Camfab - From what I can figure out the refurb valve job might have slightly increased the port volume but not sure by how much. Going to have another cylinder or two cc'd and also a bone stock head because there is way too many conflicting numbers out there. Will flow them both too.

Unraveling the mystery is almost as much fun as getting the power.

 

[Ron]

Flowchart:

 

[Ron]

For anyone interested, a flow comparison of a JM Stage III OEM ported head versus a M/Y 2000 AB casting stock head.

I suspect that the earlier GENII stock AA casting head has a smaller chamber but won't be able to confirm that for a few weeks.

http://www.lastrail.net/heads/flowcomp.pdf