I am currently running dished pistons with 76cc heads which works out to about 9:1. What would my compression be if i were to use 58cc heads? TIA.

 

I'm assuming 350. Go here to find an equation:
http://home.tampabay.rr.com/yz250/calc.htm

I already tried it out for you and estimate close to 11:1. In which case those heads had better be aluminum. But you ought to plug the numbers yourself to get a better idea.
To help you out in case you don't know the numbers off the top of your head, the bore for a 350 would be 4.000, unless it's been bored. 30 over would be 4.030
The stroke is 3.48
And the average head gasket is .039

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91 Z28
Come see The Vicious...

[This message has been edited by ViciousZ (edited December 10, 2001).]

 

You don't mention the volume of those dished pistons or the deck height or the gasket.
I get around 8.5cr for your combo. ( I'm assuming a factory .025" deck height and a factory .015" shim gasket and estimating 12cc
piston dish.)
Irregardless you will see right around a 22.5% increase in compression ratio by changing
to 58 cc heads, all else being equal.
It's going to be touchy even on the best available pump gas.

[This message has been edited by F-BIRD'88 (edited December 11, 2001).]

 

http://216.121.161.76/files/EngineCalculator.xls

There is another engine calculator for you.