[95Sally]

I have a stock 302 out of a 90' fox body with gt40p heads. can anyone give me an answer to what the compression ration might be.

[Mikey6119]

This might help its from an older thread i had with a similar question>



I have a 94 GT that I plan on rebuilding in the next month and im hoping to go with the GT40p's which have a slightly larger combustion chamber.. So this got me to thinking about how and IF i could go about changing my CR to gain HP.. I was wondering

1: how would I go about changing it?
2: how high could I go?
3: what kind of gains could I expect if i did bump my compression up from the stock (9:1) to say something like 10, 11 or 12?? oh yea my car is Naturally aspirated

If you would like i can post a full list of mods and mods that are going to be added
The GT40P heads have SMALLER chambers (58-59cc) than the stock E7's (62-63cc) so they will actually bump the CR higher by ~0.40:1. Milling 0.010" off the head surface will increase the CR by 0.13:1, and milling the same amount from the block deck will increase the CR by 0.20:1.
The limiting factor is going to be piston-to-valve clearance when you mill the heads or block, and there's a limit to how high you can take the CR with iron heads before you start to face pinging issues even with premium fuel. __________________

[EagleAutosports]

Too many variables to be able to say what the compression would be. Things like the actual cc size of the P chamber and head gasket thickness all affect the ratio. The 1990 engine had 9.0:1 compression, so if everything was equal and then you put P heads on then it'd be around 9.3:1.