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Unit 5302 · 03-31-2002, 11:58 AM

Here's the information I eventually came up with.

99PI heads, 44.75cc +/-1.5cc combustion chambers.
96-98GT pistons have a 9cc dish and/or relief.
99GT pistons have a 17cc dish and/or relief.

All told it makes for 10.65:1 CR on the stock Gen I pistons with the PI heads. There is so much tolerance in the heads, and I don't like the compression ratio being close to 10.7:1. If I go the 4.6L route, I'll have the heads cc'd to 47cc. That will bring the ratio down to 10.25:1; perfectly acceptable as far as I'm concerned.

I think those tolerances are VERY poor considering how small the combustion chambers already are. If I didn't do the work I could wind up with one cylinder over 10.9:1, and another as low as 10.3:1. That's a huge difference.

Kinda wish I could find a nice 94-95GT in blue though. So far, it's nearly impossible to find one in blue, with a stick, in good shape, with low mileage, and for less than 10k.

03-31-2002, 11:58 AM	#6
Unit 5302 Registered Member Join Date: May 1999 Posts: 5,246	Here's the information I eventually came up with. 99PI heads, 44.75cc +/-1.5cc combustion chambers. 96-98GT pistons have a 9cc dish and/or relief. 99GT pistons have a 17cc dish and/or relief. All told it makes for 10.65:1 CR on the stock Gen I pistons with the PI heads. There is so much tolerance in the heads, and I don't like the compression ratio being close to 10.7:1. If I go the 4.6L route, I'll have the heads cc'd to 47cc. That will bring the ratio down to 10.25:1; perfectly acceptable as far as I'm concerned. I think those tolerances are VERY poor considering how small the combustion chambers already are. If I didn't do the work I could wind up with one cylinder over 10.9:1, and another as low as 10.3:1. That's a huge difference. Kinda wish I could find a nice 94-95GT in blue though. So far, it's nearly impossible to find one in blue, with a stick, in good shape, with low mileage, and for less than 10k.