'65 Barracuda FS 273 4bbl 10.5 CR on pump gas in CA

Bob Jasinski said

Just measured the bore of the Fel-Pro gasket, and it measures 4.18", whereas the bore of the stock 273 is 3.625" per the FSM. Rock Auto shows this same gasket for a 318 and 273, it appears as though 273 head gaskets are not available. If the 318 gasket were used, that would reduce compression even more I would think.

Click to expand...

The difference by math and round holes, in those two gaskets, is 5.92cc; 2.84 cc for the OEM 273 and 8.77 for the FelPro. in the 273, that's a lot.
At 8.0 true Scr, on a stocker, the total chamber volume needs to be 79.9cc. Adding 5.92cc, is 85.8 and the new SCR is 7.51, so the gasket difference alone is ! a half a point in compression.
At 10.0 the the volume needs to be 62.1cc. Adding 5.92 gets 68cc, and the new Scr is 9.2, so now 8/10s of a point DIFFERENCE

Because the stock 273 gasket does not have round holes, the difference and loss may be greater than the math shows.