J (915) vs. Magnum heads

True, in regards to compression ratio the math isn't all in the chamber volume...but if you're considering swapping one head to another on an existing short motor. the chamber volume is the only thing that changes (assuming the same head gasket is used)

All I'm saying is that once fitted with large 2.02 valves, a J head is only about 3-4cc's bigger than a magnum.

When changing from magnum to 2.02 J heads on a 360 motor you're not going to drop from 9.0:1 to 8.0:1 with 4cc or even 7cc.

A quick mathematical check tells me that would require a massive 12-13cc increase in chamber volume. To my knowlege, this is unlikely to be the case.

A drop from 9.0:1 to 8.7:1 is a more realistic number to expect and that's with the same head gaskets and no surface grinding on the J heads.