taguy, As I have said before to others, for a more precise answer than mine, go to realoldspower.com. I am not trying to push members here over to there, but you can get an answer to THAT QUESTION easily over there.
What I know is this, the compression ratio will depend upon the application of the engine, ie, 2bbl or 4bbl, toro, "W" machine, 442, or just plain vanilla wrapper Oldsmobile????? My Delta was of the latter type, 2bbl plain vanilla wrapper Delta 88. The cr was 8.5:1 or perhaps a little less.
About your heads, I would say the Gs would be better than the Js, the Js were the least desireable BBO heads of all. I am not sure about the size of the valves but probably both heads had hardened exhaust valve seats. If you plan on rebuilding the heads anyway go with the Gs, if you don't, go with the Gs. If you DO rebuild the heads and replace the valves, be sure to have hardened seats installed on the exhaust side. If you can find some C heads, those were reported to have the best flow numbers of all stock BBO heads but it would really depend upon your intended use and application and the rest of the engine anyway. Lots of variables to consider.....
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Dan
'46 2 door
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