Here's something I have been wondering about. Around 1968 I think they started to put open chamber heads (906's) on 383, 440 engines. Prior to that were 516, 915 closed chamber with the higher compression chambers. When they switched to open chambers did they change the piston's? The compression would have dropped about a point or better I think and 1968 was little early in the detuning / emissions related hp reductions. Anybody know anything on this period?