I put 906s on my '67 Newport 383 4bbl and I figured it made a low-to-mid 9 CR from the spec'd 10.0CR. I believe 452s have a few more CC than the 906s? Still got trace rattle at WOT kickdown with 92 pump octane super unleaded.
The main CR loss was the result of piston compression height decrease on the 8.2 CR motors. On the 440s, the flat top piston is .125" "in the hole" at TDC.
If you bought new pistons, they might be .020" "destroked" to allow for lower octane fuels. Meaning the compression height is .020" less than the factory pistons they will replace. This started happening in the middle 1980s, as best I recall. Unless you bought factory OEM pistons from the dealership (if available).
And then there's the issue of composition head gaskets replacing the steel shim gaskets which might have been used in OEM production. .018" compressed thickness vs. .040-.060" compressed thickness.
The open chamber heads were supposed to breathe better, the larger 1.74" exhaust valves included. They were also an emissions-related situation. David Vizzard's cylinder head book (based on Chevy small block V-8s) first posed the theory of using closed chamber heads for additional "squish" in the mixture. The later pistons with particular raised areas effectively made an open chamber into a closed chamber. More ultimate power, it seems, resulted. Detonation was not a real issue as long as the squish height was less than .020" between the piston crown and the flat part of the head's combustion chamber section.
As for port flow, the closed chamber head intake runners look pretty much identical to the 906 runners. From an article in one of the Mopar magazines, the 452s flow the same as the 906s when the MP Porting Templates are used to port the runners. It seems that what we were told, incrementally over the years, might not have been exactly accurate, as technology increased and we learned more about things. IMHO.
CBODY67
