Might the higher compression in the 1958-1963 235s be due to the use of the higher lift “261” camshaft as standard equipment in those engines?
Compression ratio, as i understand it, is a basic math problem. It is the comparison of the volume of the combustion chamber, with the volume of the cylinder from BDC to TDC. This would be considered a static comparison, with no involvement with any moving parts (such as the camshaft design).
Where it can get squirrelly is when one engine is set up with TDC ending up shy of the top of the cylinder, while another engine is set up with TDC closer to the top of the cylinder. The additional volume above the piston which does not reach the top of the cylinder should then be considered as part of the combustion chamber volume.
For a 235 to go from 8.0 to 8.5, there would need to be a considerable reduction in the volume of the area above the top of the piston and the top of the deck at TDC and a reduction of the combustion chamber of the head.
0.5 is a lot of gain.
I have had a head milled .090 and it left little if any quench volume on the head near the intake valve. I can't imagine that this would aid in the swirl characteristics of this head, and it would likely shroud the intake valve even if it were recessed into the head (to avoid contact between valve and piston).
Having said that, one will not know this until one puts it on the dyno. Parts in motion often defy the "logic" of static parts diagrams and engineer's preconceived notions.
Look at an old Harley engine. On paper, it should not run. Put it on the street and hang on!
Carl
