A Chevy 350 bored .030" oversize goes to 355 cubic inches. That's 5/8 of a cubic inch per cylinder increase in volume. For all practical purposes, the increase in compression can be ignored. There needs to be a big increase in bore and/or stroke, or a dramatic reduction in combustion chamber size to make a big difference in compression pressure. That same 350 with a 76CC cylinder head has about 8.5:1 compression. A 64 CC head brings it up to around 9.5:1 or maybe 10:1. Domed pistons and a 64CC head approach 11:1.
A Chevy 230 six has flat top pistons. A 250 has a 1/4" dish in the piston top and uses the same cylinder head. That's a difference of slightly under 4 cubic inches per cylinder, from a longer stroke on the 250. The dished piston is there to keep the compression the same on both engines. Running a flat top piston for a Chevy 307 V8 in a 250 brings the compression up to about 9:1.
Jerry
