Originally Posted by MikeSammy
Does porting heads decrease compression by raising the cc.
No, porting is inside the intake and exhaust ports and behind the valve, so it does not increase the chamber volume or change the compression ratio.
These changes to the head reduce chamber volume and increase compression:
Milling the head surface.
Installing valves with larger and thicker heads or that protrude deeper into the chamber.
Installing seats that raise the stock sized valves higher in the chamber.
Using a thinner than stock head gasket on one with a smaller than stock bore diameter.
These changes increase chamber volume and decrease compression:
Cutting chambers to clear and un-shroud larger valves.
Smoothing off the interior or polishing the combustion chamber
Cutting or grinding the valve seats and lowering the valves deeper into the chamber.
Installing valves that are thinner or recessed on the combustion chamber side.
Using a thicker than stock head gasket or a gasket with a larger than stock bore opening.
Hope this helps.