The H75 is only 2cc bigger in each chamber (which equates to about one half point (0.5)) of a compression ratio, so they are different but certainly comparable. SO for every 2cc you remove you drop 0.5 of a CR unit.

I think that shaving the H75 will achieve much more CR than a H89 if you were to shave it by the same amount because alot of the H75's combustion chamber CC's are within the volume of the cylindrical dish. The big key is to get rid of that ghastly cylindrical dish as this does no favours for an effective combustion process.

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