"Block relief" - L.M. Williams book

[Parazine]

The problem with block relief is that you are removing material from the combustion chamber, inreasing its volume and therefore lowering the compression ratio. To maintain the compression ratio, you will need to skim the head, which narrows the gas path again from the valves to the piston.

This is the main "gotcha" for the sidevalve engine and limits the ultimate amount of improvement that can be made to gas flow vs compression ratio.