How much would flycutting pistons 40-70 thousanths
drop the compression ratio?

 

Depends on the volume of the relief.

Maybe 1-2 c.c. for the fly cuts...drops about 0.2:1 SCR

 

Agreed, the flycut dosen't hurt much but will change compression a bit.

 

I looked at the reliefs on the Diamond flat tops. They are ~.150" and ~2.375" diameter. Maybe someone can confirm this. They are 2cc reliefs. They must be ~1/4 the volume of the Diamonds if you are cutting 40-70 thousandths. So I would say somewhere between .5 and 1.0cc, closer to .5cc.