' My big bore drag motors even when new wont go past 135psi even with 12:1 compression....'
That can be true of course and why one goes about decking things to get that compression back up, as well as fitting better pistons that fill the chambers up better. I have no trouble getting well over 200 psi out of them when I want it. The car ones can go over 300. You can degree to get back a bunch by simply advancing the cam to trap more in motor. As well, a whopping lot of generic supposed 12/1 pistons out there couldn't make it at all, they have to hold the pop-up ODs in a certain amount and the valve notches to fit more variables in the mostly OEM engines. Why they won't hit numbers even with shorter overlaps. I personally learned to ignore claimed numbers of compression and only look at the piston design itself.
I assure you I have the claimed 12:1, I have cced everything and done the calculations. Even if I had room more decking would only raise compression ratio, large valve overlap and valve timing events affect the cranking compression.