I don't remember if the 415 hp was advertised as 11.25 or 11.50, either way they weren't what they advertised, they where lower. Bill Jenkins got NHRA to change the dome height on the 1963 415 HP only to .090 or .093, I can't remember which exactly. I used the stock pistons and ran them that way, some guys would take a set of 1964 415 HP pistons and machine the dome down to the .09? spec and get more compression. If your making a street and strip motor on pump gas with a iron head it will detonate like crazy any where near 11:0 to compression ratio and kill the motor unless you use E85 for fuel I have a 9.4 to 1 compression 446 C.I. that I can make detonate by leaning down the primary part throttle cruise AFR to 14.3 or higher That is on Oregon 92 octane at 2500 ft. above sea level