My 605 is 10.4-1, the reason I didn't go any higher is the best fuel locally is 87 octane. I have to drive 20-40 miles to buy 91. If you build a big bore hemi be very careful about using a head gasket .042 or thinner. My 528 had 4 pistons kiss the heads at zero deck and .042. I over revved it twice.Dave