The nice thing about the .509 is all the BBs I have tried will take it without any machine work. Its a good bolt-in gain. Of course, nowadays these motors are so old, you are always doing a ground-up engine, but back in the eighties, you could buy a BB car and plug a 509 & a set of hemi springs in, you were good to go. What are you running in the rest of the combo? compression ratio, heads, converter, gears, ect.?