I built a 440 16 years ago for my Boss using some KB pistons. Set the quench at .035 and compression at 9.4-1. I set the timing at 34° and it's been fed whatever is at the pump all these years. My Boss pasted away about 4 years ago and his Grandson has the truck it's in now. Still running hard! My guess would be detonation or preassemble damage. You can't hear detonation a lot of the time.