I have #62 72 cc on a non-rebuilt '69 400 4 bbl, and it won't run on less than 100 without heavy detonation. These are the stock factory cast pistons, standard bore. Original factory cam, stock valve timing. It's a slightly tired 75,000 mile engine.
All I would need to do is drop in lower compression heads or pistons and solve the problem. Factory advertised 10.75:1, it's been determined that this figure is overrated and the actual was about 10.25:1. I would target for a maximum of 9.5:1 for 92/93 octane.
