Getting closer to piston purchase for a couple aluminum head 555 builds. 9.5:1, 10.0:1 or 10.5:1. It's a rarity I would need rely on marina fuel if ever. Boating style is 5% idle, 75% cruise, 20% wfo. 10:1 (9.7cc dome) my first choice with limited shelf pistons. Just curious what NA 540-565 builds are running w/o issues. I've always liked running flat tops due to personal preference however rather not be at 9.5:1.

10-1.have a bunch running 10-10.5-1 on premium,all run real good.

That combo, with a zero decked block, 118cc chamber heads, .041 gasket, should be right at 10:1 with a flat top. What chamber volume are your heads?

Chambers are 123. That's the only problem. (Opened up Pro 1's) Well not problem just need to take in consideration. Custom Cams I have like 10:1 plus comp.

My worked over 496's are 10.5:1 however kicked around lowering a tad on new builds but no necessary. MT was on with calculations but I don't want to zero out new blocks.

If you are going to run a cam that has pretty much duration I would go with the higher C/R. It will handle better around the docks.