I’m rebuilding my 1999 merc HP500 (carb) and I’m thinking about changing the pistons to get higher CR. The pistons I have now are forged SRP (4,5”) and the CR is 8,9:1. I’m running 088-heads with new valves and springs so I don’t want to change heads. Can the 088-heads handle 10,0:1 CR? Or what CR is recommended? Cam is crane original roller HP500. My machine shop says 10:1 is fine but they usually build street engines. My friend says cast iron heads can’t handle 10:1 on pump gas. Thankful for any help

I wouldn't run anything over 9.0:1-9.4:1. Even at that you won't see any gains.

I would think it really depends on tune. What ignitions? If it’s a standard all in by 3000 or locked timing I wouldn’t. I would think if you used something like a Daytona setup where you can do a custom timing map it would be possible.

Endeavour32 has one Daytona box left for sale if I recall. Would be nice addition to your rebuild.

Just be certain your machine shop - engine builder sets bearing clearances etc to marine/hp500 specs. Post 5 good advice. If your piston to wall clearance checks out fine and pistons in good reusable condition go for the heads. Youl'll see much more of a gain than a half a point or so in CR.

Yes duration plays a role in cylinder pressure also. So higher compression would require a little more overlap to relieve cylinder pressure

Someone on here with twins mentioned they had two similar engines but one was a CR point higher and he couldn’t tell any difference between the two. All of the power gains are to be found in the head with compression and cam being a factor to support it, not the other way around. I’m a complete hack, but that’s my understanding of it.