rmbuilder

These pistons (if they are Mahle flat top -3.53 cc) with 72 cc combustion chambers, .039" total deck ht., 4.040" bore/3.48" stroke yield 9.72 CR
Bob

220BR

Not an expert, but for a marine engine I would design to Dynamic compression ratio of 8.15 to 8.25. It's the cam (intake close) that will effect low end performance the most. In your 350, this ought to be around 9.6 to 9.8 static

jafo

Good Advice! I used the same heads in my 355's on the Formula I used to own, but they were 64cc's. TRW supplied me with forged flat-top pistons that brought my static to 9.67:1, which was about where I wanted to be for using pump premium. Keeping the dynamic in the 8.25-8.5 is best.
The Sportsmen heads have a quick burn combustion chamber very similar to the vortecs, and are more forgiving to octane requirements than older designs.
Your camshaft choice is critical in this process!
Jim

rmbuilder

The math on this particular combination is:
Static CR.........9.72:1
XM 270 HR IC......65*ABDC
DCR................7.73:1
Dynamic cranking pressure.....152 PSI
Bob

rmbuilder

Formula78
DCR can be useful as an indicator, not an absolute value. There are to many variables involved (head material, combustion chamber design, load range, plug heat range, ignition advance, coolant temperature, A/F ratio, and octane) to use DCR as a design parameter. From the information you have posted your builder has done his homework and you will have a very strong combination.
Bob

Formula78

Well, I picked the parts except for the pistons, I was going to go with JE. But hey, whats another $100-$200 added to the cost of a whole re-power anyways? I know quite a bit about Ford engines, so I am familiar with engine basics, but not the specifics of a lot of this (especially from a marine standpoint). I'm all worried about quench now. I'm going to have to check the piston to deck clearance now.

Thank you for the help guys. The community here is a priceless resource. The info about DCR would have taken me weeks to find on my own.

BAD-HABIT

Thats what everyone is here for...Just try to give something back when you can.......

BH

Da Patriot

If you want I have a dart sportsman intake that would go good with the heads. PM me if you are intested and I would say at the very least a 9.0 naturally aspirated.

Formula78

Do you think that 10.0:1 is too much?

BAD-HABIT

IF and thats a BIG IF....You run good 93 octane ALL the time along with Making sure your ignition advance is conservative then your at the limit with 10-1...If you can get it down to 9.5 you should....If your stuck with 10-1 check with GOOD WELL KNOW MARINE ENGINE builders and ask what you can do to try and stay away from detenation.....If possible by a knock sensor and use it also. Im sure you can make it work it's just gonna depend on GOOD gas and Correct Timing for YOUR engine.

BH