Pro1

Looking for some HP.

If we keep the stock Iron heads, use the Crane 741 cam as an upgrade, what compression with steel heads would be ok?

plus 2cc is 9.1 or plus 7cc would give us 9.6. Stock was 8.75. What do you think would be better?

With the cam change and increase compression and bumping rev limiters to 5500 hope top get 70hp per motor.

Griff

I would guess 9.1 would be better. You could mill the heads .010-.020 and run a thinner gasket and get it to around 9.25. Thats about the most I would want to run on 93 octane. I don't think you'll gain 70hp without some head porting and even then, I think you'd be lucky to gain 70hp.

PatriYacht

Don't forget the deck hgt. when computing compression ratio. Most BBC's have the pistons down in the hole about .020 while piston manufacturers specs call for 0 deck. If you are careful with engine temp and fuel mixture, 9.5 is doable. But you have to do the math correctly.

Young Performance

About 9.3 if you run temperature in the engine or about 9.5, as Patriyacht said, if you run it cold. Even at 9.5, you will probably need a little port work to help the heads keep up with the camshaft. Just clean up the bowls and the transition between the valve seat and the head. You will also need to clean up the combustion chamber to help stop hot spots that will cause preignition. Running the engine cold will help take some of the heat out of the head and help with the preignition.
Eddie

t500hps

I'm running 9.7 with Iron Eagles but it's a completely different motor. 540's ported heads, 741 cam, all from a 500EFI platform. Been running great for a couple years now.

ROTAX454

For under 300 bucks, you can have your comb. chambers and all valve faces TBC coated. This stuff works.
http://swaintech.com/store.asp?pid=10971

Young Performance

That may be true, but you still need to get rid of the high spots and projections in the chambers. Any high point or sharp edge will collect heat and cause a hot spot, potentially causing preigniton. I do agree that the coatings will help.
Eddie

Pro1

Thanks guys.

We wound up with a plus 3.8cc piston from Wiseco. plus 7cc JE put us at 9.6, plus 1 cc put compression was at 9.0 so we should be right in between. Maybe 9.25 to 9.3 compression from the stock 8.75.

We also added the 741 cam new springs roller lifters and new exhaust. This is for a 38 Fountain. The boat ran 80 with rev limiter pinned at 5200. So with the MSD hope to get to 5500 with the little bit of compression and cam.

Still 36 degrees total?

PatriYacht

That's where I run mine. 10.0 to 1 aluminum heads though. 120 degree thermostats from Arizona Speed and Marine and safely rich fuel mixture. I always load up 93 octane on land on the trailer.

Panther

I'd probably go 36 too, maybe 34 to be conservative... Ran one on the dyno a few years ago at 38, made the most power... turned it back to 36 in the boat.