Quick2500

Dynamic compression and octane requirement depends on the cam, head material, quench..... If you are going to build a moderately high output engine, based on your questions, my best advice is to take it to a reputable engine builder. It will end up being cheaper with a lot less headaches.

ROTAX454

Don't forget that you can slightly alter the C.R. with different thickness head gaskets from Cometic. Course, who would use anything but Cometic. Ok, don't answer that question.
A good target is 112cc head volume. Use a thinner gasket to bump up the C.R. You can always go backwards with a thicker (standard) gasket. I am assuming that a very good head design is selected----in aluminum!!!

MILD THUNDER

I'd shoot for a .040-.060 quench in a N/A build. You dont wanna go too thick with head gaskets on a N/A deal.

ROTAX454

Great point. That is why you should call Cometic for their "not in the catalog" gaskets for your particular motor build. .028 is what I am using. Don't always trust what is either in print or on their website. Call. Several times this has netted me a special part that best suited my motor project. Quote: "We have those push rods but do not list them. How many sets do you need?"

MILD THUNDER

You need to measure how far the piston is in the hole. Once you get that figure, you can start looking at gasket choices, and head CC choices. A setup with the piston .020 in the hole, vs 0 deck, can alone make a difference of around .25. For example, you might use a standard felpro .040 thick gasket, 119cc chamber head, and with the piston in the hole .020, you will be 8.7:1. With the piston at 0 deck, 9.0:1.

I am a fan of a flat top piston, with a smaller chamber to get the desired compression, rather than a big chamber, with a bigger dome. Thats just me.

stimleck

Would this be a waste with stock wxhaust?

MILD THUNDER

Yes. Stock center rise cast iron manifolds that is.

dereknkathy

if you mean the stock 330-365 mag exhaust, it would be worse than a waste. the 365 motor is already pushing that exhaust to it's limit. you can do a LOT to a small block before that exhaust has to go. (same riser) but with a 454 with any mods or even a stock 502 the stock 496 exhaust is the minimum you should consider.

mach1magnum

what cam will 496 exhaust support?

stimleck

thanks for the info, I'm stuck with thru prop exhaust due to noise issues but would love to bolt on some more power to my 502 mag