I currently have 9.6:1 compression with 64cc heads. Not sure what size the head gasket is tho.

Does anyone know my approx. compression ratio with 76cc heads?

Im thinking about running nitrous so I think I may lower the compression when I do the head swap.

 

Presuming about 6cc of valve relief, 0.041" gasket compressed height, 4.120" gasket bore, standard 0.020" deck height, the change from 64 to 76cc heads should drop static compression from about 9.58:1 to 8.5:1.

 

No need to drop the compression ratio to run nitrous.

Nitrous only needs a good source of fuel at a constant pressure,
moderate ignition timing and slightly cooler plugs.

U can use 93/94 octane up to about 200+/- hp shot.

retard the timing about 2 deg per 50hp shot.

If you want to run more NOS HP shot than 200, I'd install a small separate fuel cell under the hood and run 110+ race gas from it to the Nitrous plate.

This will be true wether you use your present 9.6:1 cor drop it to 8.5:1.

 

im only planning a 75-100 shot. I have cast pistons so I wanna go easy on it. I dont plan on running tons of nitrous anytime soon.

 

Don't bother dropping the CR for a75/100shot.

Just fill up with the best octane gas available when you're going to be squeezing it. Get new spark plugs that are 2 heat ranges cooler than stock. be sure the timing is not over advanced.
NOS speeds up the combustion so less timing is required to get full power. With a 75 to 100 hp boost you should'nt really need to retard the timing but just be sure it is not over advanced. Use only the timing required. On a carbed small block its usually 32 to 36 deg at WOT. I think you;ll find that it screams at 30/32 deg , 93/94 octane gas , delivered at the proper fuel pressure.
Bottle pressure should be 900/950 psi when you start.
Follow the NOS instructions for fuel pressure settings and jetting.

We've run plain jane cast pistons up to 300 hp with no problems The trick is proper fuel and bottle pressure, high octane gas, and moderate ignition timing.