I'm looking to run pump gas, premium is fine. I would also like the option to run a small shot of NOS. How do you decide what compression ratio to build for? I thought 9.5, but I'm new at this.
My car is 9.5:1 and it behaves very well. Engine temp has never been an issue with a good stock rad. Runs on 91 octane without issue. I dont have to worry about this engine at 9.5:1. The prevous owner SVOPRO built this engine for a 125 shot. Ran an 11:89 with it and he highly suggested I put a kit back on this car. Maybe one day. My last 302 was just over 11:1 and it was a handfull compared to this motor. If I had to build another motor I would stay around the 10:1 range and build the motor to suit. Especially since we lost almost all of our Sunoco stations around here and our high octane fuel along with them. Sucks big time!!!!!!!!!!!!
im running 11.5 to 1 ratio. i use ported factory heads with biger valves. i had it dyno tuned and the dyno operator said that the heads didnt flow enough to need any more than premium fuel. i have never had any detination problems. heads play a huge factor on what compression ration you can run. so does the cam. get forged pistons if you are going to run nitrous.
I have a set of cast iron GT40 (not P) heads. Block is bored .030". There is 91 92 93 and 94 available, but I've only seen the 94 in one location so I wouldn't plan on it. Haven't chosen a cam, intake, or carb yet.
Theres also forged pistons at affordable prices. So that help outs, thanks to the popularity of our engines. I wish we had higher fuel out here. All we get is 87/89/91. But at least ours is oxigenated, which helps with fuel burn.
Does anyone have specific examples of pistons they've used with these heads to hit 9.5:1-10:1? The machine shop said their basic selection only gets them to about 9.2:1. I'm not positive about this number, but I've seen chamber sizes for these heads listed at 65.5cc. Am I going to have to shave the heads to get the compression up on this setup?
Here is a set of Keith Black pistons I was looking at. http://www.summitracing.com/parts/UEM-SKB116-030/
.200 dome that will get the compression up theres alot of variables but those with a zero decked block thin gasket and a small chamber will be more compression then you want for a pump gas motor. a set of 2 valve relief flattops in most cases with a 62-64cc head and a .040 thick headgasket will put you in a pretty safe range for pump gas assuming the block isnt zero decked. cam selection is also important as to what fuel octane you can run.
I used the summit compression ratio calculator to come up with 9.55:1 ratio. http://www.summitracing.com/experta...02-92fc-89674fbc0055&storygroup=CalcsAndTools I used 0.010 deck clearance in the calculation but that was just an estimate based off info I found for stock blocks. My block has not been decked at all. 4.030 bore, 3.00 stroke, 65.5cc chamber (again estimated from info for stock GT40 heads), and 0.040 for the compressed gasket thickness. The volume for this piston is -2.60cc. I know that these numbers are not exact, but I figured someone on here may have some insight in that area.
You need to consider cylinder pressure, not just compression ratio. The camshaft has a lot to do with it.
