What is the highest of a compression ratio I can use with flat top pistons in my 302 with stock heads, or GT40 heads?
The limit is determined by the octane and quality of the gas you use and the ignition advance curve. Normally 93 octane will take between 10 and 11 cr, with fine tuning. The head chamber volume, shape and squish area determines the real end result as well as the cam profile. The cam's effect is in the cylinder pressure it will build at lower rpm. This is determined by the closing point of the intake valve before top center on the compression stroke. To get a better handle on this, the later the valve closes the lower the cylinder pressure, the less sensitive to octane etc. So the answer is not clear cut but depends on a number of motor attributes as well as gas octane and quality.
How much you want to spend????Actually if you go much above 12.5-13:1 with gas you basically have a deisel.Cam phasing and Ignition/timing controll at this point borders on the occult.(very spendy)Cylender pressures at these levels tend to ignite the fuel before the spark plug can(even race gas)That said I have seen guys running 12.5:1 CR on VP 114 race gas with simple timing/ignition tuning.You also need to consider max effective power(BTU) that you can get from a given amount of fuel at a certain pressure level.At some point you begin to lose efficiency no matter how much compression you have.Methenol/ethenol burning engines can get away with more compression 14-15:1 due to the fact it burnes cooler but those engines burn ALOT of fuel due to the lower efficiency(BTU) output of those fuels.So I hope this helps as opposed to confuses you even more.Good quench area in the chamber/piston designe aids in allowing you to run more compression without detonation when you get into higher comp ratios just as aluminum heads allow you to run a half to whole point more compression due to there ability to reject heat rapidly to the coolant as opposed to cast iron which holds it due to the fact that it isnt a ductile as aluminum.
Highest ratio you're going to be able to get with any flat top for a 302, and this would be a 1.619 pin height piston with 2 reliefs, combined with a 54 cc head will net you at best 10.5 to 1. Anything higher and you'll need to mill the heads even more (58 cc to 54 would be about a .040 mill) and/or use domed pistons. You can easily run a 10.5 to 1 motor on pump gas. My 331 has 10.4 to 1 and is fine with 93 octane fuel with it's Z303 cam and Canfield heads (57 cc chambers)
Im using a flat top with 4 valve reliefs (-2.7cc) and with a 58cc head it comes out to about 10:1. Id say any flat top combo you find could run pump. Also a big factor is deck hight (were the piston's in the cyl at top dead center) mine is at .000 deck hight meaning the piston is even with the block.
