hello everybody! newbie here with a question on my 71 grande coupe, let me give you the rundown on this car, its a 4brl cleveland car, c6 trans, 9" rear w 350 gearset which i just put in and let me say what a difference! anyway i would like to find out the compression ratio, i had the cylinder heads off about a year ago to do a carbon and valve job, it does have the small wedge shape chambers and flat top pistons and monstrous valves. the reason why im asking is because i have a ping that i could never get rid of, regardless of timing, ive been told that this motor possibly has 10.25 compression ratio. can any of you gys confirm this? and if this is the case then it explains my pinging problem. of course i run premium gas, but it still pings. also the car has a edelbrock performer intake, 650 double pumper carb, a comp268h camshaft, hooker long tubes and flowmaster super 40 mufflers, this car runs like a scalded dog but im really concerned about the ping and poss engine damage over the long run because i drive this car about 5-7000 miles a year, i look forward to your feedback, thanx all!

 

are you sure its in the cylinders? If your running premium, and messing with timing, with no change at all, perhaps you have a valve issue. im nottoo expeireinced on this kinda thing, just some ideas id be running through my head. at least ill keep your post up top!

 

10.25 could give you problems with 93 octane gas. Why don't you run some high octane racing fuel through it and see if the pinging goes away?

 

thanx for the replys guys, i did run some sunoco race gas once i think it was 104 octane. and the ping did go away, it was mixed with some premium. but of course at 5plus bucks a gallon i dont want to run it if i dont have to. i had thought about swapping the 4 brl heads for 2 brl heads but i heard that the large open chambers can also create a ping! i have a set of 2brl heads off of a 351 modified that just need to be redone and i also have a set of 73 4 brl heads with the large open chambers and smaller valves, has anybody swapped 4brl heads for 2 brl? and did you lose any performance by doing so. im willing to sacrifice a little performance for engine longevity.

 

How thick is your head gasket, and do you know what you cam timing is and what youra/f ratiois?

 

The open chamber heads were 9:1 or 9.5:1 depending on the model. The closed quench chamber heads were either 10.7:1 or 11.0:1, which is what I have on my old cleveland now and I run 93 pump gas with no ping. You can get a close idea to your CR by doing a compression test. My cylinders are in the 215-235psi range. Here is an equation to convert compression to compression ratio (a best guess).

Compression = (Compression Ratio minus one) to the 1.2 power multiplied by Atmospheric Pressure

Given Atmospheric Pressure at sea level = 14.7 psi, we have the following:

So 11.0:1 is 235psi
10.0:1 is205psi
9.5:1 is 191psi
9.0:1 is 187psi

You can test the cylinders, then look atweatherunderground.com to see what the atmospheric pressure is in your area that day. You may need to convert from inches mercuryto psi.

Easy way to find out what your have. The compression test can be affected by cam, heads, ring wear, etc, but it will give you the affective compression ratio your are running currently.