well like the title say, i am trying to figure out my compression ratio. is there any calculator or anything that i could use? if it helps. i started with 8.5 with 76cc heads. i just got some used 62cc alum heads and had them shaved .005 to make sure they were straight. i know that the head gasket makes a difference so lets say i use one that is .039 compressed. also i have flat tops with 4 valve reliefs.

 

http://www.kb-silvolite.com/calc.php?action=comp

 

well according to that i am at 10.2. this set-up will be going on the car in a couple weeks. the rest of the set-up is in my sig. is this safe with pump gas? if not how can i lower it so it gets in the safe range?

 

you could enlarge the combustion chambers on the heads a bit by grinding and polishing, but you would need to be sure you take off the same amount from each chamber. You could get new pistons. Get the thickest gasket you can find. convert to E85...but if my memory is correct with aluminum heads you can get away with higher compression, I know the lt1's run about that ratio, but they are reverse cooling, so I'm not sure. I think you might squeek by with premium, and perhaps pull a few degree's timing out.

 

I'm running 10.5:1 on the same heads, it's really happy on 91, timing has been up to 39deg with no ping, it'd probably run good on 89 even. I say go for more compression.

Is your block decked? that makes a big difference, if not then try to get a head gasket around .020, if it is get the .039.

 

no it is not decked. it is the stock height, so do you guys think it will run good on the 89 with the either 5 or 10% mix of ethanel?

 

You cannot accuratly calculate the cr without knowing the piston to deck clearance at TDC. How far down the hole is the piston @ TDC?

 

i am not sure. i have not had the heads of off the motor yet. what is the most common? it is a pre 86 350 with flat top pistons with 4 valve reliefs.

 

H345P flat top pistons are commonly used to rebuild 350's. H345p have a non stock reduced compression height. They are at least .045" down the bore at TDC. The stock spec is .025". But even that varies.

So you really have to get in there and measure to know your cr.
With H345p pistons and a .039" head gasket and 76cc heads the cr is 8.33:1
With 62cc heads its 9.54:1

If you were to use a thin .015" shim gasket and 62cc heads the cr will be 10.13:1
When you remove your heads, measure the deck clearance and select a head gasket accordingly to adjust the cr for what you want.

 

you should be able to run pump.. i had 10.7:1 on pump before.. oh, that was with IRON heads too... but a much bigger cam.. you should be fine.. find out your piston-deck height and make sure all the measurements are correct.. any wrong measurement will throw off your calculation..

also, try keep the "quench distance" down (the piston to head distance).. try go no farther than .050".. .040" would be optimal..

 

ok, thanks for all the help guys!!!