playing with compression calculator
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TECH Regular
Joined: Jan 2009
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playing with compression calculator So playing with the 3.750 and 3.875 stroke with 6" rods. Saw somewhere on an lt1 forum some kind of equation on the deck of block minus rotating assembly, with a 6.83cc flat top piston. Just guessing not saying if it works just want to get better educated. So with an fbody gm gasket thickness of .050 I got a compression ratio between 11.60-12:1. Figuring the deck of the block is 9.020-9.025 based on search on the web.
Ok that helps now I have a complete formula to helop you with 
Take 1/2 crank throw 1.9375 for the 3.875 and 1.875 for the 3.750.
We will call them A and B respectively
A. 1.9375 + 1.062 + 6.0=8.9995 So I use 9.025 (I usually double check tho) 9.025 - 8.9995 =.0255 in the hole <----thats top of piston to deck clearance now say you use the .050 gasket your quench will be .075 which is terrible. Optimum quench is between .037-.041. You wouldnt want it any higher than .045 from a performance stand point, and with that amount of compression.
B. This combo bring the psiton in at an even 9.000 or .025 in the hole.
I see where you were trying to go with this. If you had the block zero decked it would presumably bring the total height to 9.000. In this instance either of the combos would work perfectly with a .039 gasket or the fel pro 1074's. Use that gasket in your calculations, I believe summit has the best description of the parameters.
Just on a personal note, if this combo is to be street driven more than track use I would opt for combo B, to keep the wrist pin out of the ring lands.
The only other thing that plays into the CR is the heads combustion chamber size. If ya need to know anything else or I didnt cover something ask me and I will share what I know
Take 1/2 crank throw 1.9375 for the 3.875 and 1.875 for the 3.750.
We will call them A and B respectively
A. 1.9375 + 1.062 + 6.0=8.9995 So I use 9.025 (I usually double check tho) 9.025 - 8.9995 =.0255 in the hole <----thats top of piston to deck clearance now say you use the .050 gasket your quench will be .075 which is terrible. Optimum quench is between .037-.041. You wouldnt want it any higher than .045 from a performance stand point, and with that amount of compression.
B. This combo bring the psiton in at an even 9.000 or .025 in the hole.
I see where you were trying to go with this. If you had the block zero decked it would presumably bring the total height to 9.000. In this instance either of the combos would work perfectly with a .039 gasket or the fel pro 1074's. Use that gasket in your calculations, I believe summit has the best description of the parameters.
Just on a personal note, if this combo is to be street driven more than track use I would opt for combo B, to keep the wrist pin out of the ring lands.
The only other thing that plays into the CR is the heads combustion chamber size. If ya need to know anything else or I didnt cover something ask me and I will share what I know
Thread Starter
TECH Regular
Joined: Jan 2009
Posts: 403
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that's the exact same formula I found. Reason I'm asking is because I would like to reuse the stock ported gm castings and be able to go with an aftermarket set of ported heads if I choose to down the road with just machine work without having to swap pistons.
