How to figure my compression
How to figure my compression I have a 408 with zero deck,Dart 225 heads ported to the max,stock mls headgaskets,Lunati 21cc dish pistons,and I am going to run a t88 turbo but I was wanting to figure the compression to see what kind of boost I could run.
Simply put, the compresion ratio is the volume of the entire system at BDC, divided by the volume at TDC. You'll need all of the following to calculate it:
Bore & Stroke: In your case 4.030 x 4.00
Head Gasket Compressed thickness
Head Gasket Bore
Combustion Chamber Volume
Piston Deck Clearance: in your case zero clearance, so zero volume
Piston Dome/Dish Volume: in your case 21cc
Assuming some general dimensions, we can give you the math to get there. Lets use a 72cc chamber, and a 4.060" gasket that is .040" thick.
Cylinder Volume = (Bore/2)^2*3.1416*Stroke = (4.03/2)^2*3.1416*4.000 = 51.0224
Gasket Volume = (4.06/2)^2*3.1416*.040 = .5178
Dish Volume, converted to Cubic Inches = 21 cc / (2.54^3) = 1.2815
Combustion Chamber Volume, converted to Cubic Inches = 72 cc / (2.54^3) = 4.3937
To get the volume at BDC, just add them all. The volume at TDC is the sum of everything but the cylinder volume. So, for our example, your compression ratio is:
(51.0224 + .5178 + 1.2815 + 4.3937) / (.5178 + 1.2815 + 4.3937) = 9.23:1
Now you have the math, and you can change the variables as you get the specifics for your setup.
Bore & Stroke: In your case 4.030 x 4.00
Head Gasket Compressed thickness
Head Gasket Bore
Combustion Chamber Volume
Piston Deck Clearance: in your case zero clearance, so zero volume
Piston Dome/Dish Volume: in your case 21cc
Assuming some general dimensions, we can give you the math to get there. Lets use a 72cc chamber, and a 4.060" gasket that is .040" thick.
Cylinder Volume = (Bore/2)^2*3.1416*Stroke = (4.03/2)^2*3.1416*4.000 = 51.0224
Gasket Volume = (4.06/2)^2*3.1416*.040 = .5178
Dish Volume, converted to Cubic Inches = 21 cc / (2.54^3) = 1.2815
Combustion Chamber Volume, converted to Cubic Inches = 72 cc / (2.54^3) = 4.3937
To get the volume at BDC, just add them all. The volume at TDC is the sum of everything but the cylinder volume. So, for our example, your compression ratio is:
(51.0224 + .5178 + 1.2815 + 4.3937) / (.5178 + 1.2815 + 4.3937) = 9.23:1
Now you have the math, and you can change the variables as you get the specifics for your setup.
Last edited by hammertime; Nov 23, 2007 at 12:02 AM. Reason: used wrong stroke & nobody noticed :o)
As for thickness, I've seen ranges from .043 to .047, and a GM spec from Scoggin Dickey that says .065
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I like to use this one:
http://www.csgnetwork.com/compcalc.html
Using what you gave us I got 9.616819240502426:1
4.03 bore
4 stroke
4.06 Head gasket bore diameter
.052 compressed thickness
65cc combustion chambers
-21.0cc dish
0 deck
I've always understood the GM gaskets to be .052 compressed thickness.
Now using the newer 5 layer mls gaskets with the 4.1 bore diameter I get:
9.59746390276784:1
http://www.csgnetwork.com/compcalc.html
Using what you gave us I got 9.616819240502426:1
4.03 bore
4 stroke
4.06 Head gasket bore diameter
.052 compressed thickness
65cc combustion chambers
-21.0cc dish
0 deck
I've always understood the GM gaskets to be .052 compressed thickness.
Now using the newer 5 layer mls gaskets with the 4.1 bore diameter I get:
9.59746390276784:1
