i am installing a 350/330hp crate motor in my iror.. it came with vortec 64cc heads.. the campression ratio of 9:1.1. i need to know what the compression ratio will be with a set of 76cc heads with 2.02 valves.. any help will be appreciated thanks Jason

 

Bore = 4.00 inches = 10.16 cm = 81.07 cm^2 area.
Stroke = 3.48 inches = 8.84 cm
Swept volume = 716.6 cm^3

Compression ratio = volume at BDC divided by volume at TDC.

Volume at BDC = Static volume + swept volume
Volume at TDC = Static volume

Static volume = combustion chamber volume + gasket volume + deck height volume + piston relief volume.

Since gasket volume, deck height volume, and piston relief volume are all unknown, we must use algebra to dtermine the total volume of these three variables, based on the known compression ratio with known cylinder head volume. Once we know, we can hold them constant to calculate the modified compression ratio with the new cylinder head volume.

9.1 = (716.6cc + 64cc + X) / (64cc + X)

9.1(64cc+X) - (64cc+X) = 716.6cc

8.1(64cc + X) = 716.6cc
8.1X = 716.6cc - 8.1(64cc)
X= (198.2cc)/8.1
X=24.5 cc

Again, X represents the volume of the head gasket, deck height, and piston. If we figure 9.1cc for the head gasket, and 7.4 cc for the deck height, the piston must displace about 8 cc. This is probably a wide relief, or perhaps even a dished piston?

Anyway, Let us assume you hold this constant, and you use the same type of head gasket.

Figuring in the new combustion chamber, what is out new compression ratio?

(716.6cc + 76cc + 24.5cc)/ (76cc+24.5cc) = 817.1cc / 100.5cc = 8.1:1 compression.

You'll have a nice low-compression blower motor.

What you need to do is somehow remove 12cc from the engine. You can remove 6cc by installing a .015" steel head gasket, and mill the block to a zero deck height for another 7cc. This will get you back to a net compression ratio of 9.1:1. If you overbore the engine, the swept volume will increase, and this will also increase compression ratio.

Best regards,


------------------
Daniel Burk
http://www.isthq.com/~dan/fcar.html
'84 Trans Am WS6/L69
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yay! algebra