Thread: Cam Choices

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Here's the calculator for KB pistons that I have used many times to get close on a combination.....
http://kb-silvolite.com/calc.php?action=comp

Use the following values:
Cylinder head volume 67
Piston head volume 5
Gasket thickness 0.039
Gasket bore 4.100
Cylinder bore diameter 4.03
Deck clearance 0
Stroke 3.48
Rod length 5.7
Intake closing point 56 (the calculator uses the actual intake closing point @0.050" tappet lift plus 15 degrees, so add 15 to 41 on the Comp 274 cam and you enter 56 into the calculator. When I first began using this calculator, we settled on 8.00 to 8.25 to 1 DYNAMIC compression ratio as a figure to attain for pump gas. Since then, I have been advised that these figures are valid, but that the KB calculator is a little on the conservative side. So, I think if we shoot for 8.4 to 8.5, we'd be a little closer to reality with this particular calculator.

Plugging in the values, we get 10.046 static compression ratio and 8.536 dynamic compression ratio. This is using 67 cc heads, flat-top pistons with 5 cc's of valve relief, zero deck (piston crown even with the deck at TDC) and a Fel-Pro 1074 or similar gasket that compresses to 0.039".

You can play with the calculator and enter different values to find the results, but I would shoot for somewhere between 8.4 and 8.5 dynamic compression ratio no matter what combination you use.

The other thing I would advise is to shoot for 0.035" to 0.040" squish. I have shown this build with zero deck and a 0.039" gasket for a squish of 0.039". You can achieve the same results by juggling the deck and gasket thickness. For instance, you could have a deck height of 0.024" and a gasket of 0.016" for a squish of 0.040". Or you could have a deck height of 0.012" and a gasket thickness of 0.028" for a squish of 0.040" It doesn't matter how you get there, as long as you get there. Without this tight squish, you'll never get the motor to operate properly on pump gas.