Thread: pump gas

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Everything I've read says 8.0 to 8.25 dynamic for pump gas. In reading threads on other forums, I've come to believe that the calculator I've been using (KB pistons) is a little conservative. I still believe the 8.0 to 8.25 is valid, depending on the calculator used, but I tend to favor 8.25 to 8.50 on the KB calculator. Of course any engine build that is optimizing the DCR should also include de-burring of the chamber to prevent hot spots and proper squish, which on a SBC would ideally be set at 0.035" to 0.040". Big blocks should use a little more because the bore is bigger and the piston has a better chance to present the edge of the piston above the deck as it rocks on the pin. Same with forged pistons in a small block. You'll use maybe 0.005" to 0.007" piston to wall clearance, so the piston is looser in the bore and has a better chance to present the edge of the piston above the deck. Here's the KB calculator if you want to play. Determine the intake closing point of the cam you're going to use and add 15 degrees to that figure for entry into the calculator. (ex: a cam with a closing point of 41 degrees after bottom dead center would be entered into the calculator as 56 degrees).

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

www.summitracing.com has a very complete list of head gaskets for you to find the gasket bore and compressed thickness. A common gasket used on the 350 with a zero deck would be one such as the Fel-Pro 1010, 4.166" bore, 0.039" compressed. If you don't want to cut the deck to zero and the piston in a little ways down in the bore, use a thinner gasket to reach your correct squish figure. For instance, Chevy makes a 4.100" by 0.026" shim gasket under part number 3830711 and Fel-Pro makes a steel shim gasket at 4.100" by 0.016" under part number 7733SH1. Cometic makes a custom gasket in any thickness starting at 0.027" on up and any bore size.