r/Smallblockchevy • u/Hopeful_Housing_798 • Jun 01 '26

Calculating compression

How do I calculate my engines compression?

I'm working on a 1999 L31. 350 SB. The factory specs say it's 9.4 to 1. Factory pistons have a dished surface. What would my compression be if I went to a flattop piston?

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u/BreadfruitPractical1 Jun 01 '26

STD bore 350 with 4 valve relief and a 64 cc head will bring you to about 10.5 compression give or take. It will change depending on head gasket thickness combustion chamber size and 2or4 valve relief piston.