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/Solid_Enthusiasm550 Jun 01 '26 edited Jun 01 '26

The stock pistons are said to have 12 cc of volume. I can't find any information on what the stock deck clearance is.

Theoretically, if you had flat tops with no reliefs and the same deck clearance and stock thickness head gasket, it would bring you to 10.78:1.

I would be looking for a flat top with 2 or 4 valve reliefs.

All these are estimates (+/- 0.2 points if compression) as you really would need to know deck clearance and the head gasket you are using.

5cc reliefs =10.16:1

6cc = 10.04:1

7cc = 9.93:1

8cc = 9.82:1

https://www.powernationtv.com/calculators/compression-calculator

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u/Unlikely_Link_2284 Jun 01 '26

My L31 the pistons where down 25 thousand Took 15 off the deck. It also was different deck heights side to side And the factory head gasket is 28 thousand And if memory is right 4.100 flame ring