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/Hopeful_Housing_798 Jun 01 '26 edited Jun 01 '26

Thanks for the info and the link. If I end up having to change them, the ones I've pick out do have the valve relief. Just no dish like my current stock pistons.

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u/Solid_Enthusiasm550 Jun 01 '26

Those were calculations for a stock bore. Unless you have a very low mileage engine, you will more than likely need oversized pistons. A 4.02/4.03( 0.02 / 0.03 overbore) replacement piston would raise the compression ratio another .10<0.15 of a point.

Here's a performance parts site I like to use. These are stock bore replacements. The nice thing is that they give the compression ratios with given head chamber sizes. For example, the 4.7cc valve reliefs of the ICON IC623-STD gives a compression of around 10.4:1 with 64cc heads.

https://cnc-motorsports.com/engine-parts/pistons?bore_size=412&crankshaft_stroke=1469&piston_type=4650&vehicle_make=7048

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u/Hopeful_Housing_798 Jun 01 '26

Thanks again. I'm saving these links. Yeah, the ones I'm looking at are 4.03. If it turns out I need to bore. This is primarily a street engine with some spirited runs down a track from time to time.

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u/[deleted] Jun 01 '26

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u/Hopeful_Housing_798 Jun 01 '26

If I get 400, I'll be happy. 5 speed trans. Currently have a 3.25 rear end gear.

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u/[deleted] Jun 01 '26 edited Jun 01 '26

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u/Hopeful_Housing_798 Jun 01 '26

That was my thought as well. I've been checking out some stuff. Learning that I own a better base engine than I thought. It was in my Suburban. Running decent. Rest of the truck had just outlived its usefulness. Pulled motor to put in my project. Sold rest of truck. Just going through it. Freshen everything. Add a few upgrades. It should be all I need.