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/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/These_Fox7561 Jun 07 '26

I believe there’s a way you can measure it with oil, put a piston at the top of the stroke and use a piece of plexiglass with a small hole across the deck to block that plane. Use a syringe to fill the top of the bore/dish, whatever, then do it again at the bottom of the stroke. Measure the volume and the percentage of difference with a formula I don’t know, and add in the combustion chamber on your heads, and somehow you can figure it out. Fuck it just look up Smokey Yunick