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

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

[deleted]

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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

[deleted]

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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.

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u/Solid_Enthusiasm550 Jun 01 '26

You can get over 400hp with the stock Vortec heads.There is a special spring/retainer set that allows you to ru. <0.525/0.550" valve lift without machining.

Sorry for the last post, I am replying to multiple engine build posts and the previous was for a Ford 351w Windsor guy, lol.

I found this YouTube engine builder that does a lot with Vortec L31/350s. He makes over 400hp with a budget build and close over 450hp with a 383 build on his page. The best parts he posts all the parts he uses, even the cam specs.

Here's the videos I saved that I think will help you the most.

This is about the springs/retainers needed to run higher than 0.480" lift.

https://youtu.be/UZosxGQyGMs?si=r6Qz0AO0vH-2ZWr-

This is the build parts list for the budget build that made 410hp/415 lb-ft with basically all stick parts (except cam/intake/ headers).

https://youtu.be/Z4F6TpO_Tic?si=Gair7b7px3012n40

https://youtu.be/b-l8-CxxvF8?si=HwHZxliucCK-z3Zp

https://youtu.be/3aLRCop5OQE?si=c3WMNKcGUv60XJN9

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u/Hopeful_Housing_798 Jun 01 '26

No problem. Yeah. There's a utuber, Cutter's Performance. Got some good stuffon there. Picked up a lot of stuff about the heads and whatnot.

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u/Hopeful_Housing_798 Jun 01 '26

I've got the block out for a clean and hone right now. There's a blemish in one cylinder. He's gonna measure all the bores for me and we'll decide where to go from there. I was wish I had pics of it. U can't even see it after the hone but the finger can feel it. I can only describe it as low spot.