GM claims that the factory LS1 has a 10.1 compression ratio. When I plug the numbers into my compression calculator I always wind up with 10.2 on the nose. Wondering if any of you might have some ideas why there is a discrepancy? Here are the numbers I am using to calculate compression ratio on a stock LS1 (346CI):

Bore: 3.898"
Stroke: 3.622"
Cylinders: 8
Rod Length: 6.098"
Deck Height: 9.240"
Head Chamber Volume: 66.67cc
Head Gasket Bore: 4.0"
Head Gasket Compressed Thickness: .0523"
Piston Compression Height: 1.3380"
Piston to Deck Clearance: -0.0070"
Volume Above Top Ring: .9370cc

I know a tenth of a point is not much but I'm curious why this is happening?

 

Piston to deck height could be wrong, It varies.

 

It shouldn't vary on a factory LS1 unless the block decks (or heads) have been milled and/or the pistons replaced with ones having a different compression height. From what I understand the factory piston extends out of the bore (into the combustion chamber) .0070", effectively reducing the combustion chamber volume by 1.37cc.

 

Some say -.005, some say -.006, some say -.007, some have said -.008. Whos right?

I measured mine so i could care less, I know what mine is but my block is not near stock. I have my factory block still though too, maybe Ill go measure it sometime this weekend. Ill report back later.

 

isn't the stock compressed gasket .054"?

 

Yeah, .005" to .008" is a very small amount...could be caused by a number of things. I'm guessing slightly out of spec factory tolerances could account for the difference as well as thermal cycling on a used motor.

 

that varies too. I have my old stocker gasket and its .050

 

I'm probably being too **** here but that tenth of a point is driving me crazy!

Compressed gasket thickness is certainly a variable too. If I plug in .050" I get 10.25, with a .045" gasket 10.38.

 

You wont get an accurate measurement unless you actually measure everything.

 

My understanding is that the factory tolerance on the deck height was 9.235-9.245.
Of course that only makes things more complicated.

I think the point is that the factory has a range of tolerances on most aspects, which means that if they wanted to be completely accurate with their stated compression ratio that it would be given as a range, not a single number.