1998 Corvette, stock LS1 GM advertised 10:1 mechanical compression ratio.

Now let's do the math (I used a on line calculator):

• Bore: 3.898
• Stroke 3.622
• Head Gasket comp thickness .051
• Deck Height -.007
• Piston top volume 0.00
• Combustion Chamber 66.67

I get 10.4:1 when I enter the data.....you can figure the head gasket bore, top ring down (.250), etc. but I am not getting much difference, the best I could get on the low side is 10.25:1....thoughts? What have I overlooked?

 

come on there is gotta be a smart one out there somewhere...any thoughts?

 

i got 10.75:1

 

would valves have anything to do wit it? different shaped valves would differ in head cc. i always wondered if filling in dished valves would help compression. but increase valve float greatly.

also are the stock pistons chamfered around the edges? i cant recall

 

static compression ratio is defined as volume when piston is at bottom dead center divided by volume when piston is at top dead center,
where people go wrong, and i wouldn't call it wrong actually it's all a matter of how accurate you want to be, is how you geometrically calculate the volume.
It is not just volume of a cylinder ( pi*r^2*h) because the total shape at either BDC or TDC isn't a true cylinder. above you accounted for head gasket thickness but do not have head gasket bore diameter which is around 0.030" extra than the bore diameter, so there you have another cylindrical shape that you need to account for which will lower the CR number. And from there if you want to be 100% accurate you start accounting for things like piston shapes and the volume between the top top piston ring to the piston crown to the cylinder wall.

i use this CR calculator, http://www.csgnetwork.com/compcalc.html
using 4.198" for head gasket bore diameter, it says 10.21 CR

http://www.zealautowerks.com/
this one has much more, block deck height for the LS1 is 9.24", the rod length is 6.098" and piston compression height is 1.34" this shoots out a static CR of 10.45 for your numbers,
in the end you usually end up chasing your tail with published numbers, when i looked up rod length and deck height i came across 67.3 cc as combustion chamber volume and also 0.052" for gasket compressed thickness,
so it's hard to really know what numbers were used by who to come up with whatever the published CR number was,
I generally see the bore number as 3.90" not 3.898", so which is correct?
in fact everything is in metric now, which i'd bet are the more accurate numbers for bore and stroke, so when converted to inches there's probably round off error which then prevents your calculated CR number from matching the published one.


http://www.gmhightechperformance.com...ion_ratio.html