Calculating the static compression ratio on a stock 7MGTE

[JimR]

I'm going to calculate the stock compression ratio using numbers found in different threads, and a couple other sources.

So I've read a bit-o-this and a bit-o-that.......

Sources
Compression Ratio Explained: http://www.team-integra.net/sections/articles/showarticle.asp?ArticleID=233
Calculating displacement: http://www.gm.com/explore/education/9-12/technology/ice_volume.html
Head and piston volumes: http://www.supramania.com/forums/showthread.php?t=14760

Ok, here goes with my first simplified formula:
CR = (D + PV + G + CC) / (PV + G + CC)

CR = Compression Ratio
D = Displacement
PV = Piston Volume
G = Gasket Volume
CC = Combustion Chamber Volume

To solve for displacement we need:
Bore size = 83mm
Stroke = 91mm

83/2=41.5mm(The radius)
Volume of a cylinder = pi*radius*radius*height
3.1416*41.5*41.5*91 = 492,336 = 492cc (492cc*6=2952cc, check!)

To solve for gasket volume we need:
Gasket thickness = 1.37mm
Bore size = 84mm
Using an equivalent formula to the displacement one, we get:
Gasket volume = 0.785*84*84*1.37 = 7.6cc

Piston volume = 19cc
Combustion Chamber volume = 40cc

Back to the original simplified formula:
(492cc+19cc+7.6cc+40cc)/(19cc+7.6cc+40cc) = 8.387, which is pretty close to the 8.4:1 ratio that we already know is stock.

Now, it's the deck clearance volume that's messing me up... here's the complete formula:

CR=(D + PV + DC + G + CC) / (PV + DC + G + CC)

CR = Compression Ratio
D = Displacement
PV = Piston Volume
DC = Deck Clearance Volume
G = Gasket Volume
CC = Combustion Chamber Volume

First off, the "deck clearance volume" is the volume between the piston top and the top of the block at TDC. If we use a Deck Clearance of 0.508mm (20 thou), the resulting negative volume is 2.8cc's (84mm HG bore).

So,
(492cc+19cc-2.8cc+7.6cc+40cc)/(19cc-2.8cc+7.6cc+40cc) = 8.71:1 !!!

Obviously, that's off. So what's happening? Which numbers are wonky? Do I need more accurate measurements?

Jim

 

[Dunckel]

I got 8.79

 

[nosman4]

On a 7M you shouldn't have to figure out a deck clearance volume, because stock deck clearance is 0. So that would put you back to the 8.387 that you originally had. Am I right?

 

[JimR]

On a 7M you shouldn't have to figure out a deck clearance volume, because stock deck clearance is 0. So that would put you back to the 8.387 that you originally had. Am I right?

Not according to IJ. Apparently the pistons protrude 20 thou stock.

 

[kyle1jzsupra]

i got .................................green?

 

[nosman4]

Well, as far as I can tell mine is at 0 and that's the spec that both JE and Arias pistons have, so I don't know. I would think a piston manufacturer would have to know what the deck clearance was.

 

[JimR]

Maybe the block where the '20 thou' measurement was taken had been decked in the past and they didn't realize it? :dunno:

This is one case where I'd love to have my block at a point where I could check! But it's completely disassembled at the moment. :biglaugh:

 

[selfinfliction]

Maybe the block where the '20 thou' measurement was taken had been decked in the past and they didn't realize it? :dunno:

This is one case where I'd love to have my block at a point where I could check! But it's completely disassembled at the moment. :biglaugh:

if my main studs would ever get here, i would have the machinist measure mine for everyone. hopefully they will be in this week and if so, i'll post the numbers. i've got probes though, don't know how they differ from oem

 

[IJ.]

For the life of me I can't find that spec to quote.

If it was something I measured here it would have been on an uncut brand new block with Ross and JE pistons.

If I can find it I'll post here.