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Old 06-30-2016, 11:02 PM   #11
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lookforjoe's Avatar
Join Date: May 2008
Location: Rockland County, NY

Ref this thread for oil cooler t/stat housing oring PN - Mine needed 925093 square cut oring

Originally Posted by lookforjoe View Post
Waiting for the head work to be completed. Intake valves recessed another .010", exhaust valves recessed .040". Based on the calculations, that, along with the increase in piston volume above the ring land & larger flycuts should get me down from 10.7:1 to 10.3:1.
Doing the math on the revisons to calculate new static CR:

Using Steve C's (Xweb) Formula - bolded values are my revised numbers.

Volume at BDC is (swept volume - pop up volume)+ flycut and ring land clearance volume + gasket volume + chamber volume.

Volume at TDC is (volume in gasket - pop up volume) + flycut and ring land clearance volume + chamber volume.

Volume of BDC over Volume at TDC is expressed as a ratio to one (xx:1)

Your bore is 87.00
Your Pistons are 86.15 above the top ring land and the top ring groove is 8mm down from the crown.
Your Valve Flycut measured volume is 2.1cc
Your Piston sits at 0.65mm plus deck
Your Combustion Chamber measured volume is 36cc
Your head gasket is going to be 1.5mm compressed and has an inside fire ring diameter of 87.2mm (assuming a Felpro 21195-PT2)

So lets do some math...

Swept Volume of the cylinder.
Pi x R squared x Stroke
3.14285 x (43.5 x 43.5) x 67.4
3.14285 x 1892.25 x 67.4

Volume of the piston 'pop up'
3.14285 x (43.075 x 43.075) x 0.65
3.14285 x 1855.4556 x 0.65

Volume of the head gasket.
3.14285 x (43.6 x 43.6) x 1.50
3.14285 x 1900.96 x 1.50

Volume of the ring land clearance.
[3.14285 x (43.5 x 43.5) x 8] - [3.14285 x (43.075 x 43.075) x 8]
[3.14285 x 1892.25 x 8] - [3.14285 x 1855.4556 x8]
47.576cc - 46.651cc

and then put these numbers into the equation,

Volume at BDC = (400.831 - 3.790) + 2.1 + 0.925 + 8.961 + 36 = 445.027

Volume at TDC = (8.961 - 3.790) + 2.1 + 0.925 + 36
= 43.271

Static Compression: V at BDC / V at TDC (445.027 / 43.271) =


note: there is a chamfer at the top of the piston that is not accounted for - actual compression will actually be less. Not sure how much less. Need to see if I can get the specs on that from Wiseco, I never measured the depth of the chamfer and the diameter of the crown inside the chamfer...

My XR July2013 - 446AWHP @ 8K rpm

Last edited by lookforjoe; 07-13-2016 at 10:29 PM..
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