Calculation for Compression Ratio

rbrown

Registered User
What is the calculation for CR?

If I shave the head I want to know how much will affect the ratio. I know a little has little effect but at what point does it start to change.
 
When you shave the heads you decrease the combustion chamber volume so that is the number that changes.

Not sure if anyone has documented the changes at various head shaving points.
 
Simply put as the volume of the cylinder at BDC divided by the volume of the cylinder at TDC.

For a clean-up job, the compression gain is negligible. My heads only needed about .004"-.005" clean up. The shape of the chamber determines the effect of compression whenever milling the heads. I have cut as much as .170" off of heads (not SC) only to gain 1 point of compression. I had to reshape the combustion chamber, though.
 
Simply put as the volume of the cylinder at BDC divided by the volume of the cylinder at TDC.

For a clean-up job, the compression gain is negligible. My heads only needed about .004"-.005" clean up. The shape of the chamber determines the effect of compression whenever milling the heads. I have cut as much as .170" off of heads (not SC) only to gain 1 point of compression. I had to reshape the combustion chamber, though.

Yeah this is on my little 1.6 with 7.9 CR. There is a bunch of material on the head that can be removed I am told. I want to figure if I can realistically get to 8.2-8.5 to help low speed driveability or if I am simply barking up the wrong tree.
 
That online calculator looks pretty good. Just as a test, I tried putting in a negative number for head gasket thickness, and it seemed to work. So you could use that field to enter the amount of shave ... simply deduct the amount of shave you are considering from the standard head gasket thickness, and put it in, even if it is a negative number.

I don't know what type of engine you are working with here, but one way to raise the CR significantly is a stroker crank.
 
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