Milling your cylinder head is a cheap way to raise compression, much cheaper than swapping out pistons. But then why not do this? The main reason is that when you mill the head, you are putting slack in your timing belt because essentially the head is getting "shorter". Needless to say this is a bad thing. Now I know many people that have milled their heads 30 thousandths and are fine...and 30 thousandths is a lot. Expect this to raise your compression approximately 1 whole point. So if you're at 10.2:1, you'll be at 11.2:1. This is just and approximation. Here's the formula you need to calculate true CR....




CCV = combustion chamber volume

CV = cylinder volume

MHV = milled head volume

CR = ( CCV + CV ) / CCV



To calculate CV (cylinder volume)

CV = (stroke) x (bore)^2 x (Pi/4)

CV = 7.7 cm x 8.1 cm^2 x .7853981633974

CV = 396.8 cc



To calculate CCV (combustion chamber volume)

Rearranging the equation CR = (CCV + CV) / CCV

CCV = CV / (CR - 1)

CCV = 396.8 cc / (10.2 - 1) = 43.1 cc



To calculate MHV (milled head volume)

(amount head was milled) x (bore)^2 x (Pi/4)

.030" mill off the head = .076cm

MHV = (0.076 cm) x (8.1 cm)^2 x .7853981633974 = 3.92 cc.



To calculate new CCV with milled head

CCV = CCV = 43.1 cc - 3.92 cc = 39.2 cc



To calculate new CR with milled head

CR = ( CCV + CV ) / CCV

CR = (39.2 cc + 396.8 cc) / 39.2 cc

CR = 11.12:1