How to: calculate compression ratio

[240KAT]

I know how to do a compression test, but my next project is going to be all motor w/ some serious compression. I know one can buy high compression pistons to, say, 11:1, but I'd like to run 12-12.5:1 w/ individual throttle bodies. So i imagine some cylinder head shaving is in order. How do i figure out how much to shave in order to get desired compression, and, how do i figure out my existing ratio? Thanks.

[masterbunta]

bottom dead center = BDC
top dead center = TDC

Non-flattop pistons:
volume TDC = Vtdc
This is the volume when the piston is at TDC.
volume BDC = Vbdc
This is the volume when the piston is at BDC.
Compression ratio = r
r = Vbdc/Vtdc

Flattop pistons:
clearence volume = Vc
Volume of combustion chamber.
displacement volume = Vd
(pi*(bore/2)^2)*stroke
compression ration = r
r = (Vd+Vc)/Vc

[deviousKA]

Shaving your cylinder head is not the proper way to raise compression ratio. What it essentially does is lower the combustion chamber volume, which is a factor of compression ratio (nicely listed above). The downsides of this outweigh the gains. This puts more slack in your timing chain/belt, retarding timing and causing unnecessary problems. On both sohc/dohc ka's, you can shave very little anyways before you have line up problems (also look at the intake/exhaust studs, very close to the deck). You will not be able to obtain 12:1 this way, even if starting with ka24e piston swap (11.1:1). I assume you are using ka24de? If you are looking for that compression ratio, you can either look into decreasing combustion chamber volume (welding in more quench design/cloverleafing). Or you can go with some taller or dome type pistons with flycut valve reliefs, and yes you will flycuts.

I assume you are planning to use ka24de, similar situation in both ka24e, and any other engine.