CR change by shaving heads
 

Anyone have a formula(s) for calculating what you'll get for a new compresison ratio by knowing original compression ratio and motor size and shaving some value of heads?

Stroke volume= Vs
Combustion volume = Vc

CR (compression ratio) = (Vs+Vc)/Vc (if you know the CR you can determine the Vc)

Shaved height= H
delta Volume trough shaving = Vd= (0.25**D²)*H
=3.14 and D is cylinder diameter

New Cr = (Vs+(Vc-Vd)/(Vc-Vd)

hope this will help (and this is correct...)

Yes that make sense... I was forgetting Vc in the numerator of CR in what I was doing. Below is example for 993 for shaving 0.010" to 0.020" from heads. Frankly I was surprised at how much CR went up.

http://forums.pelicanparts.com/uploa...1261574450.jpg

yes, the effect goes up exponential with the CR. I was doing this calculation on a 3.3 turbo with a CR of 7.5 and there the effect is not that big.

And then there is valve to piston clearance and deck heighth checks...