Jack Brooks wrote :
> I had figured the 87mm pistons took the CR up to around 9:1,
> maybe a bit
> higher, but with the unchamfered head, I wonder just how high it
> really is.
Jack :
I unfortunately didn't save my chamber measurements, but if the compression
with 83mm liners was 8.5:1, changing to 87mm liners should bring it up to
9.25:1, if the head and gasket remain the same.
Compression ratio is swept volume plus unswept volume, all divided by
unswept volume. S = (bore/2) * (bore/2) * PI * stroke (for one cylinder).
83mm liners give S = (8.3/2) * (8.3/2) * 3.1415 * 9.2 (measurements in
centimeters, so result is cubic centimeters) = 497.76 cc
So, stock (S+U)/U = 8.5, and S = 497.76, so stock U must be 497.76/7.5 =
66.37 cc.
For 87mm, S = (8.7/2) * (8.7/2) * PI * 9.2 = 546.91 cc/cylinder, and
CR = (546.91 + 66.37) / 66.37 = 9.24
Randall
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