A few days back I asked how my compression ratio would change when I switch
from my original 83mm pistons and liners to a new set at 87.2 mm.
I had many responses, thanks everyone for the input!
Here's what I learned:
a) The size of an engine eg. 1991 cc describes only the volume in the
cylinders that the pistons sweep through i.e. 1991 cc doesn't include the
space into which the gases get compressed in the cylinder head.
b) The compression ratio will go from 8.5 to around 9.25 in my case. If I
was concerned about the accuracy of this number I could measure the
thickness of the head gasket and the diameter of its piston hole and factor
that in.
c) With the new higher CR, I could indeed use premium gas, but it was
specified in the first place anyway. Alternatively I could retard the
timing a bit.
d) CR = (swept volume + compressed volume)/compressed volume
e) I should be sure to remove sharp edges in the combustion chamber to
reduce the chance of pre-detonation, pinging, pinking, or dieseling. Thanks
for this advice especially as I had read it long ago but needed the
reminder, and am glad to have received it while the head is off. Dremel time?
f) I could take the next step and measure the volume of each combustion
chamber to make sure they're all the same (especially after step e)).
Thanks, everyone.
Jim
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