Let me share this with you !
I set out to discover the compression ratio of my 1275 engine by measurement
so that I could decide by how much to skim the head to raise the compression a
bit.
Started with a standard engine, though re-bored to +0.030
The plasticine idea for measuring the cylinder head capacity works well and is
easy to do. I reckon it gives an accurate measurement to within at least
0.25cc
Then did all the sums for the extra capacity in the piston tops, gasket
thickness, etc and to my surprise the ratio works out at 10.5 : 1 Certainly
as high as I would want to go, possibly even a bit too high. I may need to
enlarge the cylinder head capacity a bit just to get it down. I had planned
on 9.5 or 10.0 at most.
I was puzzled by this as standard should be 8.8 : 1 I have "cleaned up" the
head chambersa bit, which if anything should reduce the ratio. After some
thought and checking I realised that the reason it is so high is that the
pistons are coming right to the top of the block at TDC, or at least within
0.007 of it. I think that in a standard engine they would be perhaps 0.050
down at TDC.
One of two things has caused this. Either the new pistons are taller between
the gudgeon pin and crown (unlikely) , or, and I think this is it, I had the
crank journals line bored, and this has effectively raised the center line of
the crank nearer to the top of the block.
Anyone "got the T shirt" who can confirm this ? Also with the pistons that
close to the top of the block am I likely to get problems ?
Guy
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