One couple of other points on this -
Boring the cylinders can change CR by a couple tenths of a point easily
so how significant a change it is depends on how close to detonation you
are at present.
A valve job will decrease CR as it causes the valves to sit deeper in
the head (assuming you're not putting new seats in, just regrinding
them). Normally a valve job won't make a significant change but on my
U20 the #4 intake valve sits 0.050 deeper than the other three with
accounts for a 2cc chamber volume difference - that's about 0.1 less
compression on my engine.
Another note on the valve job is that, if it's done carefully, grinding
the seats can be used (to a limited degree) to raise the valve stem
height which can be used to change camshaft lobe wipe patterns on the
rocker arms if cam tower shims have been used or if a reground cam has
been used. Obviously this it limited and trades some CR for improved
cam / rocker wipe pattern.
You start to figure this crap out if you do your own cylinder head work.
Brian '69 2000
Tampa, FL
web.Tampabay.rr.com/oilleak
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