Pete :
Depends on your definition of 'linear'. If you're thinking "well, if I
shave .050" off and that raises the CR by 1.5 points, shaving .100" should
raise it by 3.0 points.", that is _not_ true. But, if you just mean "If I
shave .050" off that reduces the unswept volume by 5cc, so shaving .100"
will reduce it by 10cc", that's sorta true for a stock TR6 head. As someone
already posted, it depends on how vertical the combustion chamber walls are
in the area you are removing. A head like the late TR4 with a very tapered
wall will be very non-linear.
CR is (swept volume + unswept volume) / (unswept volume). You pretty much
have to estimate or measure the change in unswept volume and work the
equation to know what the change in CR will be. Swept volume is easy, it's
bore x bore x stroke x .785398. For results in cc, use inputs in
centimeters, for results in ci, use inputs in inches.
Randall
> That all seems convincing enough, but the trouble is I don't know if
> the relationship between head thickness and compression ratio is
> linear. If it's not, then I don't think my numbers are accurate.
> Despite all that figuring, I really and truly belong in the home for
> the mathematically challenged, so I can't guess as to whether that
> relationship should or should not be linear. Anybody know if you can
> extrapolate from known specs to get thicknesses for a given
> compression ratio in this way? Assuming my arithmetic was correct
> all those years ago, should my 11.8:1 figure be correct?
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