Today was an easy day. A good day for dusting off a few cobwebs, brought
about by several notes regarding 'how much to take from a cylinder
head', in thousandths of an inch, when one wants to change the
compression. KK's book, good as it is, doesn't show how he arrived at
his numbers. Don't let the following below throw you. It's easier than
it looks, if you have a simple calculator.
The formula for determining displacement ratio is always 1.0 less than
the compression ratio. i.e. if your compression ratio is known to be
7.75: 1 the displacement ratio will be 6.75: 1
If you want to raise it (the compression) to 9.5:1 this displacement
will be 8.5: 1
So, the amt. to mill from a head to raise its compression, as taken from
the 'Auto Math Handbook' with the substituted TR numbers is as written
below:
8.5 - 6.75 X 3.74 / 8.5 X 6.75 =
1.75 / 57.4 X 3.74 = .114
8.5 is the no. which represents the 9.5 'new' compression ratio.
Subtract 6.75, the 'old' C.R. (for the '73 TR) or 6.50 if you have the
later TR
Multiply this by 3.74 (the crank stroke)
Divide this by 8.5 X 6.75 = Your answer, so far, should be -11.5475
Then punch in 1.75 (which is the difference between the old and 'new'
C.R.) divided by 57.4 X 3.74 = .114
.114 would be the amt. to be milled from a stock head, rated at 7.75: 1
to have the compression raised to 9.5: 1.
If your head has a different startng ratio, or you want to achieve a
different final C.R. change either the first or the second no. in the
formula to the one you want. Remember to subtract one whole number from
your C.R. to fit the formula number.
If you haven't gone to sleep yet, here's more of the same, this time for
an 'older' TR with an 8.5 C.R. who wants to go to a 10 C.R.
9 - 7.5 / 9 X 7.5 X 3.74 = 1.50 / 67.5 X 3.74 = .083 Easier than
reading an Ohmeter! 8 ^(=)
Now I'M going to sleep!
Dick T.
'73
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