how do I convert x psi to n:1?
The short answer is you don't. Test each cylinder, see what the range of
values might be, and hope that there isn't much more than a 10% to 12% spread.
Or 15%, or maybe 20%, depending on what you plan for the thing.
Basically, the pressure reading on the guage is dependant on several variables
that affect how the number you get relates to the compression ratio. When you
crank over the motor and start getting the puffs into the guage, you are
heating the air as it gets compressed. And the resulting pressure of the air
depends on the temperature of the ambient air, the temperature of the engine,
the humidity, the sealing of the piston rings, the speed at which the motor is
getting cranked, whether oil is draining or getting added to the cylinder under
test, the cam lobe profile, the state of the intake system gaskets, whether
or not the butterflys of the carb are open, and so on.
If you have a constant depression (vacuum) carb like those SUs and
Stromberg CDs found on various British cars or American hot rods fitted with a
Predator, the pressure also is dependant on the where the air valve is in
relation to being closed and restricting, or open and less restricting.
There is a formula that does come close, involving a number of terms, a few
powers of this and that, a logarithm or two, but it is still only an
approximation. And of course, I can't dig it up at the moment, it was around
here somewhere...
The only way to know the real compression involves measuring the internals of
the motor.
mjb.
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