At 07:50 PM 9/18/2010, Don Hanson wrote...
>how does a fellow or a gal determine the compression ratio of a motor?
>

Measurement and mathematics. You can usually find the bore x stroke
pretty easily. Then you only need to measure the head volume at TDC.
That's often done by pouring a light oil in through the spark plug hole
(but that depends on the layout, won't work for a plug on the side or
bottom). If you start with a known qty then fill the cylinder, how much
you have left will let know how much you put in. From there, it's just
calculating.

>  I've been thinking to take a reading with my compression
> tester,  then divide that by the barometric pressure.  Is that
> correct?

That won't even get you close. First, the compression is mostly
adiabatic, the pressure rise increases temperature, which in turn makes
the pressure rise further (this is how a diesel works). The pressure is
effected by valve timing and cylinder/piston/ring wear. It's effected
by cranking speed. Basically, there are too many variables other than
compression ratio.