At 04:26 PM 9/13/2007, Todd Last wrote...
>Does it matter if the throttle is held wide open or left closed?
>I recall some instructions which stated to have the throttle held open
>when checking compression.

Yes, it can make a big difference.

Ultimate compression is something greater than the intake pressure X
the compression ratio (greater because of adiabatic affects -
compression makes the air hotter, which makes the pressure even greater
than the CR would seem to indicate). If the throttle is closed, the
intake pressure will be less than atmospheric (a partial vacuum), and
therefore the indicated compression pressure will be less than you want
to measure.