For the first few thousand feet above sea level you can subtract an inch and read the vacuum as manifold pressure but when the throttle is fully open, as it always is above 8000 or so, the vacuum gauge stays at zero. No useful information there. At lower altitudes yes, it can be used as a measure of power and can also be used to diagnose certain problems with the engine - Steve Bennett has a good chart on that subject, interpreting the behavior of a vacuum gauge.
As far as its usefulness as a diagnoistic tool, I'm guessing a MP gauge would act the same way and could be used as easily as a vacuum gauge, but don't know that. At the altitudes I fly a loss of MP while in cruising flight could indicate accumulating carb ice. That's one advantage of an MP gauge over what I've currently got. There are others - when playing with different props it's a way of "seeing" how much power they are using at a given RPM. Lots of ways to use the information if the gauge is giving you any. The vacuum only gives me useful information down low where I don't spend much time. Mike KSEE ____________________________________________________________ Slash Power Bill by 70%! Brilliant Trick To Save Up To 70% On Electric Bill In California http://thirdpartyoffers.juno.com/TGL3141/562e861ef08b161e481dst03vuc
