Jeff said,

> "I don't think the manifold pressure gauge [vacuum gauge] is going to
tell you much other than your throttle setting for a given altitude."

It won't even do that if you fly wide open throttle - which if above 8K I
assume everyone does.

Steve Bennett had a chart that showed what information could be derived
from a vaccuum gauge. I just went looking for it but couldn't find it.
It's out there on the internet - behavior of the needle indicating
various possible engine problems. Steve once recommended I put one in so
I did . . . but I've never found the gauge useful at all. The way I fly,
unless I've just taken off and am climbing, it always reads zero.


Actually, a manifold pressure gauge will show absolute pressure, which changes 
with altitude even at full throttle. (30 in manifold pressure at sea level vs 
20 in manifold pressure at 10,000').  That allows you to calculate percent 
power.  FWIW, I am almost always flying above 8000' since the airport is at 
7200', but I rarely cruise at full throttle.

I suspect the initial question was asked based on the vacuum gauge 
interpretations on this web site: 
<>.  First off, the 
interpretations on that web site are pretty lose interpretations.  Secondly, 
the things they are interpreting via a vacuum gauge would be detected much 
earlier during any type of regular engine maintenance (differential compression 
test, mag drop test on run-up, and mag timing check).  The third point is that 
a manifold pressure gauge is usually installed with a restricted port, and a 
fairly long 1/8" copper line, so any of the vacuum interpretations from that 
web site would be pretty well buffered out of the manifold pressure gauge 
readings.  That is by design in order to prevent having a significant vacuum 
leak into the intake manifold should the line to the manifold pressure gauge 

-Jeff Scott
Los Alamos, NM


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