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Just a quick note on differential pressure testers.
Essentially, a differential compression tester is designed to do the same
thing as a standard compression testor, but without cranking the engine. It
uses a compressor as the air source instead of the engine's pistons. As you
can imagine, standing next to an aircraft engine while cranking it can be
rather scary, with a huge prop swinging around just a few inches away.
A differential compression tester has the additional advantage, as
mentioned above, of letting you hear air leakage sources. Plus, if it is
used correctly, it may be a little more accurate than a standard
compression tester, since cranking speed may vary with the condition of the
starter, battery, and other factors, but the pressure of the air source can
be measured pretty accurately and maintained at a constant level.
However, there is one ideosyncrasy about a differential compression tester
worth considering. Its readings, in order to be completely accurate, should
be adjusted by some factor against engine displacement. This is because a
larger displacement engine, with its larger cylinder bore and larger
valves, has more ring and valve contact area where air can leak out of the
cylinder. With a differential compression tester, you are applying a known
pressure through a measured orifice. It will tend to leak out more quickly
with a larger displacement engine, giving misleadingly low readings. With a
standard compression tester, this tends to be self-compensating, since it
is using the engine itself as a compression source. That said, this is only
a problem if you don't have any manufacturer's specs to go on, or if you
don't know what the reading should be for a "known-good" engine.
With either kind of tester, loosen the plugs about one turn, run the engine
for a minute or so, then remove them and crank the engine again before
testing. This will help avoid getting debris in the combustion chamber that
may hold valves open and give false readings.
If you have a couple of pressure gauges lying about, you can make your own
differential pressure tester. You just have to drill an accurate orifice to
allow a known volume of air to pass at a given pressure. I saw a site on
the web a while back that described how to build one, but didn't bookmark
it.
Alan
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