(Doug) wrote in message . com...
Does anyone do an "automotive" type compression test on their aircraft
engine? I think this kind of "dynamic" test might show problems (such
as valves not opening all the way), that a leakdown test would not
(and vice-versa). Anyway my annual is coming up and I want to do it. I
imagine something that screws in and has a long hose to the guage, so
no one has to stand behind the propellor.

The automotive-type compression test doesn't find valves that
aren't opening properly. It would take a high RPM to detect that.
The differential compression test used on aircraft engines is not
only safer, but any leaks can be pinpointed. For instance, exhaust
valve leaks can be heard at the exhaust stack. Intake valve leaks can
be heard at the carb or induction filter. Ring leakage can be heard at
the oil filler or crankcase breather. Cylinder leaks can be found with
soapy water around the cylinder and head. None of these are possible
with the automotive compression test, except perhaps the cylinder/head
crack test, and it won't be nearly as easy.
Further, with the differential test an exhaust or intake valve
leak can sometimes be corrected by tapping the respective rocker arm,
with the cylinder pressurized, to dislodge some bit of carbon or lead
deposit that's interfering with valve seating. Saves on unnecessary
cylinder removals.

Dan