On 22 Mar 2004 07:51:45 -0800, (Doug)
wrote:

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.

Yup. Useta do them all the time to R985AN14B's. Hangar neighbor had a
set of nine testers that resembled a stick-type tire pressure gage
that screwed directly into the spark plug holes.

Works best with a GPU that can be used for cranking, or a "fresh"
battery.

The 985's always either read 80/80 or 0-15/80 on a differential
compression check. If you had an inking of power problems/excessive
oil usage/etc. and 9 cylinders that were 80/80, it was about the only
tool available to get a better idea of what was going on.

Never took the opportunity to try it, but have an idea that it might
have pointed to the cylinders being fed by the "flat" cam lobes (with
perfect diff. compression) on the fixed-pitch Lycoming engines that
came into the shop with static rpm issues.

TC