Compression Tests

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.

(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

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

You can conduct whatever you like - however your IA will require that a
leak down test be performed since it is prescribed under Part 43.



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.

On Thu, 25 Mar 2004 22:20:47 -0800, Pat Barry wrote:

You can conduct whatever you like - however your IA will require that a
leak down test be performed since it is prescribed under Part 43.

prove it.

TC

Pat Barry
shared these priceless pearls of wisdom:

-You can conduct whatever you like - however your IA will require that a
-leak down test be performed since it is prescribed under Part 43.

That happens to be bull****.

Jim


Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com