Can you (or anyone else) explain the mechanism of this? Why a full ruch
taxi would affect compression? I routinely lean the mixture for idle
and taxi because I've learned that at my home airport (KORL) the plugs
can foul between the ram and the run up area. But I don't understand
(and I'd like to be educated) how it affects compression.

John Stevens
PP-ASEL


Not exactly the aircraft perspective you are looking for, BUT in recip race
motors
for sports cars, you do hot cuts if you want to read plugs or do compression
checks. This means you rev the motor to 4 or 5 grand, stabilize it and shut
off the ignition.
We find differences in the numbers we get just from excess fuel in the
cylinders with
idle cuts. Running at idle is a rich environment and the small washdown you
get can
make the numbers get funny on leakdowns. Another thing to consider is that
in
the trucking industry, idling loads the top compression ring gap with
sootlike
deposits that hinder compression sealing. It is something oil companys are
VERY
concerned about. So you now have 2 data points from very different places
that show you
that idling loads up the rings and effects the compression readings. YMMV

--
Have a great day

Scott