I set my engine up LOP and flew it at 2350 RPM which is my normal cruise RPM
at that altitude. Then I set the same RPM 100 degrees ROP. Since this is a
fixed pitch prop and I was in level flight, horsepower was exactly the same.
Airspeed was the same. CHT was 10 - 15 degrees cooler and 25 below where I
usually see it. The engine was rougher but it was not vibration. It was
more like the difference between listening to the upright piano at the
elememtary school play and a Steinway, very noticable if you were focused on
the quality of the sound but not objectionable in the first case.

Although the engine was rougher LOP, when I listened closely, the sound had
a hard to describe quality of "easiness". Combustion actually has to start
while the piston is still compressing. LOP slows the combustion so that
there is less pressure against the piston as it is moving up. Peak
pressures occur at a more favorable point on the down stroke. Perhaps this
accounts for the way LOP sounds if you listen closely.

At anything above 60% power, I would go ROP with my minimal engine
instrumentation but this looks like a great thing to have in your bag of
tricks for hot days or need to maximize fuel reserves without slowing way
down.

We have been leaning aggressively on the ground and about 100 ROP in the
air. Our engine was opened up at 1030 hours due to lifter failure. There
was a normal but impressive amount of crud on the piston tops and exhaust
valves. Anything that reduces that has got to be good for the engine.

Walter can drill on my teeth anytime

--
Roger Long