<<Cruising options are 24 squared or 2100
and ? MAP.  I am using 2100 and 25in.  It runs only 3~4 knots slower, AND
cooler, quieter, and burns less fuel than 2350 and 24in. Is this a problem?
Many people talk about running over square, but fixed pitch prop engines run
over square all the time.
Tom Williams>>

I'm sure George will weigh in on this, but just as a data point, I flew a
C-T182RG (parallel-valve, 8.5 compression, carbureted) for a while and the
POH showed 2100 and 25 inches as a "legal" 75% power cruising condition.
When running at that setting ROP because of the carburetor the engine would
occasionally emit a barely audible "knock" for maybe 6 consecutive firings
of one cylinder.  This would happen every few minutes, almost regularly.
Sort of like a burp and just enough to get my attention.  I asked a Lycoming
guy and he proposed that it was "abnormal combustion" caused by carbon
buildup burning away.  I'll buy that it was carbon buildup, but I think it
was actual detonation.  The inlet air temp of this engine would typically
run over 150F at altitude, way higher than a naturally aspirated engine.  I
can't image that in the IO-360 running LOP that 25/2100 could cause a
problem.  I don't see how that could be a normal operating point, though as
you would have to be cruising below 5,000 ft to be there.  Or is it a turbo
Arrow with the Continental engine?  Either way my answer would be the same -
2100, 25 inches or even higher, LOP, quiet, lower temperatures, what more
could one want?

Gary Casey