With a tsio-550 you have these interactions:

- decreasing rpm moves theta-pp back toward TDC

- decreasing MP if mixture were constant moves theta-pp away from TDC

- decreasing throttle from max reduces mixture

 

Do you have any idea of the magnitudes of these effects?

For magnitude, I can only refer to the “Landmarks” chart (see below).  It doesn’t directly address your question, though.

We’ve long suggested 200 RPM off takeoff power for noise reduction.  It’s not exactly good for the engine, but it won’t do any harm.  Better chance of harm of the TSIOs and TIOs, but still pretty benign.

Then George did some noise certification work for the FAA, which involved heavy instrumentation of the Ada airport and surroundings.  He got a lot of interesting data!  One item was with only 100 RPM off the top end you get 90% of the noise reduction you get with 200 off!

So full takeoff power from LIMITATIONs, reduced by 100 RPM for noise.  Very, very minor effect.  Now, if you’re talking big radials at 60” MP, and often operating in detonation, that’s another story!

It is also commonly thought that reducing MP to 31 inches and rpm to 2500 is easier on the engine.

So far as I can tell, this is NOT true.

You are correct!  NOT true.  Full power to cruise altitude, UNLESS there is a time limit on full power IN LIMITATIONS.