Low MAP/RPM efficiency

Hi,
I am looking for some input from the engine gurus out there. In an other thread, I mentioned my participation to a contest in which I will not be allowed to exceed 120kts.
Thus I am thinking of doing this trying to have the lower fuel consumption I can have.

The settings I am currently using for training is 14.6"/2400RPM 20°LOP resulting in a 4.25GPH fuel burn.

I think I can get even better reducing my RPM but can't get any info on how far I can go. I read about the over squaring rule ( do not set over RPM/100+4 for MAP). In my case, it would mean that running at 1100RPM is still fine...
Not sure this rule apply at low power setting.

I think decreasing below 1800RPM is not useful.
What are your thoughts about that ? I would also prefer to run the engine in safe way as I will fly approx 20 to 30hours with these settings.

Note : Engine IO-375M1S, dual LSE, all the cylinders are peaking within 5F.

Hi Bastien,

There are probably many opinions and little hard data in this area. My inclination would be to definitely reduce to rpm to 2000 - no problem running there all day long, you will be slightly more efficient - will be interesting to find out how much more. I have never run for long at 1800. I always though the over square rule was (rpm/100) +5, so max MP for 2000rpm would be 25".

Pete

My biggest concern would be that running at such a low power setting for so long, the engine might run so cool that deposits would accumulate on the exhaust valves. Is such a worry a real concern?

First of all, lets kill the OWT of over square as it is nothing more than that. We could bash on for pages about it but just accept for now that is not an issue. You could just as easily run at 1800 RPM, which I do some days on descent to keep the power output down.

When on the Lean side of peak EGT, power is determined by fuel flow, so the mixture knob is your power lever.

BSFC minimum is at such low powers around 10dF LOP, so find yourself a power setting that at around 1800-2000 RPM gives you smooth operation, adjust the MP back to where you get say 125kts and I assume this is all IAS, then lean by doing a BMP, or take it to a bit more than 10dF LOP, and this is to cool off the engine so volumetric efficiency improves, then find peak from the lean side, then set about 10 (average) so some may be just under peak some a bit more, near enough will do.

Now you have a cool and thus volumetrically efficient engine running at the best BSFC you can.

Now to adjust airspeed you will only need very minor changes in MP or RPM, to achieve your target, and as the Fuel servo is a mass airflow device, and sort of linear in its operation minor changes will be made while keeping the F/A ratio's the same.

The engine will not run too cool or cause you any trouble. It may be that temps are higher than you expect due to cooling airflow being low and AoA effects.