Are high MAP and low RPM settings safe?

[Camillo]

I finished my RV-9A nearly three years ago.
Since then I fly mainly @3.000 feet @ 21 MAP and 2.350 RPM with 120 knots indicated and 25/26 liters per hour fuel burn.
I like the low noise 2.350 RPM and don't like to see 35 liters on fuel flow.
I am also slightly disappointed to be 20+ knots away from Van's published speed.

So, now that I am building the -4 I looked again at the engine/propeller selection section and I found something I already knew but I didn't focused on. Van's says that the main advantage of a C/S propeller is selecting high MAP and low RPM, thus saving fuel and reducing engine wear, the added value of a C/S being the possibility to achieve less than 75% power at altitudes lower than 8000 feet wihout reducing MAP.

Now, I tried to have full MAP (28+) and reducing RPM but I am a bit afraid to go under 2300 RPM. There is an old story about non going with RPM below the MAP (if 25 then not below 2500; if 21 not below 2100, etc...).

Before attempting the ultimate test (28 MAP and 2200 or less RPM) to achieve the best compromise, is this procedure harmful fon the engine?

Thanks.
Camillo

[N427EF]

Couple of things:

Vans numbers are for 75% power at 7500 feet.

Since then I fly mainly @3.000 feet @ 21 MAP and 2.350 RPM with 120 knots indicated

Your setting is nowhere near 75% power and at 3000 feet your are not at optimum altitude for efficiency. Try the published numbers in your airplane and I know you'll be within a mile or two of published numbers.
Take into account some of the airspeed and pitot/static installation errors.

There is an old story about non going with RPM below the MAP (if 25 then not below 2500; if 21 not below 2100, etc...).

You are correct, it is an old story and in my book an old wife's tale.
The issue however has not been settled and probably never will, think primer war.

Flying over square is not harmful to my engine but to each his own.

[AK4x4]

I would encourage you to acquire a copy of the Lycoming O-320 Operators Manual. The part number is 60297-30, and can be purchased from AERO in stock or downloaded from various web sites. Loads of power settings both high and low are just fine including many that are "over square" and if you use the ones laid out in the operators manual the engine will live a long happy life. Good luck, Russ

[jetdriven]

The O-360 operators manual lists power settings as much as 8" over square, but also, take into account that these are ROP power settings and cylinder pressures. If you fly it LOP, the peak cylinder pressure is less, so to the cylinder it is like running lower MP. Below about 50% power, MP and RPM are meaningless, there is no setting that can harm it. Set power with FF and limit to 75% anyways. I leave the throttle firewalled and only have to back off for cruise below 1500', or if trying to get less than 50% power.

The exception being a counterweighted crank spun at 1800 RPM or less might cause issues.

[DanH]

Originally Posted by AK4x4

I would encourage to acquire a copy of the Lycoming O-320 Operators Manual.

+1 Look at the "Limiting Manifold Pressure" line on the power chart; it is MP vs RPM at best power mixture, meaning the 100-200 ROP ballpark.

The limiting factor appears to be detonation. Here's an example from a Lycoming detonation survey, an IO360 angle valve running 28.6/2400, at maximum temperatures per the certification standards. As most would expect, the worst detonation is around 50 ROP, but the zone runs from roughly 100 ROP to 100 LOP.



Note the test was run at 20 degrees ignition advance. More advance, more compression, lower octane, or a combustion chamber with less squish or turbulence would all reduce the detonation margin. Lower CHT and/or intake temperatures would increase the margin.

[Russ McCutcheon]

Also make sure you know and follow what if any power settings your propeller manufacture restricts with your engine/prop combination.

[Darren S]

Check out Mike Busch's latest webinar on flying economical in a $7 avgas world. It addresses the issue. Very good.

[rvbuilder2002]

Originally Posted by Russ McCutcheon

Also make sure you know and follow what if any power settings your propeller manufacture restricts with your engine/prop combination.

I agree with everything posted so far. I quoted Russ's post because it is an important one. Some of the metal props have specific restrictions on some high power / low RPM ranges (but I am not aware of any for the O-320).

[Camillo]

Thanks. The O-320 and the Hartzell C/S propeller, matched together, don't have RPM limitations.

I should have said also that to achieve Van's numbers I had to increase MAP. And I did. See photo below. I was conservative with RPMs. That's why I wrote.

[Camillo]

Originally Posted by Darren S

Check out Mike Busch's latest webinar on flying economical in a $7 avgas world. It addresses the issue. Very good.

Thank-you, Darren. Not on-line, yet, the article... Keep looking at it.