Manifold Pressure and RPM

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Skylarktc

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Shohola, PA
I was always taught that it is bad form to run your manifold pressure higher then your RPM. When I practice my aerobatics I generally use 25 squared or 24 squared and leave it alone. I recently observed a friend running the prop at 2350 and then sometimes depending on the maneuver add full power for a short amount of time. Is this an acceptable technique? Does anyone else do this? Is it bad for the engine?
 
I always did the same. 25 squared or 24 squared until the engine wouldn’t do it anymore. Then take whatever I could get as altitude increased. But leave the RPMs alone. Til in the pattern to land.
 
I guess I didn’t explain myself well. I mean if you are flying low enough that your engine still produces enough power to exceed Rpm. For example running 25 inches of manifold pressure with an RPM setting of 2350. Would this be harmful to the engine even if it was done for short intervals?
 
I transitioned to jets & turboprops. And that all went away. 😞 I’m currently flying a C140. Had a Citabria for a few years too. Constant speed props & turbocharged engines are not in my bailiwick….
 
Skylarktc said:
I guess I didn’t explain myself well. I mean if you are flying low enough that your engine still produces enough power to exceed Rpm. For example running 25 inches of manifold pressure with an RPM setting of 2350. Would this be harmful to the engine even if it was done for short intervals?
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It’s my understanding, from an engine standpoint, that 2500/100 = 25 being similar to 25” is just a coincidence. TIO-550K engines in the Cirrus run 2500 RPM regardless of MAP and takeoff at 36.5” MAP. You have no control of the RPM other than at low enough MAP that the prop governor hits the stop and then it functions like a fixed pitch prop (think final) which is true on most single engine piston constant speed props. At high MAP and low RPM you are maybe putting additional stress on the prop governor and or crank, but in our non turbo engines the worst case would be 25” and 2200-2300 RPM which feels significantly less stressful than 36.5” at 2500RPM.
 
We had turbo normalized engines at Beechcraft that would operate that way. I think that they were IO 550s too. They had a wastegate that would keep the engine from producing too much power down low. If I remember they would produce 25 or so inches up to altitude. Regardless of were prop rpms were.
 
I’m thinking of the Seneca 39.5 inches without wastegate protection. I had to manually adjust the power levers to keep it from over boosting. But the rpms weren’t effected.
 
I feel like a dinosaur we had 3 levers in the single to control throttle, prop, and mixture. Six in the twins. We would have wastegate controls to keep from over boosting the engine too. So you could go to full throttle. RPMs were controlled by a separate lever I think that they were around 2700 rpms at full throttle. We would bring em back to 2500 for climb….
 
I don’t know where the myth of running over square is bad. Some dumb instructor somewhere with a whopping 15 hrs in a complex airplane probably.

Running over-square is completely acceptable, and wont hurt your engine. Lycoming and Continental both have power tables for it. As well as Cessna, Mooney and Piper in their POH’s. Those are the ones I flew.

You’ll have the most efficiency with the max MP at the lowest RPMs. This will give the most air to fill the cylinder, and the slower speed will reduce rotational friction, and allow more time for a more complete combustion.

My Mooney was run with the throttle at full in until I needed to slow for approach, prop turning 2200 rpms.

My 180, I’m running 24-25” and 2250-2400 rpms, and any combo in between, depending on what kind of drag I’m pulling (floats or big tires) or what speed or fuel endurance I’m looking for.

I don’t do acro, but I’m guessing if you ask the pro’s, they set the prop somewhere between 2500 and max, and throttle is what it needs to be for the maneuver. That’s what we do for cropdusting and fire fighting.
 
I think the "keep it square" was a guideline before everybody got AFMs. Use that when you have no other information.

The Decathlon AFM, p.4-12, shows 85% power at 2400 rpm and 26.5"

We climbed the 450 Stearman at 2700 rpm and 37".

The contest I flew? They told me to leave it at 25/25 the entire time except for the spin. I did that, and it worked.
 
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