M20R 310 STC cruise minimum 2500 RPM

[Tyler G]

Verification Request – Cruise RPM Limitation in 310 HP STC (SA02193CH)

I'm seeking clarification regarding a cruise power limitation published in the AFM Supplement for the 310 HP STC (SA02193CH) applied to the Mooney M20R.

In a copy of the AFM Supplement (Document SUPP0029, dated 11/14/2017), the Log of Revisions under “Revision B” contains the following entry:

> “Revised maximum Cruise RPM to read Minimum Cruise RPM - 2500”
Approved by Charles L. Smalley – dated 3/7/2011

I would appreciate it if someone could help verify the accuracy and operational applicability of this change:

Is this 2500 RPM considered a minimum required RPM during level cruise?

Is this change still current and applicable to all aircraft operating under this STC?

I’m operating an M20R with the IO-550N (310 HP STC) and want to ensure I'm adhering to correct cruise power procedures per FAA-approved documentation.

Edited Monday at 05:58 PM by Tyler G

[Alan Maurer]

Hi, I did the STC also. Ovation 2 to Ovation 3. I take off at 2700 RPM. At 1000 AGL i dial back to 2500 for climb. . 2500 is not, in any way, a minimum.

You can be more efficient and easier on the engine at a lower RPM.   I hope that Skip will jump in. He is smarter than me

Alan

[kortopates]

Perhaps if you post page where you got this i am sure we can help you better understand it’s a recommendation and not a Limitation.

Bob, the guy that developed the STC, really felt from his testing that 2550 rpm was the most efficient cruise rpm. But this was for ROP cruise and for a LOP cruise a much lower RPM will be more efficient. After all Continentals Operating and maintenance manual performance tables shows approved RPM’s settings much lower than that. I would recommend you get a copy and look there.


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[Jeff Uphoff]

On 5/10/2025 at 11:48 PM, kortopates said:

Bob, the guy that developed the STC, really felt from his testing that 2550 rpm was the most efficient cruise rpm. But this was for ROP cruise and for a LOP cruise a much lower RPM will be more efficient. After all Continentals Operating and maintenance manual performance tables shows approved RPM’s settings much lower than that. I would recommend you get a copy and look there.
 

I'd love to learn more about the higher/lower RPM efficiency difference here between ROP & LOP!

--Up.

[PT20J]

1 hour ago, Jeff Uphoff said:

I'd love to learn more about the higher/lower RPM efficiency difference here between ROP & LOP!

--Up.

Leaner mixtures burn slower and reach peak pressure later after ignition than rich mixtures. Ignition timing is fixed and set to provide peak pressure at the crankshaft angle that produces maximum torque at full rich, maximum manifold pressure and max rpm to get rated power. LOP, the peak pressure occurs after of the optimum crankshaft angle. Since timing is fixed, your only remedy is to slow engine rotation to get closer to the optimum operating point.

[jetdriven]

But lowering the RPM reduces horsepower more than the recovery from having  the optimal cylinder pressure at the optimal crank angle. 

[Tyler G]

I added the STC doc from Minnis showing 2500 as the minimum RPM

[Tyler G]

5 hours ago, Jeff Uphoff said:

I'd love to learn more about the higher/lower RPM efficiency difference here between ROP & LOP!

--Up.

I would also like to understand the LOP best RPM

[M20S Driver]

My STC shows 2550 at max power and 2400 for lower power settings.

M20R Power Chart BP LOP from Bob Minnis.pdf

[Ragsf15e]

1 hour ago, Tyler G said:

I added the STC doc from Minnis showing 2500 as the minimum RPM

That’s the list of revisions, for better context on exactly what they changed and how, post the updated page 3.  That’s where the real info is. 

[kortopates]

That’s the list of revisions, for better context on exactly what they changed and how, post the updated page 3.  That’s where the real info is. 

And unless it’s listed in a FAA Approved Limitations section it’s not a limitation but a suggestion.


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[kortopates]

I looked it up on my copy of the AFMS and I'll just paste in the Limitations section and highlight the answer to your question in bolded text:

 

POWER PLANT LIMITATIONS
Propeller Manufacturer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hartzell
Propeller Hub/Blade Model Number . . . . . . . . . . . . . . . . . . . . . PHC-J3YF-1RF/F7693DF(B)-2
Number of Blades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Propeller Diameter: Hartzell
Min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 in.
Max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 in.
Engine Operating Limits for Takeoff and Continuous Operations:

• Green Arc - Normal Operating Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2200 - 2699 RPM
• Red Line - Maximum Limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2700 RPM
• Maximum Continuous Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 BHP

Also, in the prior Section 1 under Descriptive, it shows the following for the engine, again in bolded concerning recommended max cruise, and again this isn't a limitation but a recommendation but what "max cruise power" really means is where you can start leaning the engine to best power. But the the TCM IO-550 Maintenance and Operators manual will give much better and more detailed guidance on RPM and Leaning curves in addition to what has already be posted on the power chart. Notice in subsequent pages they say Cruise power should be 2550 rpm and 24" MAP which is actually substantially less than 262 BHP and very close to 75% power - which is a reasonable.

DESCRIPTIVE DATA
ENGINE
Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Manufacturer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continental Motors Incorporated
Model Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IO-550-G modified per STCSE02930AT
Optional Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IO-550-N
Number of Cylinders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 550 Cu. In. (9014 cc)
Maximum Continuous Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 BHP
Maximum Continuous RPM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2700
Maximum Recommended Cruise Power / RPM . . . . . . . . . . . . . . . . . . . . . 262 BHP / 2550 RPM