Welcome Guest. Sign in or Signup

2 Answers

Manifold pressure vs RPM settings

Asked by: 6484 views , , , , ,
Aircraft Systems, Commercial Pilot, Flight Instructor, Instrument Rating, Student Pilot

I know that for any given RPM, there is a manifold pressure that should not be exceeded. I tried to search the C172 RG POH but I didn't find any table that shows me the max MP for a given RPM. My question, how can I determine this limitation?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

2 Answers



  1. Kris Kortokrax on Nov 20, 2014

    If it were a limitation, it would be found in the Limitations section of the Airplane Flight Manual (or POH). It is not a limitation, although I would not pull the prop all the way back and shove the throttle to full. You will see in some manuals a warning to not let the RPM drop more than 500 RPM when exercising the prop during a runup.

    Are you talking about the Old Wive’s Tale that says that manifold pressure should never exceed RPM/100? (i.e. 25″ and 2400 would be an invalid power setting)

    Consider that on every takeoff, your manifold pressure exceeds RPM/100.

    In the Performance section, you should find a chart that lists MP/RPM combinations that will yield a certain percentage power at given altitude/temperature combinations.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes



  2. Mark Kolber on Nov 20, 2014

    There may be some ultimate limits that an engine guru can teach us all but I’m not aware of any POH “limitations” on the subject. “Oversquare” settings exist in all the power tables I’ve seen, some pretty large ones in the case of turbocharged engines. The normal takeoff in a Turbo Arrow, for example, calls for 2575 RPM and 41″ MP, more than a 15″ difference. . I have seen a few aircraft placarded against certain combinations, but other than that…

    Unless you are interested in the purely academic exercise, remaining Within the tolerances of the POH should keep you comfortably within the “safe” area. Beyond that, even if not an engine health issue it’s certainly an engine efficiency issue.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.