M20J Cruise Power Settings

[dwanzor]

In the POH for the M20J, all of the cruise power settings are listed at either 2400, 2600 or 2700 RPM's. 

Is there any reason not to cruise at 2500 RPM's? 

In another post on this forum, many commenters suggested that they always cruise at 2500 RPM with the throttle firewalled (with MP often below 25" due to altitude). 

I am a new M20J owner and just reading the POH and starting to try things in the plane. 

I will cruise most often between 5,500 and 9,500 MSL. 

[ArtVandelay]

I do it all the time, you can extrapolate from the table if you want to.

[dwanzor]

6 minutes ago, dwanzor said:

In the POH for the M20J, all of the cruise power settings are listed at either 2400, 2600 or 2700 RPM's. 

Is there any reason not to cruise at 2500 RPM's? 

In another post on this forum, many commenters suggested that they always cruise at 2500 RPM with the throttle firewalled (with MP often below 25" due to altitude). 

I am a new M20J owner and just reading the POH and starting to try things in the plane. 

I will cruise most often between 5,500 and 9,500 MSL. 

It seems odd to me that 2500 RPM is omitted from the book, but red line RPM (2700) is included for each power setting. I can't figure out why it is presented that way, but perhaps there is a good reason. 

[DC_Brasil]

I'd suggest you start by reading these (rather long) series of articles: Where Should I Run My Engine? 

They are rreally insightful and will change the mindset most CFIs still teach about engine management. 

https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-63where-should-i-run-my-engine-part-1/

 

https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-64where-should-i-run-my-enginepart-2-the-climb/

 

https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-65where-should-i-run-my-enginepart-3-cruise/

 

 

https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-66where-should-i-run-my-enginepart-4-descent/

[PT20J]

Mooney had different versions of the power charts in different POH versions. Who knows why? Here’s one that shows 2500 rpm. I use 2500 most of the time. Bob Kromer (former factory test pilot) has written that the prop on the J was optimized for 2500 rpm.

[toto]

+1 for Bob Kromer’s recommendations. I trust him more than I trust most. 
 

From an old MAPALog:

[Schllc]

On 12/18/2024 at 7:03 PM, DC_Brasil said:

They are rreally insightful and will change the mindset most CFIs still teach about engine management. 

Cfis and ferry pilots are generally unconcerned with the health of the engine beyond their time in the plane. I flew with a cfi in my ovation from 7 hours until I completed my ifr. 
beyond the conversation of take off full power, and my approach speeds, he never once had a conversation with regard to power settings or operating ranges. 
I dont want to impugn all Cfi’s, I have had some very good ones, but they are human.   After 30 years of employees and watching how they care for things they don’t have to pay for, it has made me a bit cynical, and a lot more aware of human nature. 
What I have gleaned from my limited experience, and from this forum is that there are ranges unique to each engine that you absolutely need to know to stay out of, and the rest will be determined by trial and error, your mission, your altitudes, and your own unique plane. 
the poh is a good starting point but others who share your setup, who have been flying it for years, and your experimentation will help you find the optimum place for you to fly. 

[EricJ]

If you fly around at WOT, which is a perfectly reasonable thing to do with a naturally-aspirated, fuel-injected engine, then RPM is proportional to output power.    So 2500 vs 2700 is at 2500/2700 = 92.6 % of available power.    If you want to go a little faster, crank the rpm up a bit, even to 2700 if you want, it just costs higher fuel flow.  You won't go a lot faster, but it should be noticeable.   2500 is just kind of a sweet spot for speed and efficiency, which is kinda what Mooneys are all about, anyway.

No two of these airplanes are the same after around forty-plus years, so you can just fly a lot and figure out what are reasonable settings for how you want to fly your airplane, since nobody else is likely to have your exact mission set, either.

My airplane has a Hartzell top prop, so I don't know how that fits in the picture as far as optimizing rpm, but it works very well at 2500 rpm and WOT, so that's usually how I'm cruising around.   I'm very rarely at a different setting unless I'm on initial climb or on a descent.   Other people have very good success with different settings, ymmv.   The tribal wisdom around here is often pretty good for this sort of thing.

[dwanzor]

Lots of great feedback here so far, very helpful. 

[0TreeLemur]

I've been doing a deep dive on this very topic lately.   Today on a 7-1/2 hour cross country I experimented with some of my notions.  I've learned that:

1.  Propeller technology is mature.   Since WWII all constant speed propellers in wide use have efficiencies that vary from about 81-85% over the RPM range from  2200-2700 rpm.  The efficiency doesn't vary that much.  From 2300-2600 RPM it varies only from about 83-85%

2.  There are three ways to change the amount of power that an IO-360 produces: RPM, MP, FF.  Lycoming engine manuals vary the heck out of RPM and MP, but only provide data at maximum power mixture, which is not where we usually run them.

3.   Pumping loss within the engine at high rpm are significant.   Running an engine at high rpm has a cost.  More energy is lost in the induction and exhaust systems with higher mass flow rates, probably proportional to mass flow rate squared.

4.  At higher altitudes for normally-aspirated (NA) engines, you must increase RPM to increase engine power because of the reduction in MP with altitude.

I had several legs today, and here's what I tried, with the notion that 65% power (130 HP) offers a nice compromise on speed and fuel consumption when running LOP:

Leg 1- 4000 ft.   Set MP at 24".  Did the big mixture pull (back to about 8.5 gph)- at this point with 24" MP the engine is pretty deep LOP.   Reduced rpm to get about 60% power indication on the JPI (I know, it's not perfect, but hey- it is an indication).   That resulted in about 2360 rpm.  Used the Lean Find function on the JPI to operate at peak EGT (ff=8.7 gpm).   That increased the power indication on the JPI to about 64%.

Leg 2 - 6500 ft.  WOT.  Pulled mixture back to a ff of about 8 gph.  Set RPM to 60% power on JPI- about 2450 RPM, then used LF to set to max EGT (ff=8.6 gph).  Resulting power indication on JPI 65%.

Leg 3 - 8500 ft.  WOT.  Pulled mixture back to ff=7.6 gph.  Set RPM to 60% power on JPI, about 2550 RPM.  Used LF to set max EGT (ff=8.4 gph).   Resulting power indication: 63% (ish).  increased RPM to about 2580 and that made JPI indicated power about 65%.

This last result means that it won't be possible to use RPM to adjust to 65% running at peak EGT much above 8500 ft because I don't think cruising with RPM above 2600 makes much sense because of the increased pumping loss, and the fact that at the high end of the rpm range the efficiency starts to drop off pretty rapidly.

At no point during these settings did the cylinder head temps do anything unusual.  Typ. 340-360F with cowl flaps closed.  OAT ranged from 11 to 3 C.

Oddly enough flying west from Alabama to New Mexico I had a tailwind for all but the first two hours.  Averaged about 17 MPG.

 

 

[jetdriven]

You may be surprised to find out the airplane is 5 or 6 knots faster at 2700 RPM that at 2500 RPM at about 10,000 feet or above. And this is with the same fuel flow.  You can't make enough percentage of power up there to get what you want, and 2700 is more than 2500.

[Shiroyuki]

6 hours ago, DC_Brasil said:

I'd suggest you start by reading these (rather long) series of articles: Where Should I Run My Engine? 

They are rreally insightful and will change the mindset most CFIs still teach about engine management. 

https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-63where-should-i-run-my-engine-part-1/

 

https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-64where-should-i-run-my-enginepart-2-the-climb/

 

https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-65where-should-i-run-my-enginepart-3-cruise/

 

 

https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-66where-should-i-run-my-enginepart-4-descent/

Spent some time and read through all for sections. Honestly I feel quite skeptical about it, it sounds like he’s pretty paranoid and making things extra difficult, jiggling mixture all the time…

I always leave mixture full rich after take off until over 7000 DENSITY ALTITUDE, then I will start to lean for more power. Running rich for a couple minutes in climb won’t hurt the engine, but leaning prematurely will. Yes he talks about keeping the mixture rich enough, fair enough, but for what gain? Some extra power and efficiency? If someone isn’t smart enough and lean too much they might damage the engine.

Also I don’t like how he doesn’t talk about density altitude at all. I’ve had days with -5000ft DA on the ground. Imagine what’s going to happen when someone try to lean 50 ROP (as the author stated for climb and cruise above 8000ft) at 7000ft indicated altitude?

The cruise power section is just so paranoid about “the red box”, somewhere around 50-100 ROP above 70% power. Maybe someone else can chime in but is that really such a big deal? All POH I’ve read for different airplane have best power mixture at 50 ROP, so most manufacturers write the POH in such a way that will destroy the engine?

Lastly, he seems to base most of his technique on Continental IO550 engine, with little mention of IO360 in the J… from his words it sounds like IO550 have some big issue with cylinder quality, maybe that’s why he’s so careful with the “red box”? Is it such a big deal on Lycoming?

He has a lot of good points in the article, but I find it hard to agree with some of his core arguments and practices…

[Shiroyuki]

My personal method of flying my J is pretty simple.

WOT on take off and climb, don’t touch the mixture until it’s 7000ft density altitude outside then lean for more power. How do i know where’s the most power? I feel it in my butt.

For cruise power setting below 7000 DA, I generally following the power chart for 75% power, 2400rpm, and set 50 LOP. But I rarely fly that low. 

Due to geography and fear of engine failure, i like to stay high for most of my flight, unless prohibited by cloud, wind or airspace. Above 7000ft DA, i would simply set 2600rpm with wide open throttle, ram air open, and lean for best power.

i had my plane dynamic balanced at 2600rpm as i know with all the flying i do, i need that rpm to go faster. 

During descend i would pull 5 inch manifold pressure initially, and reduce rpm by 300-400, then reduce manifold pressure further when the engine is cooled off a bit and depending on what descend rate i want. I keep my airspeed that same during descend as cruise, as an extra 20 knots in descend saves barely a minute or two in most cases… why push the yellow arc for 2 minutes…

[jetdriven]

It doesn't get simpler than leaving the power where it was and just trimming for the nose to come down at 600-700 feet per minute. The airspeed is still in the green arc and your true airspeed came up 40 kn. Which offsets a lot of the loss that you took during the climb.

[ArtVandelay]

7000’ before leaning the mixture?! You’re just dumping avgas out your muffler.

I start by 3000’, I use the rule; 19-altitude in 1000s = fuel flow, so at 3000’ I set the FF to 16. That’s more than enough to keep the mixture rich and gets you close to best power.

I also pull power on a descent, it usually starts getting bumpy at lower altitudes, why stress out my airframe, and me.

[Shiroyuki]

1 hour ago, ArtVandelay said:

7000’ before leaning the mixture?! You’re just dumping avgas out your muffler.

I start by 3000’, I use the rule; 19-altitude in 1000s = fuel flow, so at 3000’ I set the FF to 16. That’s more than enough to keep the mixture rich and gets you close to best power.

I also pull power on a descent, it usually starts getting bumpy at lower altitudes, why stress out my airframe, and me.

A little extra fuel is wasteful, but a few minutes burning two more gallon isnt going to do much harm. We dont have a catalytic converter to clog.

[Bartman]

I keep it simple and have not looked at the power charts in several years, relying on the engine monitor. Takeoff balls to the wall and lean to maintain target EGT during climb as described in the Pelican Pearch articles.  Close cowl flaps when I reach cruise altitude and don't touch anything else until speed stabilizes.  Pull back to 2500 and set the JPI to Lean Mode.  At my normal cruise of 7k - 8k set it at 10-15 deg LOP and WOT.  If I cruise any higher, I will run at peak EGT.  Trim, set the autopilot, and enjoy the view.  If I change altitude, then I lean again.  Descending nose it over and pull the throttle back as required to maintain cruise MP.  There is no need to adjust Prop RPM, but I increase the Mixture just enough so she doesn't sputter when I add throttle for taxi.

[Slick Nick]

6 hours ago, Shiroyuki said:

A little extra fuel is wasteful, but a few minutes burning two more gallon isnt going to do much harm. We dont have a catalytic converter to clog.

7000’ is insanely high to start leaning. You’ve been to Springbank, it’s only at 4000’. I have to lean my mixture for takeoff or I won’t get max rated power, already being down a couple inches of manifold compared to sea level. Taking off full rich I can feel a very noticeable decrease in performance. I continue to lean in the climb for best power. I can’t imagine going all the way to 7000’ at full rich mixture, you must be down to 55% power. How slow is your climb v/s at max weight? 

I can’t fathom a good reason against leaning in the climb, or for takeoff at a higher altitude airport. It’s bad for your engine not to. (Different story for turbo engines.) If it’s something you were taught, it’s bad practice and a little bit lazy on the part of the instructor not to explain it better. 

[Bolter]

7 hours ago, Shiroyuki said:

A little extra fuel is wasteful, but a few minutes burning two more gallon isnt going to do much harm. We dont have a catalytic converter to clog.

It is not about wasting fuel, it is about engine management.  Leaning in climb, staying rich, is proper engine management.  Before you reach 7000' you will be running *overly* rich to the point you are losing power.  If you are departing a non-sea level airport, you also need to be leaned to get max power for takeoff.  In winter, maybe not as much with the density altitude getting lower.  You are also putting more lead into the cylinders and may foul plugs, or even interfere with cylinder/piston lubrication if there is really too much fuel.

Do you track how much your EGT has changed between launch and 7000'?  In my J, I had a target of 1400-1450 EGT during climb.  This was based on 150-200F rich of peak, so I was well within max power and cooling without going overly rich.  EGT's vary between airframes based on sensor locations.  See how much your EGT's change during your climb if you stay full rich.

I did not re-read the articles, but I believe there is no talk about being LOP during climb, only properly rich.  It discourages aggressive leaning in climb.  It also showed that factory recommended 50F ROP operating point is in the middle of the highest internal cylinder pressures, and not a good operating point for long life.  Is it the Bravo's that if they follow the POH go through cylinders frequently?  POH's are out of date on modern engine management.  They even assume all the cylinders are operating identically, as they predate engine analyzers.  

The answers are in the Lycoming documentation, even if the POH chooses to documents operating points which may nto be best.  Just as they outright ignore 2500 rpm in your POH, but not other J POH's.  The attached is from the Lycoming manual.  I added the red text boxes to highlight things.  Greatest power is 150 ROP, greatest efficiency is 20 LOP.  These are the only 2 operating points we are usually interested in.  

Lycoming Power-EGT-CHT relationship.pdf

[201Steve]

One of these days, we’ll get out of the Stone Age and let a computer control these things….. or maybe not. 
 

since there’s 1000 variations depending on who you ask, 

-understand the relationship of % power FF, MP, RPM

-Hopefully you have a CHT gauge on each cylinder, don’t let them get hot. 

-understand what excess lead is doing to your spark plugs and exhaust valves. 
 

Let’r rip. 

[EricJ]

12 hours ago, 0TreeLemur said:

3.   Pumping loss within the engine at high rpm are significant.   Running an engine at high rpm has a cost.  More energy is lost in the induction and exhaust systems with higher mass flow rates, probably proportional to mass flow rate squared.

Even though these motors have reasonably high displacement, they don't turn very fast, so I suspect pumping losses may not be that bad.    2-valve V8s with similar displacement and intakes that turn much higher rpm have similiarly-sized (or smaller) intakes, and still manage pretty linear increase in hp with rpm.    So I'd think pumping losses for them would be much worse (since they're moving 2-3x more air), but they don't seem to mind.   For those reasons I've always suspected that those kinds of issues are not significant for our motors, but I've never seen measurements for it.   I can't even seem to find a relevant tq-hp vs rpm curve for an IO-360, but I seem to recall that it's pretty linear.

[0TreeLemur]

5 hours ago, Bartman said:

... lean to maintain target EGT during climb as described in the Pelican Pearch articles.

That's what I do.  My takeoff EGT (full power, mixture rich, ff~19 gph) is 1270F.  I lean in the climb several times to re-establish that target.  Probably saves a gallon of fuel on a climb up to 10,000 from sea level.  Why not?

[skykrawler]

This is my understanding of running too rich:  it washes the oil off the cylinder walls and fouls the plugs.  (and it wastes fuel and pollutes the air more)

 

I lazily copied this off the web:

 

Key points about fuel wash:

• How it happens:

  When a combustion mixture is too rich, unburned fuel can pass the piston rings and into the crankcase, mixing with the engine oil, effectively diluting it and reducing its lubricating properties. 

   
• Negative effects:

  • Cylinder wall wear: Without a proper oil film, the piston rings can start to wear down the cylinder walls due to increased friction. 
     
  • Reduced engine performance: The diluted oil can lead to decreased lubrication, impacting engine efficiency and power. 
     
  • Increased oil consumption: The diluted oil can be more easily burned in the combustion chamber, leading to higher oil consumption

Edited Thursday at 07:22 PM by skykrawler

[Shiroyuki]

5 hours ago, Slick Nick said:

7000’ is insanely high to start leaning. You’ve been to Springbank, it’s only at 4000’. I have to lean my mixture for takeoff or I won’t get max rated power, already being down a couple inches of manifold compared to sea level. Taking off full rich I can feel a very noticeable decrease in performance. I continue to lean in the climb for best power. I can’t imagine going all the way to 7000’ at full rich mixture, you must be down to 55% power. How slow is your climb v/s at max weight? 

I can’t fathom a good reason against leaning in the climb, or for takeoff at a higher altitude airport. It’s bad for your engine not to. (Different story for turbo engines.) If it’s something you were taught, it’s bad practice and a little bit lazy on the part of the instructor not to explain it better. 

You're absolutely right. I just doesn't want to bother any risk with detonation, as I was taught leaning above 75% could risk that. That's probably the flat lander inside me. Beside spring bank, which have long runways, I've never been to other high field elevation airport...

I rarely fly at max gross. I typically sees about a thousand ish at sea level, maybe a bit more, and seven hundred ish high up? I find temperature makes more difference in climb rate then anything else.

[Shiroyuki]

5 hours ago, Bolter said:

It is not about wasting fuel, it is about engine management.  Leaning in climb, staying rich, is proper engine management.  Before you reach 7000' you will be running *overly* rich to the point you are losing power.  If you are departing a non-sea level airport, you also need to be leaned to get max power for takeoff.  In winter, maybe not as much with the density altitude getting lower.  You are also putting more lead into the cylinders and may foul plugs, or even interfere with cylinder/piston lubrication if there is really too much fuel.

Do you track how much your EGT has changed between launch and 7000'?  In my J, I had a target of 1400-1450 EGT during climb.  This was based on 150-200F rich of peak, so I was well within max power and cooling without going overly rich.  EGT's vary between airframes based on sensor locations.  See how much your EGT's change during your climb if you stay full rich.

I did not re-read the articles, but I believe there is no talk about being LOP during climb, only properly rich.  It discourages aggressive leaning in climb.  It also showed that factory recommended 50F ROP operating point is in the middle of the highest internal cylinder pressures, and not a good operating point for long life.  Is it the Bravo's that if they follow the POH go through cylinders frequently?  POH's are out of date on modern engine management.  They even assume all the cylinders are operating identically, as they predate engine analyzers.  

The answers are in the Lycoming documentation, even if the POH chooses to documents operating points which may nto be best.  Just as they outright ignore 2500 rpm in your POH, but not other J POH's.  The attached is from the Lycoming manual.  I added the red text boxes to highlight things.  Greatest power is 150 ROP, greatest efficiency is 20 LOP.  These are the only 2 operating points we are usually interested in.  

Lycoming Power-EGT-CHT relationship.pdf 198.35 kB · 4 downloads

Maybe I should start leaning it then. 

But here's my question, when should I be worried about detonation? I have this thought which come from primary training that leaning above 75% may cause detonation and damage the engine, that's why I keep it rich. Above 7000ft the engine won't make 75% power anymore so I start leaning. That was my rational and I never put too much thought in it. I also see little benefit in leaning early, maybe 200fpm faster climb, saving a gallong of fuel... And I doubt three minutes of running rich will kill an engine...