Leaning for Power?

[Half Fast]

Okay, we haven't had a fuel mixture thread for at least twenty minutes, so let's try one with a different tack. Most discussions get into a back and forth regarding ROP vs. LOP and fuel economy, but what about leaning for max power rather than best economy?

In my baby Beech with a carbureted O-360, it seems to me I can lean for max power (peak RPM) and be at safe temperatures (graph below). The graph shows that the max power range has similar CHT to that in the best economy range, and even lower EGT. Furthermore, leaning to max power and not beyond doesn't require the EGT and CHT to pass through their peaks, so to me it appears even safer for the engine.

​

Sure, it burns a bit more gas, but people spend many thousands on props, gap seals, wheel pants, etc., etc., trying to coax an extra knot or two out of a fixed gear 4x4 like mine, then lean for best economy. Spending an extra ten bucks an hour on gas once in a while by leaning for max power seems like a bargain.

On a related note, when taking off from a high elevation airport, wouldn't it be best to lean for max power?

Thoughts? What am I missing?

 

[Martin Pauly]

That graph is misleading. I would have used the word "wrong" instead of "misleading" if it had actual numbers for fuel flow on it, but the way it is one could claim that it's not to scale...

What the graph suggests is that peak CHT is just a bit rich of peak EGT, while peak HP is way off to the right. That is simply not correct. Peak CHT is found at approx. 40 to 50 deg F ROP, while peak HP is at approx. 80 deg F ROP. A correct graph would depict peak CHT about halfway between peak EGT and peak HP.

That aside, as long as you are not making anywhere near full power (say, 65% power or less), by all means feel free to to go max power on your engine. If you are taking off from a high density altitude airport where your engine is only making half its rated power, that's not only OK to do - it's likely the safest option.

- Martin

 

[MauleSkinner]

Nothing in that graph shows “safe” temperatures, merely degrees below peak EGT and relative to some indeterminate CHT, whatever those happen to be.

 

[Eric Pauley]

Unless you’re already at max power MAP/RPM you’d probably go as fast by flying higher MAP/RPM LOP, or flying higher.

A single fuel stop saved from LOP will also outweigh a lot of max power flying. LOP also enhances your payload for a given range. If you really want to overthink it LOP lets you carry less fuel which may increase airspeed and eat into the ROP time savings.

 

[GeorgeC]

That's a notional chart that shows deltas; the actual absolute temperatures are the ones that matter, and those will depend on throttle, mixture, OAT, phase of moon... do you have an engine monitor? MP gauge? Fuel flow?

 

[Half Fast]

Peak CHT is found at approx. 40 to 50 deg F ROP, while peak HP is at approx. 80 deg F ROP. A correct graph would depict peak CHT about halfway between peak EGT and peak HP.

Thanks, Martin. Some follow-up:

There's no horizontal scale on the graph, so how far left or right a peak occurs is a bit arbitrary. The important parameter is the temperature. Reading the temperature number from the right vertical axis, the graph does seem to show that CHT peaks about 50F rich of the EGT peak, as you say.

The graph also shows peak power about 130F rich of peak EGT, far cooler than the 80F you mention, so maybe the graph is incorrect there. I only have a single probe EGT, so no way to check it.

In any case, if the graph is correct in showing that max power CHT falls into the same temperature range as best economy CHT, why would it be okay to lean to best economy at >65% power but not to lean to max power?

 

[Half Fast]

That's a notional chart that shows deltas; the actual absolute temperatures are the ones that matter, and those will depend on throttle, mixture, OAT, phase of moon... do you have an engine monitor? MP gauge? Fuel flow?

No monitor. I have a carb'd O-360 with a single probe EGT (cylinder #3).

I agree the chart only shows deltas, but I have no way to determine absolute temps. If the absolute temps are okay in the best economy range, why wouldn't they be okay in the max power range? The deltas are about the same, so the absolute temps would be the same, whatever they are.

 

[Half Fast]

Unless you’re already at max power MAP/RPM you’d probably go as fast by flying higher MAP/RPM LOP, or flying higher.

A single fuel stop saved from LOP will also outweigh a lot of max power flying. LOP also enhances your payload for a given range. If you really want to overthink it LOP lets you carry less fuel which may increase airspeed and eat into the ROP time savings.

Typically, when I'm going somewhere and not just doing local sightseeing, I'm at full throttle. The plane has 5+ hours at full fuel, so I make bladder stops instead of fuel stops.

 

[MauleSkinner]

If the absolute temps are okay in the best economy range, why wouldn't they be okay in the max power range? The deltas are about the same, so the absolute temps would be the same, whatever they are.

Again, nothing that chart says the absolute temps are okay anywhere.

 

[Brad W]

pretty sure the cessna 152 and 172 manuals I trained on has you "leaning for peak RPM".... so wouldn't that be peak power?

 

[Half Fast]

Again, nothing that chart says the absolute temps are okay anywhere.

I'm not getting your point. Is it unsafe to lean at all, since the chart doesn't show absolute temperatures? I think we must assume that, since the operating manual tells us how to lean for best economy, the absolute temps are safe when leaned that way.

Otherwise, how could we ever operate these engines safely without an engine monitor to tell us absolute temperatures? Yet we do.

 

[Half Fast]

pretty sure the cessna 152 and 172 manuals I trained on has you "leaning for peak RPM".... so wouldn't that be peak power?

With a fixed prop, yes.

 

[MauleSkinner]

I'm not getting your point. Is it unsafe to lean at all, since the chart doesn't show absolute temperatures? I think we must assume that, since the operating manual tells us how to lean for best economy, the absolute temps are safe when leaned that way.

Otherwise, how could we ever operate these engines safely without an engine monitor to tell us absolute temperatures? Yet we do.

Get an engine monitor and you’ll see that those settings can absolutely be detrimental to your engine under some conditions.

 

[PaulS]

To run optimum lean of peak you need to find peak, which takes about 15 to 30 seconds in the "bad" area. In the plane I'm flying, if you do it faster you aren't hitting peak. Or more accurately, you don't know where peak is.

 

[Half Fast]

To run optimum lean of peak you need to find peak, which takes about 15 to 30 seconds in the "bad" area. In the plane I'm flying, if you do it faster you aren't hitting peak. Or more accurately, you don't know where peak is.

Given the uneven fuel flow with my carb'd engine, without an engine monitor there's no way to truly find the peak.

 

[Martin Pauly]

if the graph is correct in showing that max power CHT falls into the same temperature range as best economy CHT, why would it be okay to lean to best economy at >65% power but not to lean to max power?

And there you go. The graph is not correct. Because if it was, the conclusion wouldn't make sense.

- Martin

 

[Half Fast]

And there you go. The graph is not correct. Because if it was, the conclusion wouldn't make sense.

- Martin

Thanks. So what does a correct graph look like?

 

[Eric Pauley]

Typically, when I'm going somewhere and not just doing local sightseeing, I'm at full throttle. The plane has 5+ hours at full fuel, so I make bladder stops instead of fuel stops.

Of course you cruise at full throttle. The only question is where the blue/red levers are.

 

[Half Fast]

Of course you cruise at full throttle. The only question is where the blue/red levers are.

It’s a fixed prop; only two controls. The throttle is pushed all the way in and the remaining question whether I’m risking harm to the engine by leaning to max RPM.

 

[Martin Pauly]

Thanks. So what does a correct graph look like?

 

[asicer]

Get an engine monitor and you’ll see that those settings can absolutely be detrimental to your engine under some conditions.

As evidenced by high CHT?

 

[MajorTurbulence]

Typically, when I'm going somewhere and not just doing local sightseeing, I'm at full throttle. The plane has 5+ hours at full fuel, so I make bladder stops instead of fuel stops.

What plane?
Check your POH graphs carefully. The fine print matters! Many a pilot, including me early on, assumed that I had more fuel available than I thought when flying full throttle @75% at altitude or except for takeoff. To get the max 75% endurance with or without reserve, using my Archer2 graphs/tables, one needs to lean to peak, which is something I don’t want to do at 75% for engine health. But if you are not at peak, you are burning more fuel than you think and will not have those 5+ hrs the chart indicates you have.
And while I understand your desire to get the most speed out of your engine, especially for aircraft that are not blessed with high cruise speeds, the time saved by flying your typical 3 to 3.5 hr leg can be as little as 5-10 minutes when flying at 65% versus 75%. The big thing for me is that flying at that 65% enables me to have confidence that the fuel flow is consistent with the fine print on the chart where the chart will be close to my actual burn, and will not damage my engine flying at peak. The benefit comes, as Eric Pauley stated, that you can go farther on your tank of fuel with safe reserves, and with little significant ( see above) time loss penalty. It also may be helpful when getting past weather to avoid an early fuel stop, besides saving fuel cost. Also, in general, flying higher helps out for several reasons. And it was not lost on me that you were making bladder stops and not necessarily flying longer legs to get places efficiently.

 

[GeorgeC]

It’s a fixed prop; only two controls. The throttle is pushed all the way in and the remaining question whether I’m risking harm to the engine by leaning to max RPM.

It Depends(tm).
WOT at sea level on a hot day? Absolutely.

I had my O-200 running WOT at 10,500', OAT was 35F, about 65% power. It was leaned to the point where one turn leaner would make it quit and one turn richer would send the CHTs downhill. One of my cylinders, while below 380F, was still on the warm side...

Without an engine monitor, you don't know where the edges are, which is why the POH has conservative numbers that are padded with margin like "lean above 5000' below X% power (according to the power tables)".

 

[Half Fast]

Is the vertical scale 100F per line? On that graph, it appears that leaning to peak power would put CHT at approximately the same temperature as leaning to peak EGT, which I believe would be an excessive CHT.

If I leaned to roughness and then enriched slightly, roughly where would that fall on the graph?

What's the source of that graph? Any idea why Lycoming's would be so different?

Thanks much!

 

[Half Fast]

Check your POH graphs carefully. The fine print matters! Many a pilot, including me early on, assumed that I had more fuel available than I thought when flying full throttle @75% at altitude or except for takeoff.

No, I don't go by the POH graphs. I go by my own personally measured fuel consumption. And I fly with lots and lots of fuel margin.

 

[MajorTurbulence]

No, I don't go by the POH graphs. I go by my own personally measured fuel consumption. And I fly with lots and lots of fuel margin.

So do I by switching tanks after each hour or after after each 10 gallons consumed so that I have a pretty much known and significant quantity left in one tank for alternate and reserve. But you will find that if you fly and lean to consistent parameters, especially mirroring the performance tables in your POH, that they are close to spot on if your airplane is a well maintained specimen. Of course, if you never explore or come close to the limits of your planes normal flight capabilities, then you can get away almost anything including never leaning if your flight is only an hour or two on full fuel tanks. Certainly not recommended from an efficiency, fuel expense or prevention of the consequences of too rich mixtures on you plugs, valves, etc.

 

[Martin Pauly]

Is the vertical scale 100F per line? On that graph, it appears that leaning to peak power would put CHT at approximately the same temperature as leaning to peak EGT, which I believe would be an excessive CHT.

If I leaned to roughness and then enriched slightly, roughly where would that fall on the graph?

What's the source of that graph? Any idea why Lycoming's would be so different?

Thanks much!

There are no units or numbers on this graph. This was done intentionally, because the actual numbers (fuel flow, degrees, PSI, HP) would differ greatly from one situation/engine to another. What the graph tries to show is the peaks, inclines and declines of the various items, and the approximate locations of peak HP and peak CHT relative to peak EGT.

If you lean the way you described (until roughness, then enrichen slightly), you could be in a variety of places on this graph. The reason is that our engines have more than one cylinder working on the same crankshaft, and how well these cylinders are balanced (in terms of supplying them with air and fuel) in your engine has a major impact on when (read: at what fuel flow) your engine will start running rough. In fact, if you do what you describe, some cylinders might be LOP while others are ROP. Only with a digital engine monitor can one tell.

The graph was created by me based on data from the Advanced Pilots Seminar (APS) which for a couple of decades was the go-to place to learn about piston engine management. (The class taught for many years by George Braly, John Deakin and Walter Atkinson in Ada, OK.) The graph was peer-reviewed by a small group of people, including the late John Deakin, in preparation for my "Lean-of-Peak" video on YouTube.

You'll have to ask Lycoming why theirs is different.

Regards,
Martin

 

[Checkout_my_Six]

If you need to have max power lean to max rpms.

 

[Martin Pauly]

If you need to have max power lean to max rpms.

That works in a fixed pitch propeller.

- Martin

 

[Half Fast]

The graph was created by me based on data from the Advanced Pilots Seminar (APS)...

Hmmmmm......

Well, I was hoping you were going to tell me something like "The graph is derived from EGT and CHT data taken at various mixture settings and throttle positions on X number of O-360s, Y number of O-320s,..." or something similar. Is there any real data pedigree? I'm sure Braly must have taken reams of data, if anyone did.

You'll have to ask Lycoming why theirs is different.

Maybe something like this?

"Dear Mr. Lycoming: I have this graph without any numbers that Martin Pauly gave me and I don't know what engines or tests or test conditions were used to create it, but it appears to disagree with the graph in your engine operator's manual. Could you please clarify your graph and explain why your graph is wrong?"​

I think that's gonna be just a tad, uh, weak. I'll be glad to email Lycoming if you like, FWIW, but I'd like to be able to frame a better question. Or have you or some of the folks you mentioned already had that discussion with Lycoming?

Any help?


As a practical matter, since I have a carb engine and a single-cylinder EGT, it seems like the best I can do is lean to rough, enrich to smooth, and hope.

 

[Checkout_my_Six]

They don’t like lycoming.

 

[Martin Pauly]

Well, I was hoping you were going to tell me something like "The graph is derived from EGT and CHT data taken at various mixture settings and throttle positions on X number of O-360s, Y number of O-320s,..." or something similar. Is there any real data pedigree? I'm sure Braly must have taken reams of data, if anyone did.

Indeed. George built an engine test stand to collect data from multiple engine. That data went into (among other things) their APS class. The general dependencies, peaks, inclines and declines are valid for any internal combustion engine - whether it's your lawn mower, your car or your airplane.

- Martin

 

[MBDiagMan]

Just a rhetorical curiosity question; How many pilots who are also aircraft mechanics that take apart and rebuild engines, believe in “lean of peak?”

That would be an interesting survey.

 

[Eric Pauley]

Just a rhetorical curiosity question; How many pilots who are also aircraft mechanics that take apart and rebuild engines, believe in “lean of peak?”

That would be an interesting survey.

What would be interesting about it? Would it influence your opinion? Neither being a pilot nor tearing down engines is really an authoritative qualification on engine ops. I once had an A&P pilot tell me that as long as my NA engine is “making good manifold pressure” that it’s running strong.

I’d personally value George Braly’s view (which is backed by data) over just about every other unsubstantiated opinion combined.

 

[RyanB]

No monitor. I have a carb'd O-360 with a single probe EGT (cylinder #3).

I might've missed it, but what % power are you running at during cruise? Without an engine monitor or any kind of temperature data, lean til rough, enrich til smooth is about as good as you're going to get. That said, I wouldn't want to use that method above 65%. If you're pushing the throttle all the way in and leaning for max RPM, then yes, you could be risking damage without being able to watch temperatures.

 

[Jeff767]

Just a rhetorical curiosity question; How many pilots who are also aircraft mechanics that take apart and rebuild engines, believe in “lean of peak?”

That would be an interesting survey.

Every major airline in the days of piston engine operations believed in LOP.

 

[Pinecone]

If you're pushing the throttle all the way in and leaning for max RPM, then yes, you could be risking damage without being able to watch temperatures.

It depends on what altitude. Above about 8000 feet (IIRC, I am turbo now), full throttle will be below 65%.

And Max RPM/Max Power is about 125 degrees rich of peak, which should be OK

 

[RyanB]

If you're pushing the throttle all the way in and leaning for max RPM, then yes, you could be risking damage without being able to watch temperatures.

It depends on what altitude. Above about 8000 feet (IIRC, I am turbo now), full throttle will be below 65%.

And Max RPM/Max Power is about 125 degrees rich of peak, which should be OK

Please see bolded word above.

 

[Clip4]

Here is what Lycoming says,


Leaning the Normally Aspirated Engines

• Use full-rich mixture during takeoff or climb. Careful observation of engine temperature instruments should be practiced to ensure the limits specified in Lycoming Operator’s Manual are never exceeded. Refer to the aircraft POH (Pilot’s Operating Handbook) or AFM (Aircraft Flight Manual) for more specific instructions.
• For 5,000 feet density altitude and above, or high ambient temperatures, roughness or reduction of power may occur at full rich mixture. The mixture may be adjusted to obtain smooth engine operation. For fixed-pitch propellers, lean to maximum RPM at full throttle prior to takeoff where airports are at 5,000-feet density altitude or higher. Limit operation at full throttle on the ground to a minimum. For direct-drive and for normally aspirated engines with a prop governor, but without fuel flow or EGT, set throttle at full power and lean mixture at maximum RPM with smooth operation of the engine as a deciding factor.
• For cruise powers where best power mixture is allowed, slowly lean the mixture from full rich to maximum power. Best power mixture operation provides the most miles per hour for a given power setting. For engines equipped with fixed-pitch propellers, gradually lean the mixture until either the tachometer or the airspeed indicator reading peaks. For engines equipped with controllable pitch propellers, lean until a slight increase of airspeed is noted.
• For a given power setting, best economy mixture provides the most miles per gallon. Slowly lean the mixture until engine operation becomes rough or until engine power rapidly diminishes as noted by an undesirable decrease in airspeed. When either condition occurs, enrich the mixture sufficiently to obtain an evenly firing engine or to regain most of the lost airspeed or engine RPM. Some engine power and airspeed must be sacrificed to gain a best economy mixture setting. NOTE: When leaned, engine roughness is caused by misfiring due to a lean fuel/air mixture which will not support combustion. Roughness is eliminated by enriching slightly until the engine is smooth.
• The exhaust gas temperature (EGT) offers little improvement in leaning the float-type carburetor over the procedures outlined above because of imperfect mixture distribution. However, if the EGT probe is installed, lean the mixture to 100˚ F on the rich side of peak EGT for best power operation. For best economy cruise, operate at peak EGT. If roughness is encountered, enrich the mixture slightly for smooth engine operation.
• When installing an EGT probe, the probe must be installed in the leanest cylinder. Contact the airframe or kit manufacturer for the correct location. In experimental or custom applications, multiple probe instrumentation is required, and several power settings should be checked in order to determine the leanest cylinder for the specific application.
• During normal operation, maintain the following recommended temperature limits:
  • Cylinder head temperature – limit listed in the Lycoming Operator’s Manual.
  • Oil temperature – limit listed in the Lycoming Operator’s Manual.
  • For maximum service life, maintain the following recommended limits for continuous cruise operation:
  • Engine power setting – 65% of rated or less.
  • Cylinder head temperatures – 400˚ F. or below.
  • Oil temperature – 165˚ F. – 220˚ F.