Leaning in flight - is it better, really?

[Marc_B]

12 minutes ago, jlunseth said:

But lots of pilots make the mistake that they can use the formula any time to determine percent power.

I think this is why many Mooney instructors stick with ROP when teaching.  It's easier and safer for the novice.  LOP requires more background understanding to do it safely and efficiently; although it is really pretty simple.  Also being with ROP, 100 degrees from peak means your temps are typically all fine even if you're not managing cowl flaps or you're flying at higher altitudes with less dense air for cooling.

Basic gist for anyone reading this and not understanding...on the lean of peak side, the %HP is controlled by the total amount of fuel available because fuel is the limiting factor.  To "cool" temps more you have to add "air" which means increasing throttle while keeping fuel flow the same (aka deeper LOP).  VS. on the ROP side you have an excess of fuel so the limiting factor is the amount of oxygen you have available.  So if you want to "cool" temps on the ROP side you can add more fuel which remains unburned and absorbs heat.

But using fuel flow as your "%HP LOP" only works if you're lean of peak and limited by fuel/excess oxygen.  This requires you to have an engine that can run on the LOP side as if you have a wide spread on the fuel flow for each cylinder to peak you potentially could have one cylinder on the LOP side and others on the ROP side, or even worse would be cylinders running at peak with higher %HP which would contribute to cylinders seeing peak internal cylinder pressures that could be harmful.  With modern engine monitors, with EGT/CHT probes on each cylinder, it's easier to see exactly what your engine is doing and takes more of the guess work out of it.  Of course performing a Lean Test requires an engine monitor with probes on all cylinders.  Lots of nuance and detail that pilots should learn about if they want to optimize their engine management!

14 minutes ago, jlunseth said:

As far as engine smoothness goes, you might run a true lean test and see what you get.

I have.  Lean sweep with my TCM injectors typically falls in the 0.3 - 0.5 gph range from sweep to sweep.  I've known two other Mooney pilots with stock TCM tuned injectors who considered GAMIjectors and were told that the spread was fine and there wouldn't be benefit.  Of course if you did a lean test and had a sweep of 1.1 gph, that'd be a different story...

 

As one last aside...when you're going down the path of lean tests, etc...it's important to make sure that you're actually running at baseline.  i.e. I've known one pilot who was trying to run a Lean Test and it turned out that he had a partially clogged injector that was an outlier.  But the issue wasn't that his engine had a wide GAMI spread, but that his engine wasn't at baseline.  Fuel injector cleared and Lean Spread went from 1.3 gph down to 0.5 gph.  Many things are dynamic and constantly in flux.  In fact, I also had a transient partially clogged injector once that resulted in EGT 100 deg hotter in that cylinder.  After it cleared the temps normalized.  Injectors clog, plugs foul, mag timing drifts...it's important to establish a baseline I think so that you can see outliers more easily and help guide ADMs in the field.

[Pinecone]

33 minutes ago, Marc_B said:

I think this is why many Mooney instructors stick with ROP when teaching.  It's easier and safer for the novice.  LOP requires more background understanding to do it safely and efficiently; although it is really pretty simple.  Also being with ROP, 100 degrees from peak means your temps are typically all fine even if you're not managing cowl flaps or you're flying at higher altitudes with less dense air for cooling.

Basic gist for anyone reading this and not understanding...on the lean of peak side, the %HP is controlled by the total amount of fuel available because fuel is the limiting factor.  To "cool" temps more you have to add "air" which means increasing throttle while keeping fuel flow the same (aka deeper LOP).  VS. on the ROP side you have an excess of fuel so the limiting factor is the amount of oxygen you have available.  So if you want to "cool" temps on the ROP side you can add more fuel which remains unburned and absorbs heat.

But using fuel flow as your "%HP LOP" only works if you're lean of peak and limited by fuel/excess oxygen.  This requires you to have an engine that can run on the LOP side as if you have a wide spread on the fuel flow for each cylinder to peak you potentially could have one cylinder on the LOP side and others on the ROP side, or even worse would be cylinders running at peak with higher %HP which would contribute to cylinders seeing peak internal cylinder pressures that could be harmful.  With modern engine monitors, with EGT/CHT probes on each cylinder, it's easier to see exactly what your engine is doing and takes more of the guess work out of it.  Of course performing a Lean Test requires an engine monitor with probes on all cylinders.  Lots of nuance and detail that pilots should learn about if they want to optimize their engine management!

I have.  Lean sweep with my TCM injectors typically falls in the 0.3 - 0.5 gph range from sweep to sweep.  I've known two other Mooney pilots with stock TCM tuned injectors who considered GAMIjectors and were told that the spread was fine and there wouldn't be benefit.  Of course if you did a lean test and had a sweep of 1.1 gph, that'd be a different story...

Good post.

And I don't know if I am one of them you counted, but GAMI won't sell me GAMIjectors due to low spread on the lean test.  -SB engine

[DCarlton]

Dumb question.  Someone please remind me what happens at peak EGT?  Detonation instead of a slower burn?  

[Pinecone]

It depends on the load and mixture.

Below 65% power you can run peak or even slightly rich of peak.  Higher power settings, you want to be clearly lean of peak or at least 100 - 125 degrees rich of peak to avoid the red box/fin area where you can get detonation.

One gotcha of running LOP has been pilots not understanding and setting LOP operation, then a bit richer for "safety" which puts them right into the red box/fin area of dangerous operation.

[Ragsf15e]

25 minutes ago, DCarlton said:

Dumb question.  Someone please remind me what happens at peak EGT?  Detonation instead of a slower burn?  

Especially on the non turbo engines at high altitudes (so lower power), peak doesn’t hurt anything and is actually recommended as best economy setting.  However, it’s not as good place to be as power increases.  You’ll start to see higher chts, higher internal cylinder pressure, and eventually detonation at higher power.  As @Pinecone said, below 65% it’s not doing those things.

~50 rich of peak is even worse… yet isn’t that where the Encore power setting table in the poh has you?

[Marc_B]

6 hours ago, DCarlton said:

Someone please remind me what happens at peak EGT?

The stoichiometric ratio of air:fuel (ideal ratio that burns all fuel and doesn't leave excess air) actually is a little rich of peak EGT.  So at peak EGT you still have some "air left" that you can add more fuel to burn. 

But in terms of "harmful ways to run your engine," it's been described is the "Red Box" / "Red Fin".  i.e. at high HP the area of the curve with potentially harmful ICPs (internal cylinder pressures) and temps goes from "a" degrees lean to "b" degrees rich.  Given less fuel is being burned the lower %HP you run, the smaller that "danger area" red box width is, and below a certain %HP (some use 65%) you don't have enough fuel burning to be able to generate a "dangerous" ICP/temps and the "red box" disappears.  But at higher and higher %HP, the box starts widening and widening.  This is exactly why you run full rich on takeoff...if you ran peak EGT at 100%HP you'd most definitely have dangerous temps and ICPs and would have a very short flight into detonation.

6 hours ago, Pinecone said:

And I don't know if I am one of them you counted, but GAMI won't sell me GAMIjectors due to low spread on the lean test.

That makes 3!

 

These pic are flipped from the way I usually have seen it with ROP to the right and LOP to the left, but gives you an idea of where things fall and rise.

https://aviationsafetymagazine.com/features/lean-of-peak-egt/

 

[Marc_B]

6 hours ago, Ragsf15e said:

~50 rich of peak is even worse… yet isn’t that where the Encore power setting table in the poh has you?

My POH for Encore shows 75% as maximum recommended cruise power and suggests you "LEAN TO PEAK, DO NOT EXCEED 1650 deg TIT." 

[Fly Boomer]

6 minutes ago, Marc_B said:

My POH for Encore shows 75% as maximum recommended cruise power and suggests you "LEAN TO PEAK, DO NOT EXCEED 1650 deg TIT." 

I don't have an SB engine, but that sounds like bad advice.

[DCarlton]

30 minutes ago, Marc_B said:

The stoichiometric ratio of air:fuel (ideal ratio that burns all fuel and doesn't leave excess air) actually is a little rich of peak EGT.  So at peak EGT you still have some "air left" that you can add more fuel to burn. 

But in terms of "harmful ways to run your engine," it's been described is the "Red Box" / "Red Fin".  i.e. at high HP the area of the curve with potentially harmful ICPs (internal cylinder pressures) and temps goes from "a" degrees lean to "b" degrees rich.  Given less fuel is being burned the lower %HP you run, the smaller that "danger area" red box width is, and below a certain %HP (some use 65%) you don't have enough fuel burning to be able to generate a "dangerous" ICP/temps and the "red box" disappears.  But at higher and higher %HP, the box starts widening and widening.  This is exactly why you run full rich on takeoff...if you ran peak EGT at 100%HP you'd most definitely have dangerous temps and ICPs and would have a very short flight into detonation.

That makes 3!

 

These pic are flipped from the way I usually have seen it with ROP to the right and LOP to the left, but gives you an idea of where things fall and rise.

https://aviationsafetymagazine.com/features/lean-of-peak-egt/

 

I need to read the article.  I struggle with the fact that we're dancing around values less than maybe 10 percent of peak to move from acceptable to dangerous operations.   I can see any value that triggers detonation as a bad thing but you'd think the engine would be designed to operate within all the other ranges (because even visually the curves are pretty flat accept for EGT).  There's something I'm missing.    

[Ragsf15e]

45 minutes ago, Marc_B said:

My POH for Encore shows 75% as maximum recommended cruise power and suggests you "LEAN TO PEAK, DO NOT EXCEED 1650 deg TIT." 

Yeah you’re right, mine too… i know i saw that somewhere.  Now im going to be looking everywhere!  Either way, peak/1650 tit or 50 rop don’t seem very healthy to me!

[Ragsf15e]

13 minutes ago, DCarlton said:

I need to read the article.  I struggle with the fact that we're dancing around values less than maybe 10 percent of peak to move from acceptable to dangerous operations.   I can see any value that triggers detonation as a bad thing but you'd think the engine would be designed to operate within all the other ranges (because even visually the curves are pretty flat accept for EGT).  There's something I'm missing.    

You’ve actually pretty much got it.  Look at “ICP” in the charts.  It’s already falling at peak but it starts falling much faster the leaner you go.  We are trying to manage ICP, but we don’t have an ICP gage, so we use egt and it falls off much slower.  So yes, there’s a huge difference in ICP with only a small difference in egt.  It’s not perfect, but egt is the only quick measurement we can use to get fairly close.

[Marc_B]

@DCarlton This is a good article to read.  I think it better clarifies the red box/fin.  Simplifies it to use CHTs as the boundaries.  High CHTs bad.  Under a certain %HP, you'll never be able to get up to high CHTs.

https://www.savvyaviation.com/red-box-red-fin/

 

1 hour ago, Fly Boomer said:

I don't have an SB engine, but that sounds like bad advice.

Ya, I don't think that sounds like current best practice!

[DCarlton]

6 hours ago, Ragsf15e said:

You’ve actually pretty much got it.  Look at “ICP” in the charts.  It’s already falling at peak but it starts falling much faster the leaner you go.  We are trying to manage ICP, but we don’t have an ICP gage, so we use egt and it falls off much slower.  So yes, there’s a huge difference in ICP with only a small difference in egt.  It’s not perfect, but egt is the only quick measurement we can use to get fairly close.

If you’re in the LOP camp.  I’ve always operated ROP.  Now I’m reconsidering.  Gotta read the article.  Thanks.  

[Z W]

When we first got the plane, we followed the POH and ran 75% power (28" and 2500 RPM usually) and peak TIT as it suggests for a short time. No signs of detonation and I really don't think it's unsafe, but the TITs would be right near 1650, sometimes requiring adding mixture to stay under that, and the CHTs would be pushing 400 even with the cowl flaps mostly open. Most of the internet seems to believe those temperatures are not good for turbo and cylinder longevity. It seems to be the best setting for speed, fuel burn, and range, giving roughly 140 KIAS at 11.5 GPH, which may be why the marketing department at Mooney wanted it that way to sell more airplanes.

Now I often run the same setting but add about 2 GPH, doing about the same speeds but TITs under 1600 and CHT's under 380. I know from experience this is far away from any setting that will cause detonation, the decreased CHTs are an indication that the internal cylinder pressures are well below peak conditions, and this should be a reasonable way to run the cylinders and turbo. Sometimes, depending on outside air temp and altitudes or some other factors I don't fully understand, I can lean down closer to 12.5 GPH while staying within these temperature limits, if I want a little more economy or range.

The extra 2-ish GPH paid at the pump is hopefully a tradeoff for increased cylinder and turbo life, and the plane has so much range it's easy to just add enough fuel to plan 13.5 GPH block on just about every flight.

If I need more economy, it's easy to just reduce MP a couple inches to 26 or so or remove 100 RPM to 2400 and get the fuel flow down around 10 GPH, and then run closer to peak TIT with a cool turbo and cylinders. But you do lose about 10 knots of speed.

I detect roughness at all LOP power settings I've tried and so don't use them. It's slight but there. I've wondered sometimes if different pilots are more or less sensitive to that, and if I flew in someone else's plane who prefers LOP, if I would detect roughness they do not.

I have not done the GAMI test nor installed GAMIs. I may do the test sometime for education's sake, but the plane is already so efficient it seems like a fair amount of hassle and cost for minimal gain.

[dkkim73]

1 hour ago, Z W said:

I have not done the GAMI test nor installed GAMIs. I may do the test sometime for education's sake, but the plane is already so efficient it seems like a fair amount of hassle and cost for minimal gain.

I think that a good argument for doing the profile runs and seeing cylinder spread on the data traces is to make sure you don't have an outlying cylinder that runs too close to peak, whether you're running ROP or LOP. 

It would also bear on smoothness. Smaller spreads should be better no matter where you're choosing to run it. 

[Pinecone]

I see no reason to burn extra fuel.

I get around 134 - 135 IAS on 10.1 GPH.   5 knots for 2.5 GPH???  Nope.

[Ragsf15e]

1 hour ago, Pinecone said:

I see no reason to burn extra fuel.

I get around 134 - 135 IAS on 10.1 GPH.   5 knots for 2.5 GPH???  Nope.

Yeah, I use 10.4 (~65%) and seem to get ~141kias.  But we’re both lean of peak and he said his airplane isn’t happy lean of peak.

Ill post the gami sweeps from my last saavy test flight so you guys can see one that’s matched can run lop.  It might take getting gamis if the stock injectors aren’t balanced.  The test is run at lower mp so that you can very slowly lean through peak without having high tit.  Around 21” / 2400 was comfortable.

[Ragsf15e]

Something like this for the Gami test.  I have a relatively new engine with ~200hrs on the mags.  Gami injectors and fine wire plugs.  This was a gami spread of 0.2.

You can upload and look at your data for free on the Savvy website.

[jlunseth]

10 minutes ago, Ragsf15e said:

 Gami injectors and fine wire plugs. 

 

For those on this thread who are considering experimenting with LOP, the even fuel flows are important, yes. But LOP also requires a strong spark. A mixture is "lean" because it has less fuel than what would be necessary to use up all the available O2 during combustion. In other words, the fuel in the mixture is somewhat rare. That slows the rate of combustion in the cylinder resulting in effect in a long slow push and because of the long slow push, cylinder temps will be cooler. Because fuel is a little rare, it is a little more difficult to ignite the mixture, hence the need for a strong spark. Most of us have found that fine wire spark plugs really help with that, and the mags need to be in good shape. Sometimes my engine will start to miss a little when LOP, when it is getting close to annual time. Then the annual happens and LOP performance is excellent. The reason is that the mags get checked and reset at annual every year.

[Ragsf15e]

7 minutes ago, jlunseth said:

For those on this thread who are considering experimenting with LOP, the even fuel flows are important, yes. But LOP also requires a strong spark. A mixture is "lean" because it has less fuel than what would be necessary to use up all the available O2 during combustion. In other words, the fuel in the mixture is somewhat rare. That slows the rate of combustion in the cylinder resulting in effect in a long slow push and because of the long slow push, cylinder temps will be cooler. Because fuel is a little rare, it is a little more difficult to ignite the mixture, hence the need for a strong spark. Most of us have found that fine wire spark plugs really help with that, and the mags need to be in good shape. Sometimes my engine will start to miss a little when LOP, when it is getting close to annual time. Then the annual happens and LOP performance is excellent. The reason is that the mags get checked and reset at annual every year.

I think a Surefly can help some with the spark as well. It’s strong and the timing never wanders off through the year between annuals.

[DCarlton]

On 4/15/2025 at 5:00 PM, Marc_B said:

@DCarlton This is a good article to read.  I think it better clarifies the red box/fin.  Simplifies it to use CHTs as the boundaries.  High CHTs bad.  Under a certain %HP, you'll never be able to get up to high CHTs.

https://www.savvyaviation.com/red-box-red-fin/

 

Ya, I don't think that sounds like current best practice!

I read the article.  Appreciate the technical details.  However, if I think back to my C-152 training days, I believe I was trained to lean the mixture until the engine started running rough and then put a few turns on the mixture control until it smooths out.  If you start with that basic procedure for cruise in a normally aspirated IO-360, it seems like you would always be below the red fin on the LOP side.  Keeps it simple; no EGT or Engine monitor required.  Then richen to taste with the monitor if you want more HP or to up your engine management game.  Am I missing any important nuggets?     

[Fly Boomer]

4 minutes ago, DCarlton said:

Am I missing any important nuggets?     

The only way to know if all cylinders are on the lean side of peak is to instrument all cylinders and then observe the temperatures.

[BillySpace]

Sounds like you’ve got a solid handle on your engine management. Running ROP should be fine for short bursts, but just keep an eye on the temps and pressures—especially if you’re pushing the speed. If you’re not seeing excessive temps and your engine’s happy, it’s probably okay for a quick run, but keep monitoring. Looks like you're dialed in pretty well!

[Ragsf15e]

16 hours ago, DCarlton said:

I read the article.  Appreciate the technical details.  However, if I think back to my C-152 training days, I believe I was trained to lean the mixture until the engine started running rough and then put a few turns on the mixture control until it smooths out.  If you start with that basic procedure for cruise in a normally aspirated IO-360, it seems like you would always be below the red fin on the LOP side.  Keeps it simple; no EGT or Engine monitor required.  Then richen to taste with the monitor if you want more HP or to up your engine management game.  Am I missing any important nuggets?     

You’re right that it’s probably lean of peak on all cylinders, but you don’t know unless you’ve checked each cylinder as it peaks.  If your fuel distribution isn’t good, you could have some ROP and some LOP.  It won’t hurt anything at a low enough power, but it’s not a good place to run at higher power settings.

[Will.iam]

I have a surefly and replaced both wiring harnesses with new Maggie harnesses and tempest fine wire plugs. The surefly gets me .5 less fuel for same performance and I can go about 70 degrees lop before roughness. I’m at .2 spread on my gami injectors. The gami sweep at 65% unfortunately doesn’t magnify your spread as much as higher power setting will. And if you start at poh book values for 65% they were for ROP flying so as you sweep for your gami test the power will be less than 65%. Mike Busch talks about the big mixture pull which he does at wide open throttle so he is sweeping through peak at full power but is rapidly doing it and does not even know what LOP setting he is at because he doesn’t measure his lop he measures CHT values.  He states from engine detonation testing in Ada OK that at the most destructive air fuel setting at full power they do not see detonation until the cht have climbed through 420 degrees so he knows even though he is leaning through the destructive zone because his cht’s are all below 380 he knows there is not detonation. Now if you go too slowly you will see your cht’s start rapidly rising but its not instantaneous and you do have seconds to get through it. My point is since my JPI records fuel flow and egt I did my gami sweep at 75% power and did it quickly then went back and downloaded the data to savvy aviation and used their gami spread chart to show that at 75% power it magnified my gami spread to greater than .5 so I was eligible for gami injectors which got me down to .2 my limitation is either cht 380 in the summer or 1585 tit which I hit in the winter at 31” 2300rpm 11.5 ff  gets me 75% power lop but I routinely fly 10.2 which keeps my tit below 1550. I would not be so worried about the red box as much as what the Redbox will do to your cht’s which is push them rapidly past 380 and above 400 I would really be worried that detonation is about to. begin. It’s your cht’s that dictate how close you can run to the Redbox. If you want a good wiring harness spark plug test on your next run up go to 1800rpm then lean until you get the rpm’s to drop to 1700 THEN do a left and right mag check. My mag will drop 150 more rpm but will still run with all spark plugs firing my surefly only drops by 75 rpm a testament to the hotter longer spark. If you have a weak spark plug or wire or partial fouled plug you will feel it in roughness or egt dropping (or rising higher if you are so lop that the flame front so slow it’s still burning as the exhaust valve opens up cooking your egt prob) point is the offending wire / plug will be an outliner to the other plugs wires when you go back and look at your engine data to identify your weak point.