Leaning to peak EGT

Ed Haywood

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Big Ed
I've read numerous threads on this topic that advise leaning to 100-125 degrees ROP to avoid detonation.

Was just reading the AEIO-360 Lycoming Operators Manual and it lists two leaning methods with an engine monitor: 125 ROP for best power, or peak EGT for best efficiency.

I've never used peak EGT. What concerns should I have about that method?
 
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None if you’re below 75% power.

Ok, so follow on question. I run mixture rich when operating above 75% power or when maneuvering such that frequent power changes are necessary. For example, yesterday I was doing some formation practice on a short XC. The other aircraft was slicker than mine, and I found it took 78-82% power to hold position. I was running mixture rich. Could I have leaned safely to 125 ROP?
 
I think most sources I’ve seen call for 65% or less for leaning peak egt. So for my cross counties, that is predominantly what I’ve done on my latest engine. It gives me my best combination of increased range with only insignificant loss of TAS, IMO. No info on long term engine benefits, but oil analysis consistently good.
 
Do you know your airplane well? I don’t look at EGTs. I look at fuel flows. Fly your plane enough to correlate them. Much easier and faster for me.
 
Do you know your airplane well? I don’t look at EGTs. I look at fuel flows. Fly your plane enough to correlate them. Much easier and faster for me.
That requires a fuel flow gauge. But even still, what do you trust more, the relative error produced by your egt sensors or fuel flow transducer. I have both, and while it is undoubtedly close, I think I can extract a lower fuel flow by using my egt. YMMV. The only caveat is you might be reducing your % power to lower than desired if you creep into LOP territory or allow your rpm to drop too far at peak.
 
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Do you know your airplane well? I don’t look at EGTs. I look at fuel flows. Fly your plane enough to correlate them. Much easier and faster for me.
Doesn't that mean that you have to have a specific fuel flow for each specific density altitude? And how do you determine optimal fuel flow in the first place?
 
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Every time I use the fancy LOP function on the Garmin it coincides exactly with audible power drop off. Pretty much the old lean until rough, enrich just back to smooth. So now I just quickly lean by ear.

Then, I just monitor CHTs to keep them 400 and below. Lycoming IO-360A3B6.
 
While in formation, it’s a good idea to run rich for that period if you’re having to maneuver in place. If you’re lead, then lean as normal.
 
Every time I use the fancy LOP function on the Garmin it coincides exactly with audible power drop off. Pretty much the old lean until rough, enrich just back to smooth. So now I just quickly lean by ear.

Then, I just monitor CHTs to keep them 400 and below. Lycoming IO-360A3B6.
So what do you think the lean by ear method results in? Peak, LOP, or ROP?
 
So what do you think the lean by ear method results in? Peak, LOP, or ROP?
EGT? Results in the last to peak cylinder being about 20 LOP and the other three further LOP. At this setting CHTs run 375-385, and oil temp 180-190.
 
Doesn't that mean that you have to have a specific fuel flow for each specific density altitude? And how do you determine optimal fuel flow in the first place?
Great questions!

“Everything should be made as simple as possible, but no simpler” - well, here (leaning) we have a topic where in my opinion a lot of pilots are happy with recipes which are a bit too simple.

If one leans purely based on fuel flow, then you are not accounting for altitude or outside air temperature. And if one leans purely based on CHT, then you are potentially masking high ICP (internal cylinder pressure) due to cold outside air. Plus CHTs lag behind, and can delay knowing the precise effect of your leaning by a minute or two. That's hardly a great tool for fine-tuning the mixture.

But what puzzles me most is that using EGT is dead simple. You look for peak, and then you lean or enrichen to whatever degrees from peak you want to run. It takes 5-10 seconds to do that - with no chance the cylinders will overheat during those few seconds. Then you know exactly where your engine is running. It's really not hard.

And yes, I know what Mike Busch says on leaning. And while he has written and said a lot of very good and helpful things, I believe his guidance in this case in not the best.

- Martin

 
I think if I leaned by sound to smoothness in my Lycoming O360A4M, I would likely be straddling peak with some cylinders LOP and some ROP or at peak. When done by EGT gauge stopping at first cylinder to peak, my CHTs have never been higher than 355-362 when at 65% power.
 
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Leaning by roughness will produce very different results in different engines. If carbed verses injected the difference can be dramatic.
 
Doesn't that mean that you have to have a specific fuel flow for each specific density altitude? And how do you determine optimal fuel flow in the first place?
I use fuel flow based on my MP setting, so I guess it takes care of itself. But I rarely fly above 3000' so I don't need to pay too much attention to altitude. My control of EGT is primarily to control CHT, and in my planes there's a clear correlation. Honestly I could set fuel flow based on CHT and it'd come out pretty much the same. I'm happy to run ROP in order to keep CHTs down. I can run LOP in my Cub but haven't done it much.
 
Back to the original question. I have an AEIO-360 with engine monitor and fuel flow. Lycoming says lean to Peak EGT below 75% power for best economy. Any pros, cons, or precautions to be aware of when using that method?
 
Back to the original question. I have an AEIO-360 with engine monitor and fuel flow. Lycoming says lean to Peak EGT below 75% power for best economy. Any pros, cons, or precautions to be aware of when using that method?
If by "best economy" you mean "most nautical miles per gallon of fuel burned", then running at peak EGT will not be best economy. You get more miles per gallon if you lean past peak EGT, to the lean side of peak. This will also result in cooler cylinders. If you run at peak EGT at 75% power, your cylinders will run hot - at 75% power, the red box is small but it still exists.

- Martin
 
To me, the performance graph in the POH indicating 75% economy leaned to peak egt is fools gold. You have the engine monitor with egt and more importantly cht to keep those temps 380 or lower ( my preference) so you might get away with it, or come close to 75%.
 
Another data point. My Tiger's carbureted O-360-A4K will run smoothly until the leanest cylinder is about 5-15 degrees F lean of peak (based on the display after leaning using the Lean Assist function of an EI-CGR30P engine analyzer). At lower power it seems to be able to remain smooth to a greater number of degrees LOP, and at higher power it will reach fewer degrees LOP before roughness begins. But in all cases all cylinders reach LOP (although the last one barely) and operation starts getting rough in that range when going leaner. So I typically just use the lean to rough and then enrich until just smooth method of leaning. Cylinder head temperatures are cold (in the winter 290-320 F in cruise at 75% power, in the summer 320-360F...probably colder than is best).
 
So what do you think the lean by ear method results in? Peak, LOP, or ROP?
It depends on how consistent the mixture distribution is through the intake manifold. As you lean, the engine gets rough when the leanest cylinder is too lean, though the others may be fine. So what's the difference between the leanest and richest cylinder? If that difference is small, then you can lean more, get a bigger RPM drop before the engine gets rough. In that case, "rich of rough" might go far enough to achieve LOP. If that mixture difference is big, one cylinder gets lean before the others, and leaning "rich of rough" won't go as lean and may still have the engine in the "red box", or the hottest zone of cylinder temperatures.

Engines like the O-320 and O-360 have the carb near the center so the intake runners are roughly the same length. This helps even the mixture distribution compared to asymmetric intake runners. If it's fuel injected you can size the injectors at each cylinder differently to compensate for any differences. But all of that also depends on the throttle setting - the mixture distribution can be more or less even at different throttle settings. For example with my O-360-A4M, at wide open throttle when the enrichment circuit engages, the mixture distribution becomes uneven. If I throttle back a fraction of an inch, just enough to disengage that circuit, the mixture distribution becomes more even and I can lean further for the same power level. The difference is significant - about 15% lower fuel burn rate for the same power/airspeed.

So, the short answer is, "it depends".
 
Engines like the O-320 and O-360 have the carb near the center so the intake runners are roughly the same length.

No, my O-360's carb is mounted on the back of the engine. Carb location is airplane dependant.
 
For carb aircraft, try cracking the carb heat slightly open. If running WOT, try pulling back the throttle a 1/4 inch. Both can enhance the ability to run leaner of peak.
 
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