Cruising Lean of Peak

[rodsmith]

As this is my first fuel injected aircraft, I have no experience cruising lean of peak. Today I took a 1 1/2 flight at 9000'. Full throttle MP was 22", I set rpm at 2200 and mixture 40 deg lean of peak which gave me a true airspeed of 126K, fuel burn of 10.2gph. The EGTs peaked at about 1400, so were running around 1360 with a total spread of 20deg. CHTs were mid 300s with the cowl flaps closed and OAT at 50. % power indicated 53%. My question for those who cruise lean of peak, how far lean of peak do you normally run. I know their is a trade off between speed and fuel economy. Any other considerations? Most of my cruising will be a minimum of 8000' even for short trips due to the terrain around here.

[AKKen07]

That’s a great number- 10.2. You can lean until the engine complains (stutters/skips etc.) and at your low power setting (which I think is about 60?) You won’t hurt anything. At high power settings like 75% plus my understanding is there may be risk of detonation.

[AKKen07]

Oh and I usually get about 10.5gph at 22 squared and that’s my standard cruise.

[svyolo]

I messed around a bunch with mine.  Going much further lean saves gas but costs speed.  The engine was also running so cool the cockpit heat was ineffective.  I think 30-50 LOP is 16-1 AFR which some say (not me, I do not know) is supposed to be optimum.  Again, not my opinion, as this is not my wheelhouse.

I should be back flying in a few days and should soon work on making my ignition a bit more optimum, which again is outside my wheel house.  My EI is currently running like a Mag. Fixed 25 deg.

I will be careful........

[Utah-Jay]

This LOP interests me, I have been researching it a bit the past few days, but I am carburated and have a spread of just over 100° on my EGT’s which worries me for trying to run LOP

[AKKen07]

An EGT spread is theoretically irrelevant. Absolute EGT values don’t mean much, but the trends you see for each cylinder are significant. For a plane with a fuel flow gauge it is the spread of fuel flow values present for each cylinder when the EGT peaks for that cylinder - regardless of the absolute EGT value - that matter. If you are at a conservative power setting and lean your engine, you can’t hurt it. It will quit before damage occurs. Whether or not you are lean of peak is up to the specific fuel distribution in your cylinders based on your carb. I think you should feel free to experiment at a conservative power setting without fear. If the engine starts to run rough add a little fuel and keep on trucking.

As a side note I have found that reducing my mixture veerrrrry slowly once I get past peak lets the engine keep up with the changes and run smoothly to a lower indicated fuel flow.

[rodsmith]

I have heard that with a carburated engine, you can play with throttle position to sometimes get a better fuel distribution to the cylinders.

[Hewko]

IIRC Mike Busch had seminar on lean of peak and said there was less chance of detonation LOP, as there was no extra fuel to turn into energy to do the damage. I ran a carbureted 160 hp 0320 with 1 Pmag LOP on floats for years with good fuel economy and no issues. LOP would drop my oil temp a few degrees.

[rodsmith]

When I said I had a 20 deg spread on EGT lean of peak, I meant on how far each cylinder was below peak. Actual EGT spread was 50 deg. So nice to have data logging!

[Utah-Jay]

Here are a few screenshots I’m different cruise configurations

IMG_6984.jpg IMG_6981.jpg IMG_6985.jpg IMG_6982.jpg

[Battson]

As this is my first fuel injected aircraft, I have no experience cruising lean of peak. Today I took a 1 1/2 flight at 9000'. Full throttle MP was 22", I set rpm at 2200 and mixture 40 deg lean of peak which gave me a true airspeed of 126K, fuel burn of 10.2gph. The EGTs peaked at about 1400, so were running around 1360 with a total spread of 20deg. CHTs were mid 300s with the cowl flaps closed and OAT at 50. % power indicated 53%. My question for those who cruise lean of peak, how far lean of peak do you normally run. I know their is a trade off between speed and fuel economy. Any other considerations? Most of my cruising will be a minimum of 8000' even for short trips due to the terrain around here.

At that altitude you can't really hurt it, provided you have a good balanced cylinders. So you can run as close to peak as you like - as long as you can keep the engine cool (personally, I try to keep it under 380*F, but this is very much personal preference).

At high altitude, it's really just another way of controlling power / CHT. Think of it like this, you cannot open the throttle further, but you can run closer to peak or higher RPM. Both result in more heating. Provided you can keep it cool, then you can go faster by running closer to peak or spinning the prop faster.

The settings you are using, are the same settings which I use, that's more or less my default. I do tend to run a little further away from the peak at lower altitudes though, more like 60*F colder. There's no science behind it, I just like a bigger margin of safety and to manage engine cooling, especially in summer.