Dan wrote:
Jim Petty has a fantastic little iOS app called AircraftPower
The App is just so good if you’re really interested in what your engine is doing. I used it since when I bought my plane, and for the next flight with two pilots on board have plans to understand and use it in flight. To see where I really am. We talked about this and believe that with some settings tricks we can spare another Gallon/hour on same speed.
igor wrote:
Am I understood correctly, that by leaning until roughness and then giving back 2-3 twists to enrich you should be in the ballpark of -50* F LOP EGT?
It’s impossible to say where you end with this trick. For example, on my engine, reducing to 2000 RPM I cannot really go LOP, as the EGT does not fall below peak upon leaning. When I set, say, 2500 RPM or more, I can perfectly lean down to 150°F LOP and have no roughness whatsoever (GAMIjectors installed). Another influencing factor is the output power set. For example, with my engine it’s not possible to fly far LOP during lower power like in a descent. I have to enrichen the mixture a bit, else it will run rough. And that’s all with the same engine, just different settings.
If you don’t have any precise instruments this “trick” is the best you can do, however. It should start running rough only in the region of peak EGT or maybe up to 100°F ROP. So this is more or less about where best economy is indicated in the POH. Just make sure that you stay on lower power outputs to not harm the engine, I’d say not more than 65% power, if you don’t have single-cylinder CHT. Maybe even less.
igor wrote:
Am I understood correctly, that by leaning until roughness and then giving back 2-3 twists to enrich you should be in the ballpark of -50* F LOP EGT?
You can’t be sure about that. Onset of roughness isn’t caused by leaning as such and thus has no direct relation to EGT. It’s caused by small differences in mixture between cylinders. As power drops rapidly lean of peak EGT it is likely that these small differences will give noticeably different power from different cylinders and thus roughness. That’s why you can buy “calibrated” injector sets for fuel-injected engines to ensure that all cylinders get the same mixture. It is more difficult with carburetted engines.
@igor wrote
Am I understood correctly, that by leaning until roughness and then giving back 2-3 twists to enrich you should be in the ballpark of -50* F LOP EGT?
I think what you describe is more 50-150 rich of peak?!
On a NA carb. engine you can try LOP by going high up (7000ft+), WOT and then lean as much as you can. A little carb heat helps (caution: some carbs will freeze and even full carb heat will then not fix it!).
giving back 2-3 twists to enrich you should be in the ballpark of -50* F LOP EGT?
Maybe, maybe not…
It has been my experience flying different engines in different aircraft, that some can’t be leaned LOP. Carb and mags equipped engines generally don‘t like to go into LOP. The best configuration to take advantage of the procedure is an injected/electronic ignition engine. Also note that the air inlet, air filter, etc, may also influence the capability.
Sidenote
Jim Petty has a fantastic little iOS app called AircraftPower. I use it mostly in flight to determine or set the power settings since my instrumentation is pretty basic (engine parameters on a VM1000). Sure beats reading lines on the Lycoming tables 
And the app is FREE!
Am I understood correctly, that by leaning until roughness and then giving back 2-3 twists to enrich you should be in the ballpark of -50* F LOP EGT?
Peter wrote:
Pull the mixture back, slowly, until you feel a sudden drop in power. It is very noticeable; with a fixed pitch prop especially your speed suddenly drops. Then enrich just enough to restore the bulk of the sudden drop. Now you should be at peak EGT or so.Most engines don’t run quite as smooth at peak EGT (or LOP) as they run when ROP. So, if there is too much vibration for your liking when you have done the above, just enrich a bit more.
Good description. I think the bottom line on a carburated engine in particular is that your fuel economy is set by what level of slight roughness (or margin from roughness) you’d like to tolerate, and therefore how much you enrichen from clearly rough. In that situation instrumentation is not really the key item. As with many things most of the benefit comes from doing something (attempting to lean by this method) versus doing nothing. For a carb engine that’s only burning say 8 US GPH anyway, being obsessive won’t save you a great deal of money per hour and in cruise CHT is usually not an issue for a smaller engine.
Once you’ve set the mixture in cruise attention to fuel flow over a couple of minutes will help check if your technique was about right, if you have a fuel flow gauge.
A CHT gauge is useless except for checking the max is not exceeded; usually this is possible only in slow high power flight.
The PA28s I have flown never had an EGT gauge but had CHT gauges. When I was doing the FAA IR is Arizona, 2006, the instructor showed me how to lean, by leaning for max CHT. I kept my mouth shut 
Peak CHT is reached at the wrong point.
I wrote some notes on this here and it mentions non-instrumented engines.
chflyer wrote:
That can’t be true. All Archers come with an engine gauge cluster including oil pressure, oil temp, and cyl head temp. Of those, only the cyl head temp is of any use for leaning. Its limitation is that it is only giving you info on the cyl containing the probe and relying on that one data point is risky as you don’t know if it’s the coolest, hottest, on in between.
Did you mean to write exhaust gas temp.? I’ve yet to see an Archer with an CHT gauge, but the one in my club (a PA-28-181 Archer II) has an EGT gauge. The EGT gauge is also described in the POH, while there is no mention of a CHT gauge so it is not a modification of that particular aircraft.
Anyway, a CHT gauge is not useful for leaning as the CHT doesn’t react quickly enough.
MedEwok wrote:
In fact, the PA28 doesn’t have a single temperature indication for the engine
That can’t be true. All Archers come with an engine gauge cluster including oil pressure, oil temp, and cyl head temp. Of those, only the cyl head temp is of any use for leaning. Its limitation is that it is only giving you info on the cyl containing the probe and relying on that one data point is risky as you don’t know if it’s the coolest, hottest, on in between.
MedEwok wrote:
Under such circumstances, how would you lean? Lean until engine runs rough, then enrichen the mixture a bit again?
Yes, that’s normally the recommended technique if the aircraft doesn’t have an engine monitor providing info on all cylinders. But check the cyl head temp anyway afterwards just to confirm that at least it is not over 400°F.
