Thanks! Have you noticed how you cam is shaking after you did all your adjustments? that is not an indication that the something not that good is happening? I would love to read your comments about this.
I just re-watched it and see what you mean. Looks like in demonstrating how low the fuel burn could be with leaning, I may have left it a little leaner than optimal. At the end of the day, you enrichen for smoothness as you adjust. This isn't a catastrophic problem though. A cylinder is just running leaner than the others meaning the power isn't balanced. Other than unwanted vibes, it won't cause a major issue.
I see some comments suggesting RPM & MP settings where MP is slightly higher than RPM... at the same time, I heard earlier, and I'm still learning hi-perf props, that MP (man. press.) that is higher than RPM puts excessive pressure on the engine and could damage the pistons/cylinders.. which of the two is correct? it's confusing me
This is the "over square" old wives tale I mention in the video. The simple answer is just look in the POH. Higher MP than RPM settings are in POHs all the time, even in turbo aircraft with higher than standard MP. I just looked up a turbo Piper Arrow POH and there is a 75% power setting in the table with 34.8" MP and 2300RPM. The NA Arrow POH has a 65% setting at 25.9" and 2100 RPM. These are cruise settings and not climb settings, but there is nothing wrong with them, in fact they are recommended by the manufacturer. Actually, if you want to run lean of peak EGT, it is beneficial to run at lower RPMs.
I loved the chart. I never considered that the EGT lags behind CHT. CHT is probably the more critical parameter, so I wonder why so many arrows have EGT. Maybe EGT is easier/cheaper. This may sound strange, but I tried leaning on a hot summer day at 3500' and about 75 degF and my EGT went almost full scale before I pulled back. What's the deal with that?
I think I have seen that in the past, and I think it was just not calibrated. It doesn't make sense that the exhaust gas would be way hotter in one application vs another. I'm not sure why more planes don't have cht gauges. The egt is important to understand what the mixture is though. Cht is dependent on more variables such as load and air flow over the heads, so it wouldn't be a very direct indicator of mixture.
Hmm, that makes sense, as EGT, is the actual result of what happened in the combustion chamber. In theory, one could analyze that gas and determine exactly what happened, how much fuel was not combusted, how much NOx, etc, and since the gas flows quickly the temperature should be fairly close to what it was in the cylinder. I like it.
KC do you have any idea how the rich /lean indicator correlates to air/fuel ratios? Most gas engines like to run in a fairly narrow AFR band and every kid with a tuner car has an AFR gauge to show the engine is in that band, or bad things can happen. That seems easier than than playing around with multiple factors to get a smooth, cool running engine. However it requires EFI air and fuel data sensors to calculate. Thx.
Best power mixture which is generally considered slightly rich of peak EGT (50 degrees rich I believe) is between 12 and 13 to 1. Peak EGT is at the Stoichiometric ratio of 14.7:1. We work with the instruments we have. Also most of these piston engine singles have fairly uneven fuel/air distribution and primitive fuel delivery systems. You can't really monitor fuel entering each cylinder directly like modern cars. You can monitor cylinder EGTs though.
@@kujoclips4962 KC thanks for that. I guess there is more AFR control than i would have thought. Which leaves premature engine failures an issue for those not paying att'n. As you say these air cooled engines are still in the dark ages in terms of fuel delivery. Need to pick up their game about 50 years. Lite jets have FADEC, A/T and syn vision so pilot just flies the plane, but learning pilots have to struggle with fuel mgm't. OK for good pilots like you but a risky distraction for student pilots.
@@robertrpenny It is remarkable how GA engine technology hasn't really kept up with automotive through the years. That said, a large number of new GA aircraft have FADEC options or at least electronic ignition and things like that. The majority of the fleet is still 10's of years old though so understanding this stuff is important. Probably good for students to understand how this stuff works anyway.
@@kujoclips4962 KC i guess student pilots are mostly in decades old planes since that may be all they can afford to rent. It is good for them to understand that everything is not plug and play so long as they dont have an accident trying to deal with flying and managing fuel system. Sort of like teaching a young person to drive. Easier to do in a new Honda with AT then an old Brit. sports car later. Which has 3x as many primitive issues to deal with.
I do think the camera being hard mounted to the airframe exasperates the vibration. That said, I think I leaned it a little far hence the additional vibration. I was so busy doing the video, I must not have noticed. Normally you would want to enrichen enough to smooth things out. This shouldn't be terrible on the engine other then added vibration isn't great. So here is a takeaway from the video, when leaning, enrichen for smoothness!
No we don't. I have read that Lycomings have better air/fuel distribution than Continentals so they can be generally leaned better than stock Continentals. I don't really have much experience with Continentals, but I can say the Arrow will go 25 degrees or so lean of peak without much vibration.
As a past Cedar Rapids resident it's cool to see the old airport again. I got my private and instrument ratings there. I kept my 79 Archer in the west T's. Now live in Tucson (Marana KAVQ) where we have A LOT of student traffic in the Phoenix - Tucson corridor. I miss the low traffic flying there in Iowa, but don't miss the weather.... Almost forgot my question!! Shouldn't your prop RPM always be higher, or equal to, your manifold pressure?
I kind of miss the low traffic of Iowa and my Arrow too! KARR isn't too bad though, and I have to say I love the Bonanza! Someday I plan to be up in Payson where I bought airpark land :). Your question pertains to the old over square debate. My understanding is that is a hold over from the turbo/supercharged radial days. It is perfectly fine to run over square in cruise on a NA GA aircraft. In fact, the power settings are right in the POH showing over square. You wouldn't want to climb out with RPMs set low, but in cruise it's fine. I tend to remember the Arrow POH even showed 2100RPM and 24" as an option. I'd be more concerned if the plane was turbo charged and running 43" or something like that, but by definition you would always be over square in that case.
@@kujoclips4962 I hadn't noticed that you'd moved, but I did see you moved up to a Bonanza... Everyone's dream aircraft... Traffic isn't terrible at KAVQ, but as you move north.. Casa Grande, Coolidge, Ak Chin, Williams Field and others, it really gets crazy!! Payson is a great place! Much cooler than Tucson. You will love it up there. Good luck! If you ever get down this way, KAVQ has a great diner. Sky Rider Cafe. I'll buy you breakfast.
Hey man, I've been sniffing around CZcams watching lean of peak rich of peak tutorials, how would you think an engine is controlled with things like manifold pressure fuel flow forced induction and RPM? I find it interesting because there's math out there that allow us to understand "the red box" and stochiometric ratios and perfect CHT, CHP. Math is efficient, but I still am learning it all,
I'm not sure what the specific question is, but I think you are asking how you control "power" with manifold pressure, mixture, and RPM. The simple answer is the prop pitch (RPM) controls the load on the engine. At low RPM setting, there is the most load for a given RPM on the engine, at high RPM the opposite, and there is a RPM setting where the prop is most efficient. Above ~2700RPM or so in this case, the prop tips lose efficiency due to higher speeds I believe. The manifold pressure is an indirect measure of throttle position and load. With low throttle settings, the throttle plate is mostly closed creating engine vacuum (low manifold pressure). When you firewall it, the throttle plate is wide open resulting in minimal vacuum or near atmospheric pressure. If you have a turbo, that high manifold pressure will be above normal atmosphere. The power the engine develops is a combination of load, throttle, and mixture. There are different combinations that result in a given power as you can see in the POH power settings.
I've never heard of anyone running LOP with just a single EGT gage. Sounds risky. You don't really know where the other cylinders are at temperature wise. There is a "danger zone" right around peak EGT. I don't think you want to be within 25 degrees of it. I definitely could be wrong. I've talked to a few mechanics who LOVE LOP since they get to change cylinders more often! :)
Agreed that a real engine monitor is critical and relatively cheap insurance for your engine. I generally flew that arrow around 65% power, so the red box doesn't really apply. The danger zone is really around 50 degrees rich of peak closer to 75% power and above, so if you are running pretty lean on one cylinder, you should have some margin unless your fuel distribution is way off. I'll also note that engine was rock solid and seemingly did not burn a drop of oil in the years I had it. I do plan to have a real engine analyzer in any plane I buy moving forward though.
The POH doesn't show any power settings using 1900RPM so I would probably avoid it. I don't know that it would necessarily hurt anything, but think about it as shifting your car into 5th gear. You should not be under high load conditions if you do it.
@@kujoclips4962 for economy cruise it would seem that 19k rpm and 20MP would work lean of peak. I downloaded the Operations pdf for lycoming io360 and there is nothing that I can find that precludes such operation. I expect you to pick up at least another 2gph
In fact, I am confident, you may safely operate even lower RPM, as long as the power setting is 75% or below, which is around 20MP, and you have good charging and oil pressure.
Absolutely, however engines greatly benefit under operations at designed torque loads. Any more may lead to wear that may occur under loads not designed. Another slight problem is carbon buildup at low rpms. Both of these are nitpicking however I’d consider
Just FYI, there are some myths regarding cylinder head damage from running at peak EGT at 75% power or less that carried over from radial engines and may not apply to modern engines. The AOPA has an interesting article about this.
Generally no. In the arrow I fly it really doesn't apply anyway considering the Hartzell Scimitar prop cruise RPM is 2300. If I limited myself to 23 inches and 2300 RPM, I would only be running
Full power takeoff to /400' 2850/FT, then 25 squared reduce ff to about 18 gph. At cruise 2300/23" adjust FF to 25 LOP, cht below 380 normally somewhere around 12-14 gph IO520 in 182. I cruise 65% or below, more efficient and have shutdown a few engines, so I'm paranoid.
Everyone should get out of the dinosaur era and follow Mike Busch’s leaning method whether it’s an aero club plane or not. Set for 2350 rpm at cruise, start leaning while rpm drops then gets “slightly rough” and richen just enough for smooth rpm again. This gets you 2300 rpm cruise with nice cool cyl head temps and clean plugs forever. Do it this way whether it’s a constant speed prop or not and forget that useless egt gauge, it’s all about the cyl head temps. Keep em below 380 and the engine will last a life time. Egt means wasted gas going out the pipe and that’s all it means, it has nothing to do with engine damage, that’s what your old moron flight instructor will tell you though.
Thanks for the advice. Could you provide any references or links that explain in more depth how the method works on a technical level and why it is a good way to go?
I'm not sure I understand your comment, but I would also point to the adage never say never. If you are climbing from sea level to 9500ft in an NA aircraft and never lean, you will be way too rich at those higher altitudes. In fact, my factory fuel flow gauge in the bonanza shows where to lean to at different altitudes during climb. If you are full rich at takeoff and record the EGT, you should absolutely be able to lean during climb to maintain that same EGT and you will have the same air/fuel ratio at 7000ft as you did at 1000ft which should be plenty rich. That is usually around 1300 degrees. In a turbo, this doesn't apply.
@@igor-orzhevskii Laughing. You are not a pilot. And obviously should not be touching anyones engine. Most should be leaning during taxi to prevent fouling plugs. Set the mixture usually about 1/3rd down from full rich REGARDLESS OF DENSITY ALTITUDE. Leaning in cruise is critical and most do it by FIRST setting the throttle to your desired manifold and RPM (70% power or greater), then reducing the mixture gradually until the engine runs a little rough, then slowly enrich the mixture until the engine smooths out. No one should lean in a climb as the risk of starving the engine of fuel and reducing power below a critical level is greater than achieving any kind of fuel savings... The downside to leaning is if you run the engine too lean, you can have preignition, detonation, and/or significantly reduce the life of the engine or worse, damage pistons requiring a major overhaul.
@@speedomars you can think whatever you want. We were talking about 'leaning mixture during a climb', not how it's done on the ground or during a cruise phase. If you're in a continuous climb from, say, MSL and up to the absolute ceiling of your airplane and you're not leaning, then you just wont make it up there. Your engine is running on an imbalanced fuel/air mixture as your progressively go into a high density altitude. Period. Again, I didn't mention here that a primary reference should be EGT or CHT for a multi-probe engine monitor. If cylinders getting too hot, then you obviously should reduce power/enrich/decrease pitch attitude/open cowl flaps etc. Flying full-rich really is a waste, will foul your plugs, will increase your risk of CO2 poisoning in the case of an exhaust leak, and will reduce your performance (range). You can't starve your engine on fuel, unless you abruptly pull back the lever and that's why there's a Vernier mechanism located on the knob, to precisely adjust it little by little. For additional information, one should consult his POH and engine operator's manual.
I am a flightsim enthusiast that loves props. I really enjoy this video talking about mixture controls and EGT.
The Deakin article was in-depth, but easy to understand so it didn't take too long to read. Thanks for the tip.
Great video, very detailed yet concise. Thanks.
Thanks for a very informative presentation.
Interesting...I was taught that from day one and still have instructors teaching it.
Thanks! Have you noticed how you cam is shaking after you did all your adjustments? that is not an indication that the something not that good is happening? I would love to read your comments about this.
I just re-watched it and see what you mean. Looks like in demonstrating how low the fuel burn could be with leaning, I may have left it a little leaner than optimal. At the end of the day, you enrichen for smoothness as you adjust. This isn't a catastrophic problem though. A cylinder is just running leaner than the others meaning the power isn't balanced. Other than unwanted vibes, it won't cause a major issue.
I see some comments suggesting RPM & MP settings where MP is slightly higher than RPM...
at the same time, I heard earlier, and I'm still learning hi-perf props, that MP (man. press.) that is higher than RPM puts excessive pressure on the engine and could damage the pistons/cylinders..
which of the two is correct? it's confusing me
This is the "over square" old wives tale I mention in the video. The simple answer is just look in the POH. Higher MP than RPM settings are in POHs all the time, even in turbo aircraft with higher than standard MP. I just looked up a turbo Piper Arrow POH and there is a 75% power setting in the table with 34.8" MP and 2300RPM. The NA Arrow POH has a 65% setting at 25.9" and 2100 RPM. These are cruise settings and not climb settings, but there is nothing wrong with them, in fact they are recommended by the manufacturer. Actually, if you want to run lean of peak EGT, it is beneficial to run at lower RPMs.
I loved the chart. I never considered that the EGT lags behind CHT. CHT is probably the more critical parameter, so I wonder why so many arrows have EGT. Maybe EGT is easier/cheaper. This may sound strange, but I tried leaning on a hot summer day at 3500' and about 75 degF and my EGT went almost full scale before I pulled back. What's the deal with that?
I think I have seen that in the past, and I think it was just not calibrated. It doesn't make sense that the exhaust gas would be way hotter in one application vs another. I'm not sure why more planes don't have cht gauges. The egt is important to understand what the mixture is though. Cht is dependent on more variables such as load and air flow over the heads, so it wouldn't be a very direct indicator of mixture.
Hmm, that makes sense, as EGT, is the actual result of what happened in the combustion chamber. In theory, one could analyze that gas and determine exactly what happened, how much fuel was not combusted, how much NOx, etc, and since the gas flows quickly the temperature should be fairly close to what it was in the cylinder. I like it.
Some older planes (notably the Electra 10-E, and also I think the DC-3) actually have an instrument which does that -- the Cambridge gas analyzer!
KC do you have any idea how the rich /lean indicator correlates to air/fuel ratios? Most gas engines like to run in a fairly narrow AFR band and every kid with a tuner car has an AFR gauge to show the engine is in that band, or bad things can happen. That seems easier than than playing around with multiple factors to get a smooth, cool running engine. However it requires EFI air and fuel data sensors to calculate. Thx.
Best power mixture which is generally considered slightly rich of peak EGT (50 degrees rich I believe) is between 12 and 13 to 1. Peak EGT is at the Stoichiometric ratio of 14.7:1. We work with the instruments we have. Also most of these piston engine singles have fairly uneven fuel/air distribution and primitive fuel delivery systems. You can't really monitor fuel entering each cylinder directly like modern cars. You can monitor cylinder EGTs though.
@@kujoclips4962 KC thanks for that. I guess there is more AFR control than i would have thought. Which leaves premature engine failures an issue for those not paying att'n. As you say these air cooled engines are still in the dark ages in terms of fuel delivery. Need to pick up their game about 50 years. Lite jets have FADEC, A/T and syn vision so pilot just flies the plane, but learning pilots have to struggle with fuel mgm't. OK for good pilots like you but a risky distraction for student pilots.
@@robertrpenny It is remarkable how GA engine technology hasn't really kept up with automotive through the years. That said, a large number of new GA aircraft have FADEC options or at least electronic ignition and things like that. The majority of the fleet is still 10's of years old though so understanding this stuff is important. Probably good for students to understand how this stuff works anyway.
@@kujoclips4962 KC i guess student pilots are mostly in decades old planes since that may be all they can afford to rent. It is good for them to understand that everything is not plug and play so long as they dont have an accident trying to deal with flying and managing fuel system. Sort of like teaching a young person to drive. Easier to do in a new Honda with AT then an old Brit. sports car later. Which has 3x as many primitive issues to deal with.
Hello, Arrow II owner here. Im unable to find John Deakin Mixture Magic text on Avweb. Do you have a link to share ? Thanks in advance.
www.avweb.com/features_old/pelicans-perch-18mixture-magic/
That camera really gets the shakes when you coarse out the prop. Is it noticeable in the cabin?
I do think the camera being hard mounted to the airframe exasperates the vibration. That said, I think I leaned it a little far hence the additional vibration. I was so busy doing the video, I must not have noticed. Normally you would want to enrichen enough to smooth things out. This shouldn't be terrible on the engine other then added vibration isn't great. So here is a takeaway from the video, when leaning, enrichen for smoothness!
Do you have GAMI injectors?
No we don't. I have read that Lycomings have better air/fuel distribution than Continentals so they can be generally leaned better than stock Continentals. I don't really have much experience with Continentals, but I can say the Arrow will go 25 degrees or so lean of peak without much vibration.
I run 150 degrees lean of peak.
As a past Cedar Rapids resident it's cool to see the old airport again. I got my private and instrument ratings there. I kept my 79 Archer in the west T's. Now live in Tucson (Marana KAVQ) where we have A LOT of student traffic in the Phoenix - Tucson corridor. I miss the low traffic flying there in Iowa, but don't miss the weather.... Almost forgot my question!! Shouldn't your prop RPM always be higher, or equal to, your manifold pressure?
I kind of miss the low traffic of Iowa and my Arrow too! KARR isn't too bad though, and I have to say I love the Bonanza! Someday I plan to be up in Payson where I bought airpark land :).
Your question pertains to the old over square debate. My understanding is that is a hold over from the turbo/supercharged radial days. It is perfectly fine to run over square in cruise on a NA GA aircraft. In fact, the power settings are right in the POH showing over square. You wouldn't want to climb out with RPMs set low, but in cruise it's fine. I tend to remember the Arrow POH even showed 2100RPM and 24" as an option. I'd be more concerned if the plane was turbo charged and running 43" or something like that, but by definition you would always be over square in that case.
@@kujoclips4962 I hadn't noticed that you'd moved, but I did see you moved up to a Bonanza... Everyone's dream aircraft... Traffic isn't terrible at KAVQ, but as you move north.. Casa Grande, Coolidge, Ak Chin, Williams Field and others, it really gets crazy!! Payson is a great place! Much cooler than Tucson. You will love it up there. Good luck! If you ever get down this way, KAVQ has a great diner. Sky Rider Cafe. I'll buy you breakfast.
BTW, here is a great dissertation on operating over square by Savvy - czcams.com/video/A1fmNFWNHD4/video.html
Hey man, I've been sniffing around CZcams watching lean of peak rich of peak tutorials, how would you think an engine is controlled with things like manifold pressure fuel flow forced induction and RPM? I find it interesting because there's math out there that allow us to understand "the red box" and stochiometric ratios and perfect CHT, CHP. Math is efficient, but I still am learning it all,
I'm not sure what the specific question is, but I think you are asking how you control "power" with manifold pressure, mixture, and RPM. The simple answer is the prop pitch (RPM) controls the load on the engine. At low RPM setting, there is the most load for a given RPM on the engine, at high RPM the opposite, and there is a RPM setting where the prop is most efficient. Above ~2700RPM or so in this case, the prop tips lose efficiency due to higher speeds I believe. The manifold pressure is an indirect measure of throttle position and load. With low throttle settings, the throttle plate is mostly closed creating engine vacuum (low manifold pressure). When you firewall it, the throttle plate is wide open resulting in minimal vacuum or near atmospheric pressure. If you have a turbo, that high manifold pressure will be above normal atmosphere. The power the engine develops is a combination of load, throttle, and mixture. There are different combinations that result in a given power as you can see in the POH power settings.
I've never heard of anyone running LOP with just a single EGT gage. Sounds risky. You don't really know where the other cylinders are at temperature wise. There is a "danger zone" right around peak EGT. I don't think you want to be within 25 degrees of it. I definitely could be wrong. I've talked to a few mechanics who LOVE LOP since they get to change cylinders more often! :)
Agreed that a real engine monitor is critical and relatively cheap insurance for your engine. I generally flew that arrow around 65% power, so the red box doesn't really apply. The danger zone is really around 50 degrees rich of peak closer to 75% power and above, so if you are running pretty lean on one cylinder, you should have some margin unless your fuel distribution is way off. I'll also note that engine was rock solid and seemingly did not burn a drop of oil in the years I had it. I do plan to have a real engine analyzer in any plane I buy moving forward though.
If you're at 75% power you can pretty much lean it to whatever you want...
So on a fuel injected Turbo engine you cant control mixture or manifold pressure so how do you control the prop just rpm 🤷♂️any help ?
The way I like to do it is, 50 degrees rich of peak if I want maximum power, or 25 degrees lean of peak if I want best fuel economy!
"Um" ~Kujo
Inertia drive and counter weight
Could you cruise with the rpm at 1900?
The POH doesn't show any power settings using 1900RPM so I would probably avoid it. I don't know that it would necessarily hurt anything, but think about it as shifting your car into 5th gear. You should not be under high load conditions if you do it.
@@kujoclips4962 for economy cruise it would seem that 19k rpm and 20MP would work lean of peak. I downloaded the Operations pdf for lycoming io360 and there is nothing that I can find that precludes such operation. I expect you to pick up at least another 2gph
Mooney guys have a very similar engine and do it all the time it seems
In fact, I am confident, you may safely operate even lower RPM, as long as the power setting is 75% or below, which is around 20MP, and you have good charging and oil pressure.
Absolutely, however engines greatly benefit under operations at designed torque loads. Any more may lead to wear that may occur under loads not designed. Another slight problem is carbon buildup at low rpms. Both of these are nitpicking however I’d consider
⭐⭐⭐⭐⭐
Just FYI, there are some myths regarding cylinder head damage from running at peak EGT at 75% power or less that carried over from radial engines and may not apply to modern engines. The AOPA has an interesting article about this.
Dyl Connaway where did you find it ?
ok so you dont do the "25 squared" thing then?
Generally no. In the arrow I fly it really doesn't apply anyway considering the Hartzell Scimitar prop cruise RPM is 2300. If I limited myself to 23 inches and 2300 RPM, I would only be running
Full power takeoff to /400' 2850/FT, then 25 squared reduce ff to about 18 gph. At cruise 2300/23" adjust FF to 25 LOP, cht below 380 normally somewhere around 12-14 gph IO520 in 182. I cruise 65% or below, more efficient and have shutdown a few engines, so I'm paranoid.
Everyone should get out of the dinosaur era and follow Mike Busch’s leaning method whether it’s an aero club plane or not. Set for 2350 rpm at cruise, start leaning while rpm drops then gets “slightly rough” and richen just enough for smooth rpm again. This gets you 2300 rpm cruise with nice cool cyl head temps and clean plugs forever. Do it this way whether it’s a constant speed prop or not and forget that useless egt gauge, it’s all about the cyl head temps. Keep em below 380 and the engine will last a life time. Egt means wasted gas going out the pipe and that’s all it means, it has nothing to do with engine damage, that’s what your old moron flight instructor will tell you though.
Thanks for the advice. Could you provide any references or links that explain in more depth how the method works on a technical level and why it is a good way to go?
@@n124lp czcams.com/video/_VfiPuheeGw/video.html
@@n124lp This is probably the best: czcams.com/video/h3bATVXMHQg/video.html
NEVER LEAN on ascent. Lean in cruise and adjust the mixture to prevent the engine from running or missing on descent.
I'm not sure I understand your comment, but I would also point to the adage never say never. If you are climbing from sea level to 9500ft in an NA aircraft and never lean, you will be way too rich at those higher altitudes. In fact, my factory fuel flow gauge in the bonanza shows where to lean to at different altitudes during climb. If you are full rich at takeoff and record the EGT, you should absolutely be able to lean during climb to maintain that same EGT and you will have the same air/fuel ratio at 7000ft as you did at 1000ft which should be plenty rich. That is usually around 1300 degrees. In a turbo, this doesn't apply.
@@igor-orzhevskii Laughing. You are not a pilot. And obviously should not be touching anyones engine.
Most should be leaning during taxi to prevent fouling plugs. Set the mixture usually about 1/3rd down from full rich REGARDLESS OF DENSITY ALTITUDE. Leaning in cruise is critical and most do it by FIRST setting the throttle to your desired manifold and RPM (70% power or greater), then reducing the mixture gradually until the engine runs a little rough, then slowly enrich the mixture until the engine smooths out. No one should lean in a climb as the risk of starving the engine of fuel and reducing power below a critical level is greater than achieving any kind of fuel savings...
The downside to leaning is if you run the engine too lean, you can have preignition, detonation, and/or significantly reduce the life of the engine or worse, damage pistons requiring a major overhaul.
@@speedomars you can think whatever you want. We were talking about 'leaning mixture during a climb', not how it's done on the ground or during a cruise phase. If you're in a continuous climb from, say, MSL and up to the absolute ceiling of your airplane and you're not leaning, then you just wont make it up there. Your engine is running on an imbalanced fuel/air mixture as your progressively go into a high density altitude. Period.
Again, I didn't mention here that a primary reference should be EGT or CHT for a multi-probe engine monitor. If cylinders getting too hot, then you obviously should reduce power/enrich/decrease pitch attitude/open cowl flaps etc. Flying full-rich really is a waste, will foul your plugs, will increase your risk of CO2 poisoning in the case of an exhaust leak, and will reduce your performance (range). You can't starve your engine on fuel, unless you abruptly pull back the lever and that's why there's a Vernier mechanism located on the knob, to precisely adjust it little by little.
For additional information, one should consult his POH and engine operator's manual.