Need help leaning by EGT

[kicktireslightfires]

I'm a little confused on how to lean by EGT. The POH for my Cessna 162 says to lean the mixture until it reaches peak EGT. But what is peak EGT? Is that when the EGT indicator reaches the very top of the digital EGT gauge? I thought so but when I was out flying and slowly started leaning it, the RPMs dropped long before the EGT gauge got to the top and as soon as the RPMs started to drop, I richened the mixture up a bit.

Any insight on this would be appreciated as the POH doesn't seem to define exactly what temp peak EGT is.

Thanks!

 

[luvflyin]

I'm a little confused on how to lean by EGT. The POH for my Cessna 162 says to lean the mixture until it reaches peak EGT. But what is peak EGT? Is that when the EGT indicator reaches the very top of the digital EGT gauge? I thought so but when I was out flying and slowly started leaning it, the RPMs dropped long before the EGT gauge got to the top and as soon as the RPMs started to drop, I richened the mixture up a bit.

Any insight on this would be appreciated as the POH doesn't seem to define exactly what temp peak EGT is.

Thanks!

Supposed to work like that. MAX EGT is about fuel econony, not power.

 

[Fearless Tower]

I'm a little confused on how to lean by EGT. The POH for my Cessna 162 says to lean the mixture until it reaches peak EGT. But what is peak EGT? Is that when the EGT indicator reaches the very top of the digital EGT gauge? I thought so but when I was out flying and slowly started leaning it, the RPMs dropped long before the EGT gauge got to the top and as soon as the RPMs started to drop, I richened the mixture up a bit.

Any insight on this would be appreciated as the POH doesn't seem to define exactly what temp peak EGT is.

Thanks!

Peak EGT refers to the highest you can get the EGT when leaning. It may never get to the actual top.

As you pull the mixture back, the EGT will rise. If you keep leaning it, at some point it will start to drop. That is Lean of Peak. Enrich until it gets as high as it will go and that is Peak.

 

[Salty]

Peak EGT is the point where it stops rising any more, not the top of the gauge. As you lean, EGT goes up, and then it starts going down.

 

[Hang 4]

There's no set EGT temp that equals peak. As others have said, peak is when it gets as high as it gets and then starts to drop. Note the peak temp as you lean and then enrichen to whatever temp rich of peak you want to run. Or just lean until it stumbles and then richen til it smooths out.

 

[David Megginson]

Peak power happens around 100°F rich of peak EGT, so as you keep leaning past there, power will fall, as you correctly observed. The people writing those POHs can be ignorant about how our piston engines work.

Leaning to peak EGT with a single-probe EGT gauge isn't a great idea, because there will be a temperature spread among the cylinders, so one or more might land at 50°F ROP, where the highest CHTs and internal cylinder pressures occur. There are three reasonably safe choices without a fancy engine monitor for an aircraft with a fixed-pitch prop:

1. Restrict yourself to below 65% power, and then it won't matter where you lean (as far as CHTs are concerned).
2. Lean to peak RPM (with a fixed-pitch prop) and then enrich slightly. That should land you around 150°F ROP, which gives a bit of a safety margin.
3. (Unfortunately, your POH says Continental prohibits this) leave the throttle wide open and lean to your target RPM. This will have your engine operating cool (well lean of peak EGT), using 15% less gas at the same airspeed, leaving virtually no lead deposits, and producing very little carbon monoxide. Unfortunately, not all engines run smoothly lean of peak EGT, so then you have to revert to option 1 or 2.

 

[kicktireslightfires]

Thanks ALL! Much appreciated!

So I assume then it's better to lean via engine RPM and just twist out slowly until the RPMs spike and then start to drop and then quickly twist the mixture back in a couple turns? ... Rather than using the EGT temp and twisting out slowly until the EGT temp stops rising? Because David makes a good point that this appears to be only a single-probe EGT.

David, curious if #3 is the best option, why would Continental prohibit it? Is it the more basic/dumb engines that don't work well for this and that is why and only more advanced engines do well with this technique?

 

[MauleSkinner]

Thanks ALL! Much appreciated!

So I assume then it's better to lean via engine RPM and just twist out slowly until the RPMs spike and then start to drop and then quickly twist the mixture back in a couple turns? ... Rather than using the EGT temp and twisting out slowly until the EGT temp stops rising? Because David makes a good point that this appears to be only a single-probe EGT.

It’s always worked out to be pretty close to the same place for me. One airplane I flew had both a single probe EGT and an all-cylinder engine monitor. Leaning by either of those or engine roughness worked out the same.

that doesn’t mean it’ll be universal, but it seemed to work out on all of the airplanes I tried it. The only way to know for sure on your engine is to try it both ways.

 

[David Megginson]

Thanks ALL! Much appreciated!

So I assume then it's better to lean via engine RPM and just twist out slowly until the RPMs spike and then start to drop and then quickly twist the mixture back in a couple turns? ... Rather than using the EGT temp and twisting out slowly until the EGT temp stops rising? Because David makes a good point that this appears to be only a single-probe EGT.

David, curious if #3 is the best option, why would Continental prohibit it? Is it the more basic/dumb engines that don't work well for this and that is why and only more advanced engines do well with this technique?

There's a long history behind that. There was a 20+ year debate about running lean of peak (even though the old propliners did it routinely in the 1950s); the debate is pretty-much over now, and most people accept that LOP is a legitimate way to operate an airplane engine, assuming can run smoothly that way.

The engine manufacturers used to be convinced that we GA pilots were too ignorant to operate an engine LOP, and until about 20 years ago, there wasn't good enough instrumentation in the typical GA light piston to do it safely with a constant-speed prop (it's much easier with a fixed-pitch prop). Also, a lot of old timers — not just pilots, but GA mechanics — repeated the old myth that "leaner" always means "hotter", despite the fact that the charts published in Lycoming's and Continental's own manuals contradict that.

Lycoming eventually relented a bit, and shows LOP as "Best Economy Range" in the O-320 Operator's Manual, even though they still don't explicitly recommend it in the written procedures; Piper went a step further in the early 1980s and actually recommended lean-of-peak in new revision of the PA-28-161 (Warrior II) POH, but that's still a rarity. AFAIK, Continental is still adamant that we're all too stupid to run their engines any way but very rich, though the instructions quoted from the C-162 POH would actually put the engine near the worst possible place (hottest CHTs are at 50°F rich of peak EGT, not lean of peak).

 

[flyingcheesehead]

So I assume then it's better to lean via engine RPM and just twist out slowly until the RPMs spike and then start to drop and then quickly twist the mixture back in a couple turns? ... Rather than using the EGT temp and twisting out slowly until the EGT temp stops rising? Because David makes a good point that this appears to be only a single-probe EGT.

Try it both ways. The EGT peak may be more noticeable. The RPM will peak before (rich of) where the EGT peaks. That's because best power mixture is going to be roughly 100ºF rich of peak.

Scientifically, theoretically, here's what's happening. You're pulling a mixture of fuel and air into your engine, and it's burning. Peak EGT is the point where you've got the perfect stoichiometric mixture of fuel and air, and there will be neither any oxygen nor any unburned fuel coming out of your exhaust. If you're rich of peak, all the oxygen will be used up but you're putting more fuel in than you have oxygen to burn, so some of the fuel will come out the exhaust unburned. If you're lean of peak, the opposite is happening - All the fuel is burned, but there's still some left-over oxygen.

The best economy mixture, therefore, is usually going to be roughly at peak EGT. The reason the best power mixture is rich of peak is that the same amount of fuel is being burned (because we've got about the same amount of air to burn it with), but because there are more fuel molecules hanging around near the oxygen molecules, the burn happens faster. Now, we say our fuel burn is higher, but in reality, we're USING more fuel, not BURNING more fuel because we're blowing some fuel out the exhaust. (* Yes, if you're truly pedantic - and frankly, I am - This may not be 100% true, but I'm trying to avoid making this even more complex by talking about flame propagation.)

David, curious if #3 is the best option, why would Continental prohibit it? Is it the more basic/dumb engines that don't work well for this and that is why and only more advanced engines do well with this technique?

I hate to break it to you, but all of our airplane engines are basic/dumb for the most part. We're using 1950's engine technology, there's nothing "smart" about any of the engines except the ones that have an "F" in the model number, but those literally never get used in certified airplanes... And they wouldn't have a mixture control to worry about if you did have one.

 

[David Megginson]

Try it both ways. The EGT peak may be more noticeable. The RPM will peak before (rich of) where the EGT peaks. That's because best power mixture is going to be roughly 100ºF rich of peak.

The the technique in the POH extract that the OP shared in the first post—lean to peak EGT and then enrich slightly—will put the engine exactly on the place where hottest CHT occurs, around 50°F rich of peak EGT. There is no worse way to operate your engine than that. Unfortunately, it has been in POH's and earlier operating manuals for many decades and brainlessly copied forward. Fortunately, as long as you're operating below 65% power, you'll be OK with any type of leaning, but still, this one will give you higher CHTs than any other.

I'm generally a fan of following the recommendations in a POH, but this one is so obviously wrong that you need to walk away from it. This diagram is for my Lycoming engine (the overlays in colour are mine), but all it applies to all aircraft piston engines, and shows what happens if you follow the POH recommendation:

 

[Clip4]

Due a number of factors, peak EGT is not an absolute value that the manufacturer can tell you to use.

 

[David Megginson]

Due a number of factors, peak EGT is not an absolute value that the manufacturer can tell you to use.

Right — peak EGT is a relative value, the point where it stops rising and starts falling again as you lean at any given power setting. As you point out, the exact temperature is meaningless (unlike with CHT).

As a relative value, peak EGT is useful only if you have probes in all cylinders and you know what to do with it (when EGT peaks, you need to keep leaning to move away from peak CHT; not enrich to move towards it).

 

[GeorgeC]

I find my 4 point CHT/EGT to be of limited value with the O-200.

Level off, throttle back to 2400-2450, lean until it's rough, enrich until it's smooth and sounds good, ends up being around 5gph if you have fuel flow. Don't worry, be happy.

 

[David Megginson]

I find my 4 point CHT/EGT to be of limited value with the O-200.

Level off, throttle back to 2400-2450, lean until it's rough, enrich until it's smooth and sounds good, ends up being around 5gph if you have fuel flow. Don't worry, be happy.

Agreed that leaning with a fixed-pitch prop doesn't need to be complicated (and doesn't require an engine monitor), but instead of leaning to roughness, I'd suggest leaning to peak RPM before enriching slightly.

You're probably fine at a low power setting like that regardless, but leaning to roughness isn't a good idea, because where that happens depends on the mixture spread among cylinders and varies from engine to engine (the O-320-D3G on my Piper PA-28-161 will lean right to idle cutoff without significant roughness; other engines might run rough while some cylinders are still rich, so enriching slightly would put them right in the red zone around 50°F ROP).

Since my engine happens to run smoothly lean of peak EGT, I normally operate it LOP-WOT, which also doesn't require an engine monitor (with a fixed-pitch prop): I just leave the throttle wide open, then lean until I hit my target RPM. Also a case of "don't worry; be happy."

Unfortunately, many engines won't operate smoothly there, so it's not an option. And things get much more complicated with a constant-speed prop.

 

[flyingcheesehead]

The the technique in the POH extract that the OP shared in the first post—lean to peak EGT and then enrich slightly—will put the engine exactly on the place where hottest CHT occurs, around 50°F rich of peak EGT. There is no worse way to operate your engine than that.

My point wasn't that the OP should operate in any particular range - I don't disagree at all with your assessment (which I think should be clear after you read my whole post).

My point was that leaning solely via RPM, it may be harder to find the peak than it is with EGT. It'd be interesting to see, presuming a resolution of 10 RPM on the tach display, what the range of fuel flows and EGTs is going from peak RPM - 10 through peak and down to peak RPM -10 again, and how far below peak RPM the peak EGT is.

In either case, I think it would hold some value for the OP to try finding both peaks, and use the knowledge we've imparted here to decide where to operate the engine relative to whichever of those peaks is easier to find on his airplane.

 

[David Megginson]

My point was that leaning solely via RPM, it may be harder to find the peak than it is with EGT.

Yes, interesting point, because the power curve is flat at the top.

Back in 2002, my first instructor told me for my student-pilot cross-countries to lean my rental 172 to max RPM, then enrich until I see the first sign of a drop. That's actually perfect advice for ROP ops with a fixed-pitch prop and no advanced engine monitor, because it makes sure that you're at the rich side of the top of the power curve, well away from the red zone.

I doubt he'd ever read any of the (then-new) articles about leaning and better engine operations, and I certainly hadn't. But it does show that not all of the old-school advice was wrong.

 

[Piper18O]

The best economy mixture, therefore, is usually going to be roughly at peak EGT.

peak EGT is useful only if you have probes in all cylinders and you know what to do with it

In regards to peak egt and running lean of peak, I had the leaning discussion with someone at a fly-in once. His unusual method was somewhat intriguing, but difficult for me to accept. He flew a Comanche 250 with constant speed prop and Lycoming O-540. He only had a single prob egt and cht gage. He said he would try to run rich of peak to his assigned or chosen altitude keeping cht temps below 400. (as he climbed he would have to lean a little obviously) Once at altitude he would level off, set his mp and rpm’s for the percent of power he wanted to run. Then he would pull mixture somewhat quickly to engine roughness and then enrichen just until it smoothed out. He then would then re-adjust his mp and rpm’s slightly if necessary. He claimed that as soon as the engine ran smooth, he would still be lean of peak and could run there economically and safely without having to install a “fancy temperature gauge” on a normally aspirated engine, which he didn’t think was useful unless you had fuel injection anyway. Does anyone have any thoughts on this method?

 

[Daleandee]

Pull the red knob out until it quits. Push it back in until you hear noise ...

 

[Piper18O]

Pull the red knob out until it quits. Push it back in until you hear noise ...

Or in some cases, once the noise stops. (the screaming if you are riding with an inexperienced passenger.....)

 

[kicktireslightfires]

Yes, interesting point, because the power curve is flat at the top.

Back in 2002, my first instructor told me for my student-pilot cross-countries to lean my rental 172 to max RPM, then enrich until I see the first sign of a drop. That's actually perfect advice for ROP ops with a fixed-pitch prop and no advanced engine monitor, because it makes sure that you're at the rich side of the top of the power curve, well away from the red zone.

I doubt he'd ever read any of the (then-new) articles about leaning and better engine operations, and I certainly hadn't. But it does show that not all of the old-school advice was wrong.

I'm confused. Is that what I should be doing with the O-200-D in my C162? And what does "lean to max RPM" mean? Are you saying to go full throttle in straight and level flight, then lean the mixture slowly until ... when? When it reaches 2750 RPM, the start of the red zone (max RPM?), and then slowly richen the mixture until it drops just slightly below that, and then pull back the throttle to get back to normal cruise? Also, doesn't what altitude you do this at matter, as I've noticed I have a little different RPM speeds at full throttle depending on what altitude I'm at.

 

[kicktireslightfires]

Pull the red knob out until it quits. Push it back in until you hear noise ...

Is this actually safe to do in flight? I've never tried it -- nervous to do so. But if I pull the mixture and the engine quits, pushing the mixture back in before the prop stops spinning will cause it to fire right back up right?

 

[kicktireslightfires]

One more question for you guys: What's the downside of just always leaving it full rich on a small carburated engine like the O-200-D? Am I just looking at having to change the spark plugs more often (due to extra carbon deposits), a tiny bit more fuel burn (like the difference between 5.5 GPH and 6 GPH?), and a tiny bit less power (like 5%?) ? Would that be accurate? Are there any other negatives?

Before I bought my C162, I actually learned on a C162 and on the C162 I learned on it was actually the flight school's policy for some reason to always leave their C162s full rich during ground and flight operations. I'm assuming this is because they deemed the negative effects of not leaning to be less than the chances of having a student screw up and go full power before remembering to go full rich again.

 

[GeorgeC]

More lead on your plugs and valves, more carbon monoxide, more than a tiny bit more fuel usage... plus full rich won't do the job at all for sufficiently high DA ops.

 

[Ed Haywood]

Choices:

A. Do what your aircraft and engine manufacturers recommend in the official FAA-approved handbook for your aircraft.

B. Do what some guys on the internet say, because they are really emphatic that the manufacturers are wrong, and they have impressive graphs to back it up.

Tough call.

 

[sourdough44]

With a simple engine, leaning until rough, then smooth out works. It can be one of those measuring with a micrometer then cutting with an axe deals.

 

[Daleandee]

Is this actually safe to do in flight? I've never tried it -- nervous to do so. But if I pull the mixture and the engine quits, pushing the mixture back in before the prop stops spinning will cause it to fire right back up right?

I was being facetious to say the least.

In a certified plane with low compression and a big hunk of long spinning prop it likely will restart when you push the mixture in.

On my high compression Corvair conversion with the light weight and relatively short wooden prop it won't as there isn't enough inertia to keep the prop turning and too much compression for the engine to windmill.

Perhaps if mine had more prop it would but my airframe/engine is limited as to length and weight for the prop.

 

[Daleandee]

One more question for you guys: What's the downside of just always leaving it full rich on a small carburated engine like the O-200-D? Am I just looking at having to change the spark plugs more often (due to extra carbon deposits), a tiny bit more fuel burn (like the difference between 5.5 GPH and 6 GPH?), and a tiny bit less power (like 5%?) ? Would that be accurate? Are there any other negatives?

I lean aggressively for all ground operations. Running full rich fouls the plugs and allows lead to build up on valves causing all sorts of undesirable things to happen. If the flight school wanted it ran full rich I'd be fine with that as they would be paying to have the repairs done from lead damage to the engine.

In my plane I use some 100LL & mix it with 90-93 E-free Mogas to help keep the lead out of the engine. Most experimental pilots use Decalin to help scavage the lead out. Certified pilots use TCP (Decalin is not allowed in certified planes) to help prevent lead build up.

Bottom line ... I try to keep the lead out of mine as much as possible. If the flight school wants it full rich ... well that's what we'll do!

 

[David Megginson]

I'm confused. Is that what I should be doing with the O-200-D in my C162? And what does "lean to max RPM" mean? Are you saying to go full throttle in straight and level flight, then lean the mixture slowly until ... when? When it reaches 2750 RPM, the start of the red zone (max RPM?), and then slowly richen the mixture until it drops just slightly below that, and then pull back the throttle to get back to normal cruise? Also, doesn't what altitude you do this at matter, as I've noticed I have a little different RPM speeds at full throttle depending on what altitude I'm at.

No, I'm not suggesting that, because your POH explicitly recommends against lean of peak operations.

I'm just suggesting that you set your desired RPM (or maybe 50 rpm below) with the throttle, then start leaning. As you lean, at first the RPM will rise, then it will pause, then it will start to fall again. You've found the lean side of peak power. Now enrich slightly again until the RPM rises and starts to fall slightly, and you've found the rich edge of peak power.

It sounds complicated reading it, but it takes only a couple of seconds once you're used to it. It will land you a bit over 150°F rich of peak EGT (averaged across the cylinders), which is a good place for ROP operations.

 

[David Megginson]

With a simple engine, leaning until rough, then smooth out works. It can be one of those measuring with a micrometer then cutting with an axe deals.

It might or might not. Where roughness occurs depends on the mixture spread among the cylinders, and that can vary even among engines of the same type.

My O-320-D3G will usually go right to idle cutoff without significant roughness. Another engine, with a bigger mixture spread among cylinders, might start running rough when some cylinders are still rich, so you will land them right in the hot zone.

Peak power is more reliable, because it happens at the same place for every engine.

 

[David Megginson]

Choices:

A. Do what your aircraft and engine manufacturers recommend in the official FAA-approved handbook for your aircraft.

B. Do what some guys on the internet say, because they are really emphatic that the manufacturers are wrong, and they have impressive graphs to back it up.

Tough call.

I almost always agree with this, but some of the guidance on engines is so bad that I make an exception. The graphs, by the way, are from Lycoming and Continental (part of the irony).

But I won't advise the OP to go against their POH and run LOP; I'll just advise them to ignore the misguided advice about leaning to peak EGT and enriching slightly, which every chart from Lycoming or Continental shows will land them on peak CHT. :-/

 

[David Megginson]

One more question for you guys: What's the downside of just always leaving it full rich on a small carburated engine like the O-200-D? Am I just looking at having to change the spark plugs more often (due to extra carbon deposits), a tiny bit more fuel burn (like the difference between 5.5 GPH and 6 GPH?), and a tiny bit less power (like 5%?) ? Would that be accurate? Are there any other negatives?

Maybe a lot more fuel burn. That won't matter for short, local flights, but for cross-country planning, it could mean running out of fuel because you're using a lot more than the POH performance tables say. At 75% power, my O-320 burns about 10 gph ROP (but still leaned), 8.5 gph LOP, or maybe 12–13 gph full rich.

So with my 48 gal usable, if I planned a 4-hour flight assuming 10 gph and really burned 12–13 gph, I might not just land with less than the minimum legal 30 minute day VFR fuel reserve in my tanks, but might even run out of fuel before my destination.

There's nothing wrong with flying full rich all the time — aside from (as you mentioned) lead-fouled plugs, extra CO, and extra $$$ for fuel — but before you do a long cross-country, try lots of short flights to get a feeling for how much you're burning, because they number's probably not in your POH. Most engine failures you read about in the accident reports are from fuel exhaustion, and most fuel exhaustion is pilot-induced (either mismanaging the tanks, not sumping them to get the water out, or most often, just not having enough fuel on board for the flight).

No power loss, BTW, because with a fixed-pitch prop, e.g. 2,400 rpm corresponds to exactly the same power output lean of peak, rich of peak, or full rich (at any given density altitude). You're just throwing more dead dinosaurs at the engine to get that power.

 

[kicktireslightfires]

No, I'm not suggesting that, because your POH explicitly recommends against lean of peak operations.

I'm just suggesting that you set your desired RPM (or maybe 50 rpm below) with the throttle, then start leaning. As you lean, at first the RPM will rise, then it will pause, then it will start to fall again. You've found the lean side of peak power. Now enrich slightly again until the RPM rises and starts to fall slightly, and you've found the rich edge of peak power.

It sounds complicated reading it, but it takes only a couple of seconds once you're used to it. It will land you a bit over 150°F rich of peak EGT (averaged across the cylinders), which is a good place for ROP operations.

That's the first explanation I've heard of ROP and LOP that makes sense to me. Thank you!!

 

[kicktireslightfires]

Maybe a lot more fuel burn. That won't matter for short, local flights, but for cross-country planning, it could mean running out of fuel because you're using a lot more than the POH performance tables say. At 75% power, my O-320 burns about 10 gph ROP (but still leaned), 8.5 gph LOP, or maybe 12–13 gph full rich.

So with my 48 gal usable, if I planned a 4-hour flight assuming 10 gph and really burned 12–13 gph, I might not just land with less than the minimum legal 30 minute day VFR fuel reserve in my tanks, but might even run out of fuel before my destination.

There's nothing wrong with flying full rich all the time — aside from (as you mentioned) lead-fouled plugs, extra CO, and extra $$$ for fuel — but before you do a long cross-country, try lots of short flights to get a feeling for how much you're burning, because they number's probably not in your POH. Most engine failures you read about in the accident reports are from fuel exhaustion, and most fuel exhaustion is pilot-induced (either mismanaging the tanks, not sumping them to get the water out, or most often, just not having enough fuel on board for the flight).

No power loss, BTW, because with a fixed-pitch prop, e.g. 2,400 rpm corresponds to exactly the same power output lean of peak, rich of peak, or full rich (at any given density altitude). You're just throwing more dead dinosaurs at the engine to get that power.

Wow, a 30% fuel savings from full rich to LOP is huge! I think it's a different story for the O-200-D if my POH is right as it says to expect "fuel savings in excess of 5% when compared to typical flight operations at full rich mixture," which I take to mean maybe 6 or 7% savings, which at 6 GPH if rich would work out to 5.6 GPH if leaned. So the fuel savings seems very small on this particular engine.

 

[William Pete Hodges]

Is this actually safe to do in flight? I've never tried it -- nervous to do so. But if I pull the mixture and the engine quits, pushing the mixture back in before the prop stops spinning will cause it to fire right back up right?

I haven't read this entire thread, but all your questions are normal. The Best way to fully understand them is to take the Advanced Engine Managment class. Here is a quick blurb about the RED BOX, or the danger zone where engines run at extreme internal engine pressure. https://www.advancedpilot.com/redbox.html
Do a little research on the class. It was the best $500 I ever spent.

 

[Hang 4]

Wow, a 30% fuel savings from full rich to LOP is huge! I think it's a different story for the O-200-D if my POH is right as it says to expect "fuel savings in excess of 5% when compared to typical flight operations at full rich mixture," which I take to mean maybe 6 or 7% savings, which at 6 GPH if rich would work out to 5.6 GPH if leaned. So the fuel savings seems very small on this particular engine.

Careful, there's a big difference between full rich and lean of peak and leaned, but still rich of peak. David was giving an extreme example to make a point. Your POH is likely referring to running ROP, but leaned from full rich for the 5% difference. His point remains the same, one has to be careful to understand actual fuel consumption vs manufacturer or other data, like a pilot who flies a similar plane and says they run at X GPH. Just using that number for your fuel planning without knowing your own plane's actual performance could be problematic on a long XC.

 

[GeorgeC]

I think it's a different story for the O-200-D if my POH is right as it says to expect "fuel savings in excess of 5% when compared to typical flight operations at full rich mixture," which I take to mean maybe 6 or 7% savings, which at 6 GPH if rich would work out to 5.6 GPH if leaned.

Leaning will take the O-200 from 6 gph down to 5 gph.

 

[David Megginson]

Wow, a 30% fuel savings from full rich to LOP is huge! I think it's a different story for the O-200-D if my POH is right as it says to expect "fuel savings in excess of 5% when compared to typical flight operations at full rich mixture," which I take to mean maybe 6 or 7% savings, which at 6 GPH if rich would work out to 5.6 GPH if leaned. So the fuel savings seems very small on this particular engine.

Again, I haven't measured the full-rich fuel burn (I'd never cruise there), so it's just a guess. The 15% difference between ROP (leaned) to LOP, however, is documented in the PA-28-161 POH and elsewhere. I personally see a little more fuel savings, but I stuck with the book number (as I do for flight planning).

 

[David Megginson]

Wow, a 30% fuel savings from full rich to LOP is huge! I think it's a different story for the O-200-D if my POH is right as it says to expect "fuel savings in excess of 5% when compared to typical flight operations at full rich mixture," which I take to mean maybe 6 or 7% savings, which at 6 GPH if rich would work out to 5.6 GPH if leaned. So the fuel savings seems very small on this particular engine.

I shared an annotated version of this chart before, but here's the original from Lycoming's O-320 Operator's Manual (the chart looks the same for all piston engines, including Continental's).

It has a lot of information, and takes a while to really understand. The right side represents the mixture all the way in to full rich, and the left side represents the mixture pulled almost to idle cutoff.

The four curves are mostly relative (except for % power) — they don't show absolute values, but how EGT, CHT, power, and fuel consumption relate to each-other at different fuel-air mixtures. Along the bottom, Lycoming has labeled LOP as "Best Economy Cruise", ROP as "Max Power Cruise," and the right edge as "Full Rich Take Off" (not everyone adds those labels).

The curve to watch here is the bottom one, "Specific fuel consumption" — note how much it rises from LOP to ROP, then how much it rises again from ROP to full rich. The latter difference is much bigger, so there's a significant in fuel consumption from leaned ROP to just rich of max RPM (say, 150–200°F rich of peak EGT) and full rich.

Note that there is a 5% loss of power from max RPM (around 150°F ROP EGT) to full rich — maybe that's what the person was trying to tell you. The extra fuel consumption is likely much, much higher than 5%.

 

[exncsurfer]

I shared an annotated version of this chart before, but here's the original from Lycoming's O-320 Operator's Manual (the chart looks the same for all piston engines, including Continental's).

It has a lot of information, and takes a while to really understand. The right side represents the mixture all the way in to full rich, and the left side represents the mixture pulled almost to idle cutoff.

The four curves are mostly relative (except for % power) — they don't show absolute values, but how EGT, CHT, power, and fuel consumption relate to each-other at different fuel-air mixtures. Along the bottom, Lycoming has labeled LOP as "Best Economy Cruise", ROP as "Max Power Cruise," and the right edge as "Full Rich Take Off" (not everyone adds those labels).

The curve to watch here is the bottom one, "Specific fuel consumption" — note how much it rises from LOP to ROP, then how much it rises again from ROP to full rich. The latter difference is much bigger, so there's a significant in fuel consumption from leaned ROP to just rich of max RPM (say, 150–200°F rich of peak EGT) and full rich.

Note that there is a 5% loss of power from max RPM (around 150°F ROP EGT) to full rich — maybe that's what the person was trying to tell you. The extra fuel consumption is likely much, much higher than 5%.
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David,
You suggested he lean just rich of peak RPM, based on the chart you posted, why not use peak RPM? Just to be on the 'cool' side of CHT? Isn't the RPM peak represented by the Percent Power curve on the chart?

Thanks for sharing.