Why I No Longer Fly Lean of Peak For Now

[denverpilot]

O-470s don't like LOP. Fuel distribution sucks.

Beat me to it. It’s really rare to find an O-470 that won’t run like crap LOP. Ours won’t do it.

 

[NordicDave]

O-470's generally cannot fly LOP due to uneven intake manifold runners which creates uneven fuel distribution, which is the corner stone to run LOP.

Yes, many pilots have tricks like very slightly canting the throttle and adding carb heat. This helps but does not solve the fuel distribution problem of the O-470. I'd definitely never run these engines LOP with out a full engine monitor.

 

[James331]

ROP all the things, but the pistons I fly (FI or carb) are 1500AGL or less 85% of the time

 

[Juliet Hotel]

The fuel burn difference in my 182 is three gallons per hour at 100 ROP versus 50 LOP. It goes faster ROP.

In all the ROP/LOP discussions I've read, there is lots of talk about how wonderful it is to burn less fuel. 'Yeah it goes a bit slower but oh the fuel I save...' I always end up wondering how much fuel is really saved from the mixture setting itself because it seems like every discussion contains the phrase 'it goes a bit faster...' or 'it goes a bit slower...' depending on whether ROP or LOP is being referenced.

This is where the wheels always fall off for me. Its an argument for one type of operation vs another with better performance (speed vs fuel burn) being the advantage, but its an argument being made with very non-specific performance data i.e. yeah it goes a bit slower but so what?

I'd really like to see hard numbers comparing LOP/ROP operations done in the same plane at the same altitude on the same day but with throttle adjusted to keep speeds identical. I would suspect that when speeds are matched. If you match the speeds and can still show enough fuel savings to make LOP operations and the associated risks worth while, then you would have a winning argument. To date, I've never seen anyone make such an argument.

 

[tspear]

I'd really like to see hard numbers comparing LOP/ROP operations done in the same plane at the same altitude on the same day but with throttle adjusted to keep speeds identical. I would suspect that when speeds are matched. If you match the speeds and can still show enough fuel savings to make LOP operations and the associated risks worth while, then you would have a winning argument. To date, I've never seen anyone make such an argument.

Been done many times on BeechTalk and on COPA that I have seen.
I have done so with an SR22 also.

Note: it often takes about five minutes for the speed and temps to settle out.

Tim

 

[genna]

In all the ROP/LOP discussions I've read, there is lots of talk about how wonderful it is to burn less fuel. 'Yeah it goes a bit slower but oh the fuel I save...' I always end up wondering how much fuel is really saved from the mixture setting itself because it seems like every discussion contains the phrase 'it goes a bit faster...' or 'it goes a bit slower...' depending on whether ROP or LOP is being referenced.

This is where the wheels always fall off for me. Its an argument for one type of operation vs another with better performance (speed vs fuel burn) being the advantage, but its an argument being made with very non-specific performance data i.e. yeah it goes a bit slower but so what?

I'd really like to see hard numbers comparing LOP/ROP operations done in the same plane at the same altitude on the same day but with throttle adjusted to keep speeds identical. I would suspect that when speeds are matched. If you match the speeds and can still show enough fuel savings to make LOP operations and the associated risks worth while, then you would have a winning argument. To date, I've never seen anyone make such an argument.

You definitely get better % reduction in fuel/h vs nm/h. SR22 I flew lost about 15kts(little less than 10%) in cruise at WOT LOP in 6-7000ft range. That corresponded to about 3 GPH(little less than 20%). The net result will be about 10% increase in mpg.Edit: Which over 2000h(TBO) of that SR22 comes out to about $18,000 in fuel savings(at $5/g)

 

[Kenny Phillips]

How does the fuel in a cylinder know to adjust how it burns if the fuel was provided by a carb or injections?

Tim

The premise is that fuel/air mixture distribution is poorer in a carbureted engine. And it usually is. So unless you have six EGT and/or CHT probes, so you can make the hottest jug your datum, you risk smoking the hot ones by going with the average.

 

[Juliet Hotel]

You definitely get better % reduction in fuel/h vs nm/h. SR22 I flew lost about 15kts(little less than 10%) in cruise at WOT LOP in 6-7000ft range. That corresponded to about 3 GPH(little less than 20%). The net result will be about 10% increase in mpg.Edit: Which over 2000h(TBO) of that SR22 comes out to about $18,000 in fuel savings(at $5/g)

Put it into plain terms. Fuel burn at speed X ROP, Fuel burn at speed X LOP.

 

[Juliet Hotel]

Been done many times on BeechTalk and on COPA that I have seen.

Not a member of either so doesn't help much.

 

[genna]

Put it into plain terms. Fuel burn at speed X ROP, Fuel burn at speed X LOP.

Never tested it like that. In SR22 I would expect about 1-2 GPH more use at ROP with power pulled back to match speed of LOP. Which is still about 10% less MPG.

This really make sense.
Logically you are adding some never-to-be burned fuel in the cylinder. While it may be useful for cooling and such, it is obviously not very efficient at converting fuel to power/speed

 

[Tantalum]

SR22 I flew lost about 15kts

That's huge. I assume NA? The 22T and 22TN are flown LOP, and at 10K you'll see around 183-186 KTAS on about 15.8-16.2 gph and 30 inches MP, generating around 72-75% power. You can keep leaning from there, but you'll see the power, and subsequently speed, drop way off.

 

[Tantalum]

Logically you are adding some never-to-be burned fuel in the cylinder. While it may be useful for cooling and such, it is obviously not very efficient at converting fuel to power/speed

Yup. People can ROP if they want more power and speed, but it's worth it for folks to understand the interplay and what LOP *actually* is vs ROP, and that in ROP you are effectively pouring some gas out the window. I bet many people don't totally get it, and end up running their engines (or atleast a few cylinders) close to that peak red zone.

 

[Weekend Warrior]

ROP. LOP is bad juju with a carb.

 

[genna]

That's huge. I assume NA? The 22T and 22TN are flown LOP, and at 10K you'll see around 183-186 KTAS on about 15.8-16.2 gph and 30 inches MP, generating around 72-75% power. You can keep leaning from there, but you'll see the power, and subsequently speed, drop way off.

NA G2. Best Economy(LOP) was topping out at about 165. Best Power(ROP) was just about hitting 180. The GPH difference was actually a bit better than 3. IIRC, 18.something @ Best Power vs just under 15 at Best Economy. Speeds are in TAS

 

[Tantalum]

NA G2. Best Economy(LOP) was topping out at about 165. Best Power(ROP) was just about hitting 180. The GPH difference was actually a bit better than 3. IIRC, 18.something @ Best Power vs just under 15 at Best Economy. Speeds are in TAS

Okay, that makes sense.. in a g1(maybe g2?) SR22 I have some time in it was similar. Weird, 15 knots sounded high, but in looking at the numbers it ties out to what I saw too

 

[tspear]

Put it into plain terms. Fuel burn at speed X ROP, Fuel burn at speed X LOP.

In my current N/A SR22 G1. Roughly 65% at 165 KIAS; 50 degrees LOP will be around 13GPH and 50 degrees ROP will normally be around 15GPH.

Tim

 

[genna]

Okay, that makes sense.. in a g1(maybe g2?) SR22 I have some time in it was similar. Weird, 15 knots sounded high, but in looking at the numbers it ties out to what I saw too

Sounds high, but it is not really. It's 8.3%. I think this corresponds to about 10% less power use(but I'm not sure on that). I flew that plane mostly at rental power setting . Best Economy was only used if fuel quantity was in question.

 

[genna]

In my current N/A SR22 G1. Roughly 65% at 165 KIAS; 50 degrees LOP will be around 13GPH and 50 degrees ROP will normally be around 15GPH.

Tim

i.e. 13% MPG savings. Or @$5/g, 2000h TBO =$19,500 less with LOP. And that doesn't even take into account that many run at 100 degrees ROP using more fuel.

That's a pretty significant chunk of change. Even if you have to replace a few cylinders. And that assumption is very arguable with other benefits of LOP out there as well

 

[Fearless Tower]

for me....

Carb'd = ROP

Injected, and one I owned; LOP in cruise only.

700-800hrs on cylinders is about all you are going to get on large bore Conti's. If you were to look at a bell curve, not listening to the occasional data point that someone will pipe in with.

Bingo. -470s are great running engines, but not exacting known for having cylinders make it all the way to overhaul.

 

[Fearless Tower]

O-470's generally cannot fly LOP due to uneven intake manifold runners which creates uneven fuel distribution, which is the corner stone to run LOP.

Yes, many pilots have tricks like very slightly canting the throttle and adding carb heat. This helps but does not solve the fuel distribution problem of the O-470. I'd definitely never run these engines LOP with out a full engine monitor.

FWIW, the carb heat to run LOP originated with Lindbergh. That was what he was doing out in the Pacific with the Army: showing them how to run radials LOP. Works fine to get max range from a B-17. Not really worth it on a -470....

 

[Chuck Buchanan]

All right, I’m late to the party but would like to continue to beat this dead horse.

Is it fair/correct to say that lower EGTs and CHTs always equate to lower internal cylinder pressures?

 

[David Megginson]

The whole lean of peak (LOP) versus rich of peak (ROP) discussion seems to be broken down into teams for a carbureted engine. Strong believers on each side.

I was heavily in the LOP club for several years and enjoyed the reduction in fuel-guzzling and happily accepted the slightly reduced cruising speed.

With 882 hours on my O-470 in four years, I was a maniac with the JPI in adjusting the LOP setting just so. But I've now been through two cylinders.

Lately, I've been asking every higher time GA piot I meet and the engine shops what setting they recommend. Interestingly the feedback has been ROP is much better for your engine. One Baron pilot just told me he had 700 hours on both engines, sold the plane, and in a year the new pilot flying LOP had to pull three cylinders. The well-respected machine shop said only to fly the O-470 at least 50 ROP. I've had other similar stories as well.

The fuel burn difference in my 182 is three gallons per hour at 100 ROP versus 50 LOP. It goes faster ROP.

I'm sure this thread will have vigorous fans on each side of the argument. It always does. It's almost like talking politics but with pilots.

So I'm curious what other frequent flyers have experienced LOP versus ROP and if others have now abandoned the LOP route as well based on actual experience.

Your shop seems a bit confused. 50°F ROP EGT is exactly the worst place to operate your engine, because it's the point where peak CHT occurs. Literally any other mixture setting, leaner or richer, is better than 50°F ROP, as you can see in this diagram from Lycoming. Given that they flunked that very simple point, I'd take anything else they tell you about engine operations theory (vs the practical side of engine maintenance, which I'm sure they're very good at) with an enormous grain of salt.

 

[David Megginson]

I've never heard of doing LOP on a carbureted engine. Always thought that was for fuel injected engines.

LOP works on certain 4-cylinder carbureted engines, because it's possible still to get fairly-even fuel/air distribution to all 4 cylinders. Lycoming's O-320 is a very good example (to the point that Piper even recommends LOP operations in my PA-28-161 POH).

LOP is much more difficult with 6-cylinder carbureted engines, because by definition some cylinders have to be located physically further from the carburetor than others, so you're less likely to get even distribution. I have heard success stories, but I think it would be very-much hit or miss, and would definitely require a good engine monitor.

In any case, it's easy enough to find out. Just lean your engine past peak EGT — if it starts vibrating so much that objects bounce up and down on your dash, then the fuel/air distribution among cylinders is too uneven, and you can't operate your engine LOP; if it just switches to a throaty growl (like the husky voice of a chain-smoking 1940s starlet), then you're fine.

If you can't fly LOP, then at 75% power, you'll want to enrich to at least 150° ROP, to give the red zone a wide margin. Below 65% power (normally aspirated), it won't matter where you set the mixture.

 

[David Megginson]

I fly behind a Lycoming O-540-J3A5D. When flying at “55%” or “65%” power settings, I lean until a stumble and then enrich until smooth operation. Correlation with my engine monitor shows that this puts 5 out 6 cylinders lean of peak and my richest cylinder #6 near or at peak. When flying at 75% power, I lean to a 125 degree ROP fuel flow. CHTs are in the 350’s across the board except #2 when running ROP 75% which knocks on the door of 380ºF (there’s a modification for my particular aircraft that I’ll ask my mechanic to perform to take care of that). In short, it’s possible to run LOP or close to it with a carbureted engine, but the real question is can you do this with all carbureted engines and the answer to that is “no.” The Continental O-470’s induction system is notorious.

That's an OK technique with some specific engines, but not for general use, because the place where an engine "stumbles" will depend on how even the fuel/air distribution is among cylinders. With your engine, you've confirmed it's OK; for another engine, the stumble might happen somewhere that enriching slightly puts you right on 50°F ROP (peak CHT). Some other engines will go right to idle cutoff without stumbling.

 

[Tom-D]

Answer-- neither, because you can't get all cylinders to be equal.

 

[David Megginson]

In all the ROP/LOP discussions I've read, there is lots of talk about how wonderful it is to burn less fuel. 'Yeah it goes a bit slower but oh the fuel I save...' I always end up wondering how much fuel is really saved from the mixture setting itself because it seems like every discussion contains the phrase 'it goes a bit faster...' or 'it goes a bit slower...' depending on whether ROP or LOP is being referenced.

This is where the wheels always fall off for me. Its an argument for one type of operation vs another with better performance (speed vs fuel burn) being the advantage, but its an argument being made with very non-specific performance data i.e. yeah it goes a bit slower but so what?

I'd really like to see hard numbers comparing LOP/ROP operations done in the same plane at the same altitude on the same day but with throttle adjusted to keep speeds identical. I would suspect that when speeds are matched. If you match the speeds and can still show enough fuel savings to make LOP operations and the associated risks worth while, then you would have a winning argument. To date, I've never seen anyone make such an argument.

A good place to test the difference is a simple engine with a fixed-pitch prop. For those, the best way to operate LOP is to leave the throttle wide-open and lean to your target RPM. That means that there's no speed loss from LOP (unless you're above the ceiling where the engine can make your desired power setting), and all the fuel savings come from LOP operations alone (not partly from flying at a slower speed).

That's the case with my PA-28-161, and I save 15–20% by operating it LOP (again, with no speed loss unless I'm at higher altitudes).

You could also avoid the speed loss LOP with constant-speed propellors, but it would involve increasing the MP to replace the speed you lost by leaning. Some pilots have techniques for doing that, but they're all trickier (you want to make sure you don't increase power back past 75%). In fact, many operators of constant-speed props also leave the throttle wide open, just like with the fixed-pitch props.

 

[David Megginson]

Answer-- neither, because you can't get all cylinders to be equal.

The question isn't "are they equal", but "are they close enough"? The answer is "yes" for some carbureted engines, but "no" for many others.

 

[David Megginson]

All right, I’m late to the party but would like to continue to beat this dead horse.

Is it fair/correct to say that lower EGTs and CHTs always equate to lower internal cylinder pressures?

No, and sort-of. At any given power setting, there's close-enough to a 1:1 relationship between internal cylinder pressures and CHT. That relationship doesn't exist with EGT, because EGT peaks at a different point than CHT (CHT peaks when EGT is about 50°F below peak on the rich side). And if you change your power setting (e.g. reduce from 75% to 65% power), you also change the level that the pressure peaks at.

 

[Tom-D]

The question isn't "are they equal", but "are they close enough"? The answer is "yes" for some carbureted engines, but "no" for many others.

You tell me which 4 cylinder engine has the same length intake pipe. and will have the same BMEP. for each cylinder.

 

[Chuck Buchanan]

David Megginson, that a fair question/statement. Assuming the same power setting (65% on the JPI) is it accurate to interpret lower CHTs and EGTs to lower internal cylinder pressures? If CHTs and EGTs continue to drop, can one go TOO lean?
Many Thanks

 

[David Megginson]

David Megginson, that a fair question/statement. Assuming the same power setting (65% on the JPI) is it accurate to interpret lower CHTs and EGTs to lower internal cylinder pressures? If CHTs and EGTs continue to drop, can one go TOO lean?
Many Thanks

If you Google "leaning red fin", you'll find much-more expert info than my amateur answers can give, but for what it's worth, yes, at 65% power the red zone is very narrow according to everything I've read, and by 60% power it has vanished altogether. Most of the expert advice is that you don't have to worry about where you lean with a normally-aspirated engine below 65% power (definitely not at 60% power or lower), except possibly for secondary issues like lead-fouled plugs or fuel endurance.

 

[David Megginson]

You tell me which 4 cylinder engine has the same length intake pipe. and will have the same BMEP. for each cylinder.

It doesn't have to be exactly the same, again, just close enough (which is the case with the O-320).

My own experience with the O-320-D3G in my PA-28-161 is that it always operates smoothly LOP at 2,500 RPM or high, and often at 2,400 RPM or lower.

Here's what Lycoming writes in the O-320 Operator's Manual:

Induction System - Lycoming O-320 series engines are equipped with a float type carburetor. Particularly good distribution of the fuel-air mixture to each cylinder is obtained through the center zone induction system, which is integral with the oil sump and is submerged in oil, insuring a more uniform vaporization of fuel and aiding in cooling the oil in the sump. From the riser the fuel-air mixture is distributed to each cylinder by individual intake pipes.

Here's what Piper writes in the PA-28-161 POH:

For Best Economy cruise, a simplified leaning procedure which consistently allows accurate achievement of best engine efficiency has been developed. Best Economy Cruise performance is obtained with the throttle
fully open. To obtain a desired cruise power setting, set the throttle and mixture control full forward, taking care not to exceed the engine speed limitation, then begin leaning the mixture. The RPM will increase slightly but will then begin to decrease. Continue leaning until the desired cruise engine RPM is reached. This will provide best fuel economy and maximum miles per gallon for a given power setting. See following CAUTION when using this procedure.

CAUTION​

Prolonged operation at powers above 75% with a leaned mixture can result in engine damage. While establishing Best Economy Cruise Mixture, below 6,000 feet, care must be taken not to remain in the range above 75% power more than 15 seconds while leaning. Above 6,000 feet the engine is incapable of generating more than 75%.

So what I have to offer as evidence is my own 18 years of experience operating a carbureted O-320 engine lean of peak EGT successfully, together with statements from both Lycoming and Piper in their official publications backing that up. You are, of course, free to continue to disagree on theoretical grounds.

Cheers, David

 

[N1120A]

LOP has become more popular in the Tiger world since the SureFly came out. My mechanic (also a pilot) and I used it on a very hot day where the cylinders didn't want to cool down and were impressed by the results. Running WOT and about 20 LOP the engine actually runs quite smoothly, thanks to the advance, and a well set up engine will still pull 2600 RPM.

 

[Chuck Buchanan]

I’ll attempt to post some snap shots of my JPI-930, each of them at 65% power, and let the comments fly.

One significant caveat. The engine is an O-520 Texas Skyways conversion mounted on a ‘65 ‘H’ model C-182. This was two days ago, 7,000 msl between Chattanooga and Atlanta. TAS was 128knots.

 

[Chuck Buchanan]

Perhaps this is easier to read...

 

[Checkout_my_Six]

High time TCM cylinders usually fail rotorcoils.....and most folks don't know what those are....and LOP gets the blame. Rotorcoils are a cheap item, about $25 ea, and they reside on the top of the exhaust valve spring stack. With each spring compression they "rotate" the exhaust valve....allowing the valve to evenly heat and cool. Once the valves stop rotating the classic burnt valve begins.....and LOP gets the blame.

If this is your first time hearing this .....make sure you replace the rotorcoils every 500-600 hrs. Or you will burn a valve and be into repairing cylinders.

http://www.aircraftspecialties.aero/roto-coil-assy-652112/

 

[Tom-D]

It doesn't have to be exactly the same, again, just close enough (which is the case with the O-320).
Cheers, David

Then tell me why #3 cylinder will fail first when you try to lean that way.
#3 will always be the richest/hottest and fail first. True in 0-360 and 0-320

 

[schmookeeg]

I wonder if you could produce a GAMIjector-like effect on carbureted engines by selling some sort of restricted orifice ring between cylinders, say on the little induction tube couplers.

By having a set of "metered intake rings", one could calibrate the airflow to each intake port and maybe cobble together some sort of balanced charge effect.

Whether the science was valid or not, I bet you'd sell a ton of em if you could get them certified.

 

[David Megginson]

Then tell me why #3 cylinder will fail first when you try to lean that way.
#3 will always be the richest/hottest and fail first. True in 0-360 and 0-320

Sorry to hear you've had that experience. That could certainly happen if #3 cylinder is running richest, the operator routinely operates at 75% power, and they lean only a little bit (not enough) because they're trying to avoid roughness. In the absence of an engine monitor, you have to lean as far as physically possible (LOP-WOT), so that you'll know if one of the cylinders is still rich (if so, the engine will vibrate like crazy, because of the steep power drop-off on the lean side).

Or even better, install an engine monitor, as some O-320 and O-360 owners do, then you'll be able to be certain that you're not operating any of your cylinders in the red zone around 50°F ROP.

I'd also be curious to know how many of those engine owners weren't actually operating LOP, but just "leaning to roughness and then enriching slightly", or "leaning to peak RPM and then a little more", both or which can be recipes for landing right on 50°F ROP and peak CHT.

 

[David Megginson]

I wonder if you could produce a GAMIjector-like effect on carbureted engines by selling some sort of restricted orifice ring between cylinders, say on the little induction tube couplers.

By having a set of "metered intake rings", one could calibrate the airflow to each intake port and maybe cobble together some sort of balanced charge effect.

Whether the science was valid or not, I bet you'd sell a ton of em if you could get them certified.

It's an interesting idea. The question is whether it would have the same effect at every power setting, since the fuel and air are already mixed together; you might need rings that expand and contract dynamically, which would be too complicated.

Personally, if my O-320 didn't run smoothly LOP-WOT, I'd just operate it at 150°F ROP (peak power/RPM), which is always clear of the red zone at cruise power settings, and then just learn to live with the elevated fuel consumption, reduced range/endurance, and higher risk of CO poisoning when I'm using the heater in winter. It wouldn't be any more harmful for the engine itself, aside from occasional lead-fouled plugs.