LOP (or ROP) vs. peak at lower power

Jim_R

Jim_R

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Jim
I have a Cherokee 180 with a Lycoming O-360-A4A (carbureted).

I have an engine monitor installed, so I can see my individual cylinder behavior.

With lots of futzing around in the air, I can often manage to run the engine lean-of-peak, but it involves messing with carb heat and there is some mild engine roughness in that condition...enough that passengers don't like it. Not sure I really do either, for that matter.

It's lots easier to find peak operation. Even using the engine monitor, I can't really improve over the manufacturer's procedure: Lean until engine stumbles, then enrich until smooth again. Quick, simple, and doesn't upset the pax if I warn them before I do it.

So, now I'm trying to answer two questions for myself.

  1. I know peak operation is "safe" for the engine if done at low enough power settings. How do I know what power is "low enough" for my engine?
  2. How can I determine the difference in performance of my engine at "safe" LOP or ROP conditions at higher throttle settings vs. "safe" peak conditions at lower throttle settings?

Are those data easily available, or do I need to hire a test pilot to fly my plane for a while (and hope he doesn't ruin my engine while collecting the data!)?
 
Jim_R said:
I have a Cherokee 180 with a Lycoming O-360-A4A (carbureted).


So, now I'm trying to answer two questions for myself.

  1. I know peak operation is "safe" for the engine if done at low enough power settings. How do I know what power is "low enough" for my engine?

Are those data easily available, or do I need to hire a test pilot to fly my plane for a while (and hope he doesn't ruin my engine while collecting the data!)?
Click to expand...

Peak allowable RPM varies with altitude and temperature. The AFM should have a chart displaying this. As long as you keep the RPM under the recommended settings for the current T&A you should be able to lean to your heart's content. Keep an eye on the CHT for each cylinder and your valves will thank you, as will the guy that pays for the fuel.
 
Jim_R said:
I know peak operation is "safe" for the engine if done at low enough power settings. How do I know what power is "low enough" for my engine?
Click to expand...
Look in your Lycoming O-360 Operator's Manual. It tells you that you can lean any time engine power is below 75%. If you don't have that book, you can order it from Lycoming. See Lycoming Service Letter SL114AW for how to do that.

How can I determine the difference in performance of my engine at "safe" LOP or ROP conditions at higher throttle settings vs. "safe" peak conditions at lower throttle settings?
Click to expand...
If you're talking about leaning at power settings above 75%, there's no good way to do that short of trying it yourself and seeing what happens, and that risks engine damage. With your full engine analyzer, you can at least monitor CHT's and stop leaning if they get too high. As for determining performance differences, just see what happens to speed or climb rate as you change power and mixture settings.
 
JOhnH said:
Peak allowable RPM varies with altitude and temperature.
Click to expand...
On his Lycoming O-360-A engine, peak allowable RPM (red line) is 2700 at all altitudes and temperatures. OTOH, RPM for the 75% recommended power limit for leaning does vary with altitude and temperature, and...
The AFM should have a chart displaying this.
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...you can determine the RPM for 75% power in cruise by using the cruise power charts in your AFM. However, there is no chart in the AFM to determine 75% power at other speeds, and with that fixed pitch prop, the RPM for 75% power varies also with airspeed. If you have a manifold pressure sensor installed with your engine monitor, you can determine engine power % from MP, RPM, altitude, and temperature at other-than-cruise speed with the power charts in the Lycoming O-360 Operator's Manual.

Some engine monitors also have a %HP function, but again, you need that engine operator's manual to calibrate that function based on inflight MP/RPM readings along with observed temp/altitude at the time of those readings. The manual for your engine monitor explains how to do that.

As long as you keep the RPM under the recommended settings for the current T&A you should be able to lean to your heart's content.
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If by "recommended settings" you mean the RPM for 75% power in cruise from the cruise power charts in the AFM/POH, that's true in cruise. Otherwise, you will need the Operator's Manual and a MP gauge, too.

Keep an eye on the CHT for each cylinder and your valves will thank you, as will the guy that pays for the fuel.
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Absolutely true. When leaning, follow Lycoming's advice about CHT's:


  1. The Operator's Manual on the O-360-A engine says the max CHT at the bayonet location is 500F in all flight phases. However, the note to that says "for maximum service life of the engine maintain CHT between 150F and 400F during continuous operation."
  2. The Lycoming Flyer Key Reprints book (1996 edition) says "the engines have benefitted during continuous operation by keeping CHT below 400F in order to achieve best life and wear of the powerplant. In general, it would be normal during all year operations, in climb and cruise to see head temperatures in the range of 350F to 435F."
 
Be careful, a friend of mine ran his Cherokee lean of peak and the FAA gave him life in prison.
 
taters said:
Be careful, a friend of mine ran his Cherokee lean of peak and the FAA gave him life in prison.
Click to expand...

I don't think it was the LOP that got him, but more likely the treadmill he was trying to take off from.
 
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Ron Levy said:
On his

If by "recommended settings" you mean the RPM for 75% power in cruise from the cruise power charts in the AFM/POH, that's true in cruise. Otherwise, you will need the Operator's Manual and a MP gauge, too.
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Yes, I meant keeping power to 75% or less in cruise.

I don't think too many fixed pitch planes carry an MP gauge, although I have thought about putting one in mine.
 
JOhnH said:
Peak allowable RPM varies with altitude and temperature. The AFM should have a chart displaying this. As long as you keep the RPM under the recommended settings for the current T&A you should be able to lean to your heart's content. Keep an eye on the CHT for each cylinder and your valves will thank you, as will the guy that pays for the fuel.
Click to expand...

There's nothing useful in the cherokee AFM, not that I've found. Some emergency procedures and examples of W&B. it's why they invented useful POH for later models.

+1 to Ron's comments about using the Lycoming engine manual. If you don't have it, PM me and I'll email you a PDF copy that came with mine. The Reprint is free on Lycoming's website.
 
JOhnH said:
Yes, I meant keeping power to 75% or less in cruise.

I don't think too many fixed pitch planes carry an MP gauge, although I have thought about putting one in mine.
Click to expand...

Probably not worth the expense since you really don't have much control over it.
 
murphey said:
Probably not worth the expense since you really don't have much control over it.
Click to expand...
Still useful if you want to know exactly how much power you're really developing, and it's not a very expensive option if you're installing a JPI engine monitor. It's particularly useful for my airplane as I have a non-standard prop installed, and the POH numbers are not accurate with anything other than the stock prop. Plus, even if you think you have a stock prop installed, prop pitch can vary an inch either side of the pitch stamped on the hub, and that makes a difference, too.
 
JOhnH said:
I don't think too many fixed pitch planes carry an MP gauge, although I have thought about putting one in mine.
Click to expand...

Ron Levy said:
Still useful if you want to know exactly how much power you're really developing, and it's not a very expensive option if you're installing a JPI engine monitor. It's particularly useful for my airplane as I have a non-standard prop installed, and the POH numbers are not accurate with anything other than the stock prop. Plus, even if you think you have a stock prop installed, prop pitch can vary an inch either side of the pitch stamped on the hub, and that makes a difference, too.
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I love the MP gauge in my C-172N-180 (fixed pitch). It makes precise power settings a snap at varying altitudes. I also notice a drop in manifold pressure from carb ice even before it shows up in rpm.

SAM_0137.JPG
 
murphey said:
There's nothing useful in the cherokee AFM, not that I've found. Some emergency procedures and examples of W&B. it's why they invented useful POH for later models.

+1 to Ron's comments about using the Lycoming engine manual. If you don't have it, PM me and I'll email you a PDF copy that came with mine. The Reprint is free on Lycoming's website.
Click to expand...
Maybe it was the Lycoming engine manual that I saw the chart. I don't remember. But since the numbers were in even 2,000 ft increments and I fly vfr (n+500) increments, I took the numbers from the manual and put them in excel and extrapolated the numbers for 500 ft intervals.
 
murphey said:
Probably not worth the expense since you really don't have much control over it.
Click to expand...
I'd still like to have one just to see what is going on and make it easier to calculate %HP. I already have a JPI 700 monitor with everything but carb temp and %HP, but I have a separate carb temp gauge. I haven't put one in yet because I keep thinking about selling and buying something faster, but it is hard to justify that much money for a few extra kts. And my plane (172n) is really nice and in good shape with a low time 180 and great cosmetics.
 
JOhnH said:
Maybe it was the Lycoming engine manual that I saw the chart. I don't remember. But since the numbers were in even 2,000 ft increments
Click to expand...
That would have been from a POH, not the Lycoming manual, which has linear graphs, not incremental tables.
 
JOhnH said:
I'd still like to have one just to see what is going on and make it easier to calculate %HP. I already have a JPI 700 monitor with everything but carb temp and %HP, but I have a separate carb temp gauge. I haven't put one in yet because I keep thinking about selling and buying something faster, but it is hard to justify that much money for a few extra kts. And my plane (172n) is really nice and in good shape with a low time 180 and great cosmetics.
Click to expand...

I have an 0-320 fixed pitch prop with a manifold gauge and a digital tach. I purchased a "power wheel" from Far West Aviation that gives you % of power at a glance. Google Far West Aviation power wheel for more info.
 
Jim_R said:
I have a Cherokee 180 with a Lycoming O-360-A4A (carbureted).

I have an engine monitor installed, so I can see my individual cylinder behavior.

With lots of futzing around in the air, I can often manage to run the engine lean-of-peak, but it involves messing with carb heat and there is some mild engine roughness in that condition...enough that passengers don't like it. Not sure I really do either, for that matter.

It's lots easier to find peak operation. Even using the engine monitor, I can't really improve over the manufacturer's procedure: Lean until engine stumbles, then enrich until smooth again. Quick, simple, and doesn't upset the pax if I warn them before I do it.

So, now I'm trying to answer two questions for myself.

  1. I know peak operation is "safe" for the engine if done at low enough power settings. How do I know what power is "low enough" for my engine?
  2. How can I determine the difference in performance of my engine at "safe" LOP or ROP conditions at higher throttle settings vs. "safe" peak conditions at lower throttle settings?
Are those data easily available, or do I need to hire a test pilot to fly my plane for a while (and hope he doesn't ruin my engine while collecting the data!)?
Click to expand...

Good luck getting a carbed motor to run lean of peak, smoothly and safely.IMHO.
 
Ron Levy said:
Look in your Lycoming O-360 Operator's Manual. It tells you that you can lean any time engine power is below 75%. If you don't have that book, you can order it from Lycoming. See Lycoming Service Letter SL114AW for how to do that.
Click to expand...

Aha--yeah, that's pretty much the bit I was looking for, for that question. Thanks!

Ron Levy said:
If you're talking about leaning at power settings above 75%, there's no good way to do that short of trying it yourself and seeing what happens, and that risks engine damage. With your full engine analyzer, you can at least monitor CHT's and stop leaning if they get too high. As for determining performance differences, just see what happens to speed or climb rate as you change power and mixture settings.
Click to expand...

Okay, that's what I thought (and why I made the comment about the test pilot ruining my engine!). Would be nice to know that, but it's not necessary.

I am a little bit hesitant about relying solely on CHTs for two reasons: 1) it's my understanding that CHTs are just a proxy for how much stress (in the general sense, not in the mechanical engineering sense) the cylinder was under, and 2) my CHT sensors are at the spark plug washer location due to the presence of Tanis heater elements in all my bayonet locations, so I'm not sure how much difference that makes in the CHT readings.

So if I can establish some safe power level criteria, and use that in conjunction with an eye on CHTs, that's something of a belt-and-suspenders approach that makes me more comfortable.
 
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Geico266 said:
Do you have a fuel flow monitor on your airplane?
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Yes. It's part of the Insight G3 package I installed.

(And it also has a manifold pressure port, but I didn't connect it to anything since I've got a fixed-pitch prop. I use that reading in my plots as a proxy for altitude, since the port measures ambient pressure.)

Many thanks for all the comments. I've got some more studying to do... :)
 
This is the super simple method.

1. Set 2500 RPM. That is 75% power at sea level and less at any standard altitude above.

2. Give the controls to your wife. Close your eyes. Pull the mixture back slowly. In a Cherokee 180 the exhaust is just forward of your feet and loud in the cabin. LISTEN to the exhaust as you pull it back. It will hit a crescendo of sound. Leave it there. Don't worry! Be happy!
 
N801BH said:
Good luck getting a carbed motor to run lean of peak, smoothly and safely.IMHO.
Click to expand...

This is why we are changing the cylinder on the PA-28, they tried to run lean of peak, ended up leaving one in the red box running at best power.

NO CHT, and they never knew they were smoking the right rear cylinder. you can buy a lot of fuel, and clean a lot of plugs for what they are paying to get the cylinder replaced.
 
Tom-D said:
This is why we are changing the cylinder on the PA-28, they tried to run lean of peak, ended up leaving one in the red box running at best power.

NO CHT, and they never knew they were smoking the right rear cylinder. you can buy a lot of fuel, and clean a lot of plugs for what they are paying to get the cylinder replaced.
Click to expand...

I've replaced a few #3 cylinders for similar reasons, seems to be a trend.
 
mondtster said:
I've replaced a few #3 cylinders for similar reasons, seems to be a trend.
Click to expand...

The simple fact is, that's the rich one, You lean until you feel the vibration of the leanest cylinder going into the lean misfire state, and the #3 is not the one misfiring, it is still on the rich side of peak, then you richen the leanest cylinder you push the #3 into the best power range.

and it burns up.
 
Jim_R said:
I am a little bit hesitant about relying solely on CHTs for two reasons: 1) it's my understanding that CHTs are just a proxy for how much stress (in the general sense, not in the mechanical engineering sense) the cylinder was under, and 2) my CHT sensors are at the spark plug washer location due to the presence of Tanis heater elements in all my bayonet locations, so I'm not sure how much difference that makes in the CHT readings.
Click to expand...
It makes a significant difference. Tanis ran some tests of the “actual” temperatures (i.e., those measured at the tip of the thermocouple in the CHT well – the “official” values used in certification) versus those measured other ways. They found that the spark plug gaskets read up to 80F high, with the largest difference at the top of the climb. Spark plug gaskets on the lower plugs read hotter than those on the top plugs.

Tanis' newer "washer" type thermocouples (Tanis P/N TAS 2144K, $60 each) that fit onto the Tanis preheater probes in the CHT wells were accurate at climb, idle, and descent, and read 30F colder than the tips at "high speed cruise." A guy I know with a PA-28R-200 got those probes and is very happy with them. However, if I were installing both systems at once, I'd get the Tanis dual probes (Tanis P/N TAS 100-K, $165 each) that replace both the preheater probes and the other thermocouples. They read the temperature at the tip in the well, and so are totally accurate.

So if I can establish some safe power level criteria, and use that in conjunction with an eye on CHTs, that's something of a belt-and-suspenders approach that makes me more comfortable.
Click to expand...
Replacing those spark plug gasket thermocouples with more accurate probes would be one step in the right direction for that.
 
Jim_R said:
(And it also has a manifold pressure port, but I didn't connect it to anything since I've got a fixed-pitch prop. I use that reading in my plots as a proxy for altitude, since the port measures ambient pressure.)
Click to expand...
It wouldn't be much work to add the MP sensor and line.
 
Lets get real with Lean Of Peak operations on any engine with a Natural aspirating engine using a carb, or throttle body fuel injection on individual intake pipes.

You could install every instrument known to man on it, the cylinders will still have a wide variation in mixture resulting in a wide variation in EGTs.

This gives us a very high chance of leaving one or more cylinders on the rich side of peak, and damaging your engine.

The advantage of instrumentation ? you can watch it happen.
 
Tom-D said:
Lets get real with Lean Of Peak operations on any engine with a Natural aspirating engine using a carb, or throttle body fuel injection on individual intake pipes.

You could install every instrument known to man on it, the cylinders will still have a wide variation in mixture resulting in a wide variation in EGTs.

This gives us a very high chance of leaving one or more cylinders on the rich side of peak, and damaging your engine.

The advantage of instrumentation ? you can watch it happen.
Click to expand...

I certainly see your point if you run the engine with a hot cylinder. You can, however, use the instrumentation to see if the cylinders are balanced and if the engine likes running LOP. Certainly, it is a crap shoot until you are monitoring all cylinders, but I know lots of carbed engines that have all cylinder monitors and run LOP quite nicely. The problem is trying to do any adjustments.
 
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Ron Levy said:
It probably wouldn't, but if you're not flying an Archer, you probably need to check your own plane's POH.
Click to expand...

Props get repitched every day by shops all across this nation, and I've yet to see them place any thing in the POH. You haul your prop over to the local prop shop and tell them to twist it up a inch, they will do it and hand it back, and you will see a change in MAP

You best have a STC or an AD if you want to change the POH or Flight Manual for the aircraft,
 
Geico266 said:
I certainly see your point if you run the engine with a hot cylinder. You can, however, use the instrumentation to see if the cylinders are balanced and if the engine likes running LOP. Certainly, it is a crap shoot until you are monitoring all cylinders, but I know lots of carbed engines that have all cylinder monitors and run LOP quite nicely. The problem is trying to do any adjustments.
Click to expand...

Like I said, you can see it happen, that won't stop an unbalance fuel flow to the cylinders built into the Lycoming 4 bangers. If you can get an even fuel flow and even EGTs you re better than the factory.

Why do you believe they adopted fuel injection? and GAMI injectors. answer= To cure the problem of uneven fuel flow and uneven EGTS.
 
Tom-D said:
Lets get real with Lean Of Peak operations on any engine with a Natural aspirating engine using a carb, or throttle body fuel injection on individual intake pipes.

You could install every instrument known to man on it, the cylinders will still have a wide variation in mixture resulting in a wide variation in EGTs.

This gives us a very high chance of leaving one or more cylinders on the rich side of peak, and damaging your engine.

The advantage of instrumentation ? you can watch it happen.
Click to expand...

The whole point of my original post was, "Hey, it's not practical to run LOP in my plane. I want to know what power level is safe to run at peak, instead. How do I make sure I'm staying safe?"

Sorry if that wasn't clear. Ron seemed to understand what I was asking, and has provided some helpful info, as have others.
 
Jim_R said:
The whole point of my original post was, "Hey, it's not practical to run LOP in my plane. I want to know what power level is safe to run at peak, instead. How do I make sure I'm staying safe?"

Sorry if that wasn't clear. Ron seemed to understand what I was asking, and has provided some helpful info, as have others.
Click to expand...
Ron was right in post 4, that's why I did not elaborate on it.

simply remember that carburetors do not deliver fuel to all cylinders equally.
 
Tom-D said:
Ron was right in post 4, that's why I did not elaborate on it.

simply remember that carburetors do not deliver fuel to all cylinders equally.
Click to expand...


And that is the perfect take away message......:yes:
 
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