LOP or Peak?

201er · October 10, 2011

Once the power setting is low enough that CHTs are acceptable regardless of peak or LOP (generally below 65%), is it ideal to fly at peak or LOP?

Also when flying at peak, should you have the leanest cylinder at peak at the rest ROP or the richest cylinder at peak and the rest LOP? Does it even matter? Is there something wrong with having a couple cylinders ROP while the others are LOP when aiming for peak?

Jeev · October 10, 2011

Being in the SW I often fly between 7000 and 10000 in much higher than standard temps. From what I have read when under 65% power on a N/A engine you really cant do wrong where ever the mixture is and when running LOP high I get an occasional engine hiccup and give up too much speed for my liking. I get 9.6ish GPH 155-160 kts true in my J between those alts (ball park) at peak. When operating over 70% power I am 20ish LOP or 100-125 ROP. Not scientific just how I fly it and how the engine has been flown for the last 550hrs with no probs and minimal carbon buildup.

**jetdriven** · October 11, 2011

Once below 65% power (Lycoming says peak is authorized at 75% power and below) you can put the mixture where you wish. The BSFC curve is lowest between ~15-50 LOP. So for mose efficiecy, you can cruise at around 15 LOP. If you wish to go a little faster, peak may be better. A little faster still, maybe some more RPM.

I prefer to have all the cylinders on the lean side of peak for most clean and efficient operation. At that low of power setting it shouldnt harm it either way. Our oil filter has zero carbon in it, when our first oil change had a tablespoon or two.

Interesting thing we noticed on our OSH trip is that 15 LOP and 2400 RPM was about the same speed but lower fuel flow than 40 LOP and 2500 RPM. We may have been cruising too lean as a matter of operating procedure.

AndyFromCB · October 11, 2011

Quote: jetdriven

Once below 65% power (Lycoming says peak is authorized at 75% power and below) you can put the mixture where you wish. The BSFC curve is lowest between ~15-50 LOP. So for mose efficiecy, you can cruise at around 15 LOP. If you wish to go a little faster, peak may be better. A little faster still, maybe some more RPM.

AndyFromCB · October 11, 2011

Quote: jetdriven

Once below 65% power (Lycoming says peak is authorized at 75% power and below) you can put the mixture where you wish. The BSFC curve is lowest between ~15-50 LOP. So for mose efficiecy, you can cruise at around 15 LOP. If you wish to go a little faster, peak may be better. A little faster still, maybe some more RPM.

**jetdriven** · October 11, 2011

Our testing has not revealed a red box at and below 65% power in an M20J with good baffles and seals. Others feel free to put your results here. At 75%, Lycoming authorizes operating at peak EGT for economy, but I will not do that, I either run 20-30 LOP or >50 ROP.

I cannot comment on any other engine but an IO-360-A series. But I think a turbonormalized IO-360 is going to have a higher intake air charge temperature compared to a NA engine, and this does lower detonation margins. Theory here suggests that the "red box" area is going to be bigger. I would not operate a TN IO-360 anywhere near peak at 75% power. Of course, testing will tell you what the engine likes.

Quote: astelmaszek

I thought that magic "where you wish" percentage was 60% and below and at 65% that there is a redbox between peak and 100ROP. So goes the lore as I understand it.

Regarding what you said jetdriven and others - percent power is percent power - does it matter if you are turbo or not regarding where is your redbox? Only difference is that power does not drop off wot with altitude. So you have to move the black knob. But I am free to put the red knob as I wish at and below 60%. I am happy to be corrected if wrong.

Erik

jlunseth · October 11, 2011

Quote: 201er

Once the power setting is low enough that CHTs are acceptable regardless of peak or LOP (generally below 65%), is it ideal to fly at peak or LOP?

Also when flying at peak, should you have the leanest cylinder at peak at the rest ROP or the richest cylinder at peak and the rest LOP? Does it even matter? Is there something wrong with having a couple cylinders ROP while the others are LOP when aiming for peak?

201er · October 11, 2011

Thanks. That is definitely an important point to keep in mind.

But once below 65% where is the goal to be mixture wise? You can put it anywhere, but is it ideal to be at peak or LOP? Can't it be argued that outside the red zone and with manageable CHTs, peak is the ideal compromise of power/gas efficiency? So is there any reason to be at 55% power and LOP anyway? At low power settings, are you definitely immune from any risks of improper leaning or poor gami spread?

AndyFromCB · October 11, 2011

Quote: jlunseth

As everyone has said, below 65% you can fly wherever you want to, LOP or peak or ROP. But your question also raises the issue of whether it works to lean to any setting so long as the CHT's are ok, and that is not true. For example, if you went to 75% power and leaned to peak and got CHT's of 380 or less, that would be leaning to a "power setting ... low enough that CHTs are acceptable... ." But it would probably not be a good power setting.

The "red box" is dictated by internal cylinder head pressure, not by CHT. CHT is in part a function of cylinder head pressure, but it is also a function of cooling. So if the cooling in your aircraft is superior, or you are flying in cold temps that keep the CHT's low, you can still have high internal cylinder pressures, and even detonation, without it showing up in the form of high CHT's. This is what they teach at the APS seminar.

The moral of the story is that you need to stay away from the "red box" regardless of your CHTs. Below 65% you are ok, above that, there is a red box regardless of CHT's.

**jetdriven** · October 11, 2011

As our boy Deakin says from the link in the previous post, Mixture for efficiency and RPM for power. Set the mixture to 10-30 LOP for most efficent operation, Peak for a little more speed, 50 ROP for most speed. RPM, 2700 for highest speed, and less for less. The BSFC curve (# of fuel per HP per hour) for our Lycoming IO-360s is best at ~15 LOP to 50 LOP. It doesnt change musch in that range.

at 555 power, the differences become really small. For example, you fly 600NM at 55% power, and 10 LOP, instead of 65% power 10 LOP. You fly an extra 20 minutes and save 2 gallons of gas. 55% at peak is a little faster, burns a little more fuel. But these are small numbers. Its a wash, after accounting for fixed costs. it still comes out ahead, but you are saving a couple dollars an hour, not 20. The old 50 ROP, 11 GPH , 155 TAS in a 201 you can really beat by using 15 LOP, 8.5 GPH, and 147 TAS.

Quote: 201er

Thanks. That is definitely an important point to keep in mind.

But once below 65% where is the goal to be mixture wise? You can put it anywhere, but is it ideal to be at peak or LOP? Can't it be argued that outside the red zone and with manageable CHTs, peak is the ideal compromise of power/gas efficiency? So is there any reason to be at 55% power and LOP anyway? At low power settings, are you definitely immune from any risks of improper leaning or poor gami spread?

**jetdriven** · October 11, 2011

Quote: astelmaszek

I repeat, at least from my reading

http://www.avweb.com/news/pelican/182583-1.html

I have been running on the rule of thumb that 60% or below is the set the mixture where you wish. At 65% there is a redbox from peak to 100ROP.

fantom · October 11, 2011

Quote: jetdriven

Deakin and APS run Continental IO-520s and IO-550s. their red box may be that, but the Lycoming IO-360 does not have such a wide restriction. I wouldnt run peak at 75% in mine, but I would at 70% or less.

jlunseth · October 11, 2011

I am just telling you what they said at the APS seminar. 65%. Not very important to me, the math is definitely different for a turbo which is what I run.

201er · October 11, 2011

Quote: jetdriven

**jetdriven** · October 11, 2011

Here is a detonation survey for the Lycoming IO-360-A done by the factory.Note the conditions. 26.8" of manifold pressure, 2700 RPM, and a CHT of 450-500 degrees. NO DETONATION.

Graph 2 is 28.5" of manifold pressure, 2700 RPM, and CHT of 450-500. You can see this is a pretty extreme test, and there is light detonation at peak. Nobody is going to cruise at this condition. Your climb setting for reference EGT is 250 ROP.

The third graph is a little different. It shows the same 28.5" of manifold pressure, 2400 RPM, and 450-500 CHT. Detonation begins at 50 ROP and is already subsiding at peak, and is gone at 60 LOP. This is due to increased cylinder pressure at 2400 RPM with the same manifold presure.

We can all speculate on where detonation occurs, or we can look at hard data.

201er · October 11, 2011

Fascinating.

Does age and wear/tear of an engine affect real world results to tests?

Is chart #3 a demonstration of the danger of being oversquare?

**Shadrach** · October 11, 2011

That's odd, My a1a is timed to 25 BTDC. I think that I would need to be flying rather slowly to see CHTs in that range at that power setting. That being said, I almost never run 2400 rpm so I don't know.

It would be nice to have the data regarding cooling air flow and to know where the inlet air temp is being measured.

**jetdriven** · October 11, 2011

From the 1E10 type Certificate Data Sheet * All models except IO-360-A1B6D and -A3B6D have optional timing of 20°BTC. News to me as well..

It doesnt say so but I would guess this was done in a test cell to simulate "most adverse conditions". 450-500 CHT is way above what I would allow in an aircraft engine I operate.

**jetdriven** · October 11, 2011

55% or 65%, the LOP value is more efficient than ROP because the BSFC is lower LOP. However, apples to apples it is a small number. When your fixed costs are 30$ an hour and you are running 7.0 GPH and then you go to 6.5 GPH and give up two knots it may become a wash. Now if that extends your range just enough to skip a fuel stop, then it saves a bundle. At 55% power the CHT is a non issue, ours runs around 300-330 at this power setting in the summer, where 65% is more like 360 degrees. At the lower speeds, fixed costs become a larger part of the total costs to fly a trip and your savings rapidly stop increasing.

RPM is also quite important at higher altitude because it makes a large difference in speed and fuel flow.

For example, recent flight, WOT, 6500' altitude, 8800' pressure altitude, LOP 30:

2500 RPM 122.5 KIAS, 8.4 GPH, 139.2 TAS, 16.57 NM/gal

2400 RPM 121.6 KIAS, 8.15 GPH, 138 TAS, 16.94 NM/gal

2300 RPM, 119.9 KIAS< 7.8 GPH, 135.8 TAS, 17.41 NM/GAL

2200 RPM, 118 KIAS, 7.4 GPH, 133.6 TAS, 18.05 NM/gal

2100 RPM, 117.3 KIAS, 7.35 GPH, 132.4 TAS, 18.01 NM/GAL

A good wag for an LOP power setting is 2" greater than a ROP power setting. For us its always WOT so this method is only used below 2000' or so. IE 25" MP and 2500 RPM is ~75% power ROP but around 68% when LOP set to 9 GPH. THe FF method is more accurate. You can lean to 55% power at 25" by leaning to somewhere like 90 LOP. MP reallly is just an indicator of potential power, not what it is putting out. It is possible to ignore RPM and MP and simply set percentage of power by fuel flow. We are starting to do just that.

Quote: 201er

DaV8or · October 11, 2011

Quote: jetdriven

Here is a detonation survey for the Lycoming IO-360-A done by the factory.

DaV8or · October 11, 2011

Quote: 201er

Fascinating.

Does age and wear/tear of an engine affect real world results to tests?

**jetdriven** · October 11, 2011

I got these from a Vans Air Force forum I read. Those guys talk engines at a post-graduate level over there. I saved copies of them. Unfortunately thats as good as the resolution gets for us.

EDIT: source: http://www.vansairforce.com/community/showthread.php?t=75132&highlight=bsfc+detonation&page=9'>http://www.vansairforce.com/community/showthread.php?t=75132&highlight=bsfc+detonation&page=9

http://www.vansairforce.com/community/showthread.php?t=75132&highlight=bsfc+detonation

http://www.vansairforce.com/community/showthread.php?t=74897&highlight=bsfc+detonation

http://www.vansairforce.com/community/showthread.php?t=57435&highlight=bsfc+detonation

http://www.vansairforce.com/community/showthread.php?t=9718&highlight=bsfc+detonation

http://www.vansairforce.com/community/showthread.php?t=7827&highlight=bsfc+detonation