Big gap between LOP and ROP

[varlajo]

2 hours ago, bigmo said:

I can never get my #1 & #4 to 300F in cruise during the winter. Oil temps are totally fine. Correct me if I'm wrong, but I thought Lycoming specs 150F-400F as the range for continued operations??? I've not been at all concerned with CHTs around 250F (especially when it's -10C out - or colder).

Same. I routinely see 240..260 dF CHTs operating deep LOP in winter, and I am not particularly concerned about lead scavenging as I see zero evidence of deposits on spark plugs. 

[A64Pilot]

Just for kicks yesterday I climbed to 6500 on the way back from Lunch, Running full throttle and roughly 100 ROP I had from memory about 23.5 “ MP with ram air closed, fuel flow maybe 11- 12 GL and hour, but Cyl head temps with full open cowl flaps was in the high green so if I had stayed there it would have cost I guess about 2 more GPH to get them back in the middle of the green. Altimeter setting was 30.07

Reducing to my normal 23 squared and 8 GPH which is real deep LOP cost me at least 10 kts but cyl head temp was in the middle of the green with cowl flaps fully closed.

Cyl head temp was my issue running full throttle and 2500 RPM at 6500, high green is within limits but it’s my belief that it reduces engine life, so I slowed down, saving the engine and cut fuel burn by 1/3, once you factored in the lower speed maybe only 1/4 but still significant. To keep the speed I think I would have had to add another 2 GPH to get the temps down. 14 GPH when I could be burning 8 is to me foolish, I’m just not in that big of a hurry

[A64Pilot]

20 hours ago, varlajo said:

Same. I routinely see 240..260 dF CHTs operating deep LOP in winter, and I am not particularly concerned about lead scavenging as I see zero evidence of deposits on spark plugs. 

I think unless mistaken they may allow as high as 475 continuous, my gauge anyway I don’t think has a yellow range just red and green with 475 annotated at the red.

400 I believe has become the accepted number for continuous if you factor in engine life. Is there hard data to back that number up? I doubt it but don’t dispute that the closer to the middle of the green the better. I think it’s a sliding scale and not some one number, by that I mean that it’s my belief that the hotter, the shorter the life, but that there is also too cool, hence my belief that middle of the green is optimal.

Some want the most speed they can get, period and they can afford it too I guess, and there is nothing wrong with that. Just as I have gotten older I guess the need for speed has lost its appeal. The RV guys in my neighborhood fly wide open all of the time, only reducing power to land. Another with a 182 does also, but he denies it comically. The Bonanza crowd mostly seem to but some of them have backed off it seems, I think form listening to me, some haven’t.

I get a little lead, not much and it’s not hard to get out.

Edited February 13 by A64Pilot

[AndreiC]

Interesting. What I take from this is that my MP gauge may be mis-calibrated, despite having been to the shop not so long ago. Is there a way to check it out somehow? It is the classical steam gauge with fuel pressure on one half and MP on the other.

My CHTs were not a problem at all at the power setting I was at. It's true here in WI temps are much lower than in FL, but I was below 300 dF CHT on all cylinders, I could barely keep them in the green. But my impression from reading the Mike Busch articles (and others), when it is cold outside low CHTs are not the whole story -- you want to control internal cylinder pressures, and when it is very cold outside CHT does not measure this accurately.

[Hank]

24 minutes ago, AndreiC said:

Interesting. What I take from this is that my MP gauge may be mis-calibrated, despite having been to the shop not so long ago. Is there a way to check it out somehow?

Before engine start, your MP should equal the reporter barometer setting.

At altitude, WOT should not be higher than shown on your Performance Tables. 

Other than that, I can't help much.

[Pinecone]

6 hours ago, A64Pilot said:

I think unless mistaken they may allow as high as 475 continuous, my gauge anyway I don’t think has a yellow range just red and green with 475 annotated at the red.

400 I believe has become the accepted number for continuous if you factor in engine life. Is there hard data to back that number up?

380 max for Continentals.

Talk to GAMI or Savvy about what is a good max CHT for you plane.  Assuming you want max life.

[A64Pilot]

17 hours ago, AndreiC said:

Interesting. What I take from this is that my MP gauge may be mis-calibrated, despite having been to the shop not so long ago. Is there a way to check it out somehow? It is the classical steam gauge with fuel pressure on one half and MP on the other.

My CHTs were not a problem at all at the power setting I was at. It's true here in WI temps are much lower than in FL, but I was below 300 dF CHT on all cylinders, I could barely keep them in the green. But my impression from reading the Mike Busch articles (and others), when it is cold outside low CHTs are not the whole story -- you want to control internal cylinder pressures, and when it is very cold outside CHT does not measure this accurately.

Even in Fl in Winter at 23 squared I have trouble keeping my Cyl head temps out of the bottom of the green with cowl flaps fully closed. When that happens usually just a gl an hour richer brings the temps up to where I’m comfortable. With middle of the green cyl head temps and power well below 75% I don’t think you can harm a Lycoming with mixture myself.

Its been long ago since I learned to fly, but back then what was taught in the trainer aircraft was don’t lean below 5,000 ft, and above in cruise slowly lean until it got rough, then very slowly enrichen until it just got smooth, many aircraft didn’t even have an EGT, but that leaning procedure put them usually just barely ROP, right at what many swear is the absolute worst mixture, yet those engines almost always ran well past TBO. They were carbureted aircraft and just simply wouldn’t run LOP.

In 2003 as part of the Army test activity we did Winter testing in the UP of Wisconsin, air density there in Winter is something us Southern guys have to experience to believe, it may be that your MP gauge isn’t that far off?

Edited February 14 by A64Pilot

[CL605]

I’ve been told by a reliable source that aluminum loses 1/2 its strength at about 400. That is my personal limit, and I’ve run every engine I’ve owned well past TBO. It may just be coincidence. 

[A64Pilot]

I don’t think it’s from strength as piston crown temp is much higher.

I think cyl head temp is the way we have to measure cyl stress, it might be better if we could measure peak pressure, but we can’t. I’ve not seen a blown head on a flat motor, I bet it’s rare.

One big way in my opinion that LOP reduces cyl stress is that it effectively is retarding timing, which I believe has a significant effect on peak pressure, another way of course is it reduces power, which further reduces stress.

Only real way to directly compare LOP fuel savings is to establish a cruise LOP, record speed, then go ROP and reduce MP until your speed is identical to what it was LOP. It just might be not as big a difference as many think. But I think there are other reasons to run LOP myself, you just have to get out of that I want to go as fast as it will go or it’s likely you won’t like LOP ops.

 

Edited February 16 by A64Pilot

[Pinecone]

65% power is 65% power.

With NA, you can be at an altitude where you cannot reach 65% power LOP.  So you may need to run at peak or ROP to make the power.

Turbo can make the % power LOP or ROP.

[Tmack201]

Interesting!

[NewMoon]

On 2/10/2025 at 5:58 PM, Ragsf15e said:

That is the benefit of lop and seems reasonable, although I wouldn’t use 50 rop. Compare 100rop to 20lop.

How much speed did you lose?  Should’ve been no more than ~5kts.  Cooler chts too.

 

Exactly, 50ROP will burn it up

[icurnmedic]

I suppose this may not apply, but my  550tn states need to be at least 100 rop, or 50 lop. I usually fall around 75 lop, at 15 gph-ish. That is wot 2450 rpm, and she moves. I really have never checked the rop speed numbers. LOP is 175 tas ,so I am good with that.

[Doug G]

I’ve nearly driven myself crazy with all the data we can get from a good engine analyzer. Bob Kromer was chief Mooney test pilot, see if you can find his articles. His recommendation is cruise at 2500 rpm and WOT leaned to 50 ROP. Mike Busch says LOP in cruise, ROP in climb, avoid the red box by not lingering near peak as you seek LOP. Lycoming says peak EGT for best economy. I think there is a large operating range we can run our Lycoming IO 360s without shortening engine/cylinder life as long as you keep CHT below 400. I would add that too cold isn’t good either and my J runs cool if either too rich or too lean. I had a Bo with a big Continental, that’s another story unless you like replacing cylinders.

I’d be interested in hearing from anyone who has had to replace cylinders or failed to reach TBO in a Lycoming IO360 from a reason other than lack of regular use. That would be more helpful to hear than another ROP vs LOP recommendation. Mike Busch documented one making 5000 hours before overhaul, it was a club plane so while well maintained I’m sure it wasn’t run at optimal mixture very often.


Sent from my iPad using Tapatalk

[Doug G]

I’ve nearly driven myself crazy with all the data we can get from a good engine analyzer. Bob Kromer was chief Mooney test pilot, see if you can find his articles. His recommendation is cruise at 2500 rpm and WOT leaned to 50 ROP. Mike Busch says LOP in cruise, ROP in climb, avoid the red box by not lingering near peak as you seek LOP. Lycoming says peak EGT for best economy. I think there is a large operating range we can run our Lycoming IO 360s without shortening engine/cylinder life as long as you keep CHT below 400. I would add that too cold isn’t good either and my J runs cool if either too rich or too lean. I had a Bo with a big Continental, that’s another story unless you like replacing cylinders.

I’d be interested in hearing from anyone who has had to replace cylinders or failed to reach TBO in a Lycoming IO360 from a reason other than lack of regular use. That would be more helpful to hear than another ROP vs LOP recommendation. Mike Busch documented one making 5000 hours before overhaul, it was a club plane so while well maintained I’m sure it wasn’t run at optimal mixture very often.


Sent from my iPad using Tapatalk

[Shadrach]

On 8/6/2025 at 10:40 PM, icurnmedic said:

I suppose this may not apply, but my  550tn states need to be at least 100 rop, or 50 lop. I usually fall around 75 lop, at 15 gph-ish. That is wot 2450 rpm, and she moves. I really have never checked the rop speed numbers. LOP is 175 tas ,so I am good with that.

It applies to the 550TN. NA Lycoming? Not so much

[Shadrach]

22 hours ago, Doug G said:

I’ve nearly driven myself crazy with all the data we can get from a good engine analyzer. Bob Kromer was chief Mooney test pilot, see if you can find his articles. His recommendation is cruise at 2500 rpm and WOT leaned to 50 ROP. Mike Busch says LOP in cruise, ROP in climb, avoid the red box by not lingering near peak as you seek LOP. Lycoming says peak EGT for best economy. I think there is a large operating range we can run our Lycoming IO 360s without shortening engine/cylinder life as long as you keep CHT below 400. I would add that too cold isn’t good either and my J runs cool if either too rich or too lean. I had a Bo with a big Continental, that’s another story unless you like replacing cylinders.

I’d be interested in hearing from anyone who has had to replace cylinders or failed to reach TBO in a Lycoming IO360 from a reason other than lack of regular use. That would be more helpful to hear than another ROP vs LOP recommendation. Mike Busch documented one making 5000 hours before overhaul, it was a club plane so while well maintained I’m sure it wasn’t run at optimal mixture very often.


Sent from my iPad using Tapatalk

I run my Lycoming IO360 LOP, wide open throttle, at low altitude (1000’-5000’) cruise. This usually equates to about 25-30°LOP.

In the middle altitudes (5000’-10,000’),I run as close to peak as is possible on the richest cylinder. 

Above 10k I will run whatever yields the highest speed and smoothest operation.
 

My CHT guidelines for all of the above are under 350° in the summer and under 330° in the winter for cruise ops.

My cylinders have ~ 750hrs on them and look great on the borescope. They yield cold compressions of 77/80 or better every year. Oil consumption is about 1qt in every 6hrs in the summer. It burns less oil in the winter months. 

[blaine beaven]

8 hours ago, Shadrach said:

I run my Lycoming IO360 LOP, wide open throttle, at low altitude (1000’-5000’) cruise. This usually equates to about 25-30°LOP.

In the middle altitudes (5000’-10,000’),I run as close to peak as is possible on the richest cylinder. 

Above 10k I will run whatever yields the highest speed and smoothest operation.
 

My CHT guidelines for all of the above are under 350° in the summer and under 330° in the winter for cruise ops.

My Cylinders have ~ 750hrs on them and look great on the borescope. They yield cold compressions of 77/80 or better every year. Oil consumption is about 1qt in every 6hrs in the summer. It burns less oil in the winter months. 

Now that I’ve got an engine monitor (G3X with EIS) I’m starting to experiment with LOP. 
 

But I don’t understand how you’re running LOP at 1000’ WOT. If I do that in my 78J with an IO360 I’m still making 27”. At 2500 RPM that’s like 80% power - and my understanding is that it’s not ideal to run LOP when above 75% power. 

I feel like this might be the missing piece of the puzzle for me, so I’m all ears. 

[AndreiC]

3 hours ago, blaine beaven said:

But I don’t understand how you’re running LOP at 1000’ WOT. If I do that in my 78J with an IO360 I’m still making 27”. At 2500 RPM that’s like 80% power - and my understanding is that it’s not ideal to run LOP when above 75% power.

The confusion probably stems from the fact that when LOP power is measured by fuel flow, not MP. The graphs and tables that Lycoming provides assume you are at peak or ROP. 

To understand this, it is quite simple. At peak, you have exactly as much air and fuel needed for a perfectly balanced burn. LOP you have more air than fuel, ROP the other way around. So ROP you need to measure how much air goes in (since anyway you have more fuel than needed, so the power produced depends on air, i.e. MP), while LOP you need to measure how much fuel you put in. 

I’ve been told that at peak or below (LOP), for an IO360, the power is calculated as 15.1 x (fuel flow in gph). (The constant 15.1 depends on the exact compression ratio of the cylinders, so for other engines it is different.) What that means is that if you run at 9.9 gph fuel flow or less you will be below 75%. Below about 9.3 gph you will be below 70%. 

This may or not be achievable while getting your engine to run smoothly. If your injectors are not well balanced, the engine may start to run rough as you lean, before you reach 9.3 gph. This would happen because the various cylinders get uneven amounts of fuel, so one cylinder may be too lean even though the total fuel flow is high. My engine nevertheless can run smoothly quite deeply LOP. I can probably go below 60% power LOP while still smooth. 

The danger zone, as I understand it, is running above peak by a little bit, at a high power setting. Probably at 1000’ altitude, WOT, if you lean to 25 degrees ROP that equates to 80-85% power. This is the kind of situation Mike Busch warns you against. If you want high power (100%), like on takeoff and climbout, you must be full rich which means 200-250 degrees ROP. In level flight, if you want to run at high power (above 75%) Lycoming says you should still be full rich. Around 75% you should be quite rich (125-150 degrees ROP min). Depending on how cautious  you want to be, you can decide to go LOP to 75% power (lean to 9.9gph), 70% (9.3 gph) or less. Thus will probably mean peak, 10-15, or 30-40 degrees LOP, depending on the altitude. Above about 5-6000’ density altitude the air density becomes the dominating factor and you can run at peak without making more than 70% power. Above about 9000’ or so, you cannot make more than 70% even ROP, so you’re safe at all settings.  

Hope this clarifies things. 

[Shadrach]

3 hours ago, blaine beaven said:

Now that I’ve got an engine monitor (G3X with EIS) I’m starting to experiment with LOP. 
 

But I don’t understand how you’re running LOP at 1000’ WOT. If I do that in my 78J with an IO360 I’m still making 27”. At 2500 RPM that’s like 80% power - and my understanding is that it’s not ideal to run LOP when above 75% power. 

I feel like this might be the missing piece of the puzzle for me, so I’m all ears. 

I’m of the opinion that high power settings are well suited to lean of peak operations. My engine has been run in excess of 80% power on the lean side of peak for many hours. Indeed, I used LOP power settings during the break in of my current cylinders.

i’m not recommending that anyone run the way that I do. My SOP has evolved over about 10 years of operations and observations. I’m convinced that it’s the best way to run these engines, but I’m not trying to evangelize. I really don’t have any hard and fast rules on where I set my engine other than maintaining conservative CHT limits. I will use the full mixture spectrum to achieve whatever my goal is under the circumstances. That goal is usually to go as quickly as possible on the least amount of fuel while maintaining cool CHTs. 
 

I do a lot of flying around the DC SFRA.  My mission frequently takes me on trips that are under 100nm were direct routing through the Bravo can often be facilitated by offering  to stay “at or below 2000’ at the end of a request.

[Shadrach]

@blaine beaven

Here is an example of what I am talking about. This was a late fall flight with cool OAT.  To keep CHTs in my desired range on the rich side would have taken in excess of 17gph, maybe more. I set the engine on the lean side of peak at just a hair under 11gph with the throttle wide open, the ram air open and 2500rpm.

 

 

[TangoTango]

14 hours ago, Shadrach said:

I run my Lycoming IO360 LOP, wide open throttle, at low altitude (1000’-5000’) cruise. This usually equates to about 25-30°LOP.

In the middle altitudes (5000’-10,000’),I run as close to peak as is possible on the richest cylinder. 

Above 10k I will run whatever yields the highest speed and smoothest operation.
 

My CHT guidelines for all of the above are under 350° in the summer and under 330° in the winter for cruise ops.

My Cylinders have ~ 750hrs on them and look great on the borescope. They yield cold compressions of 77/80 or better every year. Oil consumption is about 1qt in every 6hrs in the summer. It burns less oil in the winter months. 

This is basically what I do, however I now target closer to 380CHT on the hottest cylinder in cruise.

I ran around 350 CHT for the first 100 hours I had my J model, and I fouled the #4 bottom plug several times (brand new fine wires). After cleaning a surprising amount of lead out of the plugs, rotating them, and changing my target to 380 CHT, I haven't fouled it once in the subsequent 100 hours. I always lean to roughness for ground ops, so I doubt that was the culprit.

[Shadrach]

15 minutes ago, TangoTango said:

This is basically what I do, however I now target closer to 380CHT on the hottest cylinder in cruise.

I ran around 350 CHT for the first 100 hours I had my J model, and I fouled the #4 bottom plug several times (brand new fine wires). After cleaning a surprising amount of lead out of the plugs, rotating them, and changing my target to 380 CHT, I haven't fouled it once in the subsequent 100 hours. I always lean to roughness for ground ops, so I doubt that was the culprit.

That is odd. We have never had an issue with lead fouling and the plane was ground run at full rich for the first 30 years of its life. What kind of finewires? There are pluge with multiple heat ratings available from both Tempest and Champion.

  The only time I see anything close to 380° is in hot, protracted climbs at heavier weights.  YOu might consider moving 

[Ragsf15e]

7 hours ago, blaine beaven said:

Now that I’ve got an engine monitor (G3X with EIS) I’m starting to experiment with LOP. 
 

But I don’t understand how you’re running LOP at 1000’ WOT. If I do that in my 78J with an IO360 I’m still making 27”. At 2500 RPM that’s like 80% power - and my understanding is that it’s not ideal to run LOP when above 75% power. 

I feel like this might be the missing piece of the puzzle for me, so I’m all ears. 

@AndreiC gave you the full details but the cliff notes are here… when lop, % power only depends on fuel flow.  If you can run smoothly at wot and ff is below ~9.9gph, then you are less than 75% no matter your mp.   It is important to ensure that you’re actually lop on all cylinders before using ff as the guide.

[TangoTango]

1 hour ago, Shadrach said:

That is odd. We have never had an issue with lead fouling and the plane was ground run at full rich for the first 30 years of its life. What kind of finewires? There are pluge with multiple heat ratings available from both Tempest and Champion.

  The only time I see anything close to 380° is in hot, protracted climbs at heavier weights.  YOu might consider moving 

Tempest URHM38S, and they now have a couple hundred hours on them. At the time they were brand new.

#4 is my coolest cylinder by quite a bit, so it was hovering around the 300dF mark when I was targeting 350 on the hot cylinder. The fouling was never a problem (it always burned off with a high power runup for a minute or two) but failing a before takeoff mag check will make an impression on the passengers. I haven't failed one since increasing my target CHT.

As far as moving, my summers aren't too much warmer than yours. I know there are several AZ based members here; I flew out to KCHD this May, and that was hot. That trip was the only time I couldn't maintain under 380CHT in a protracted Vy climb.