Leaning in flight - is it better, really?

[Red Leader]

Good morning guys! I have a 1981 M20k with the GB engine. We all know this model engine is prone to getting hot fast if you don't watch the temps, even with the intercooler and Merlyn (which I have)  Usually I run LOP and watch my pressures and temperatures - running about 29" and keeping it under 400 degrees. At these settings, I don't get the of speed our model is known for but I do get good fuel burn (9 - 9.5gps). On my last trip, I was running a little late on the return leg so I ran it ROP. I was burning around 11gph, going noticeably faster while not seeing the high cylinder temperatures I notice on my typical flights. Other than fuel burn (and higher speed) is there anything I am doing wrong by running it ROP? 

[Pinecone]

No, ROP is fine.  I run ROP when I need the speed.  Just make sure you are at least 100 - 125 degrees ROP.

But you can also run a higher manifold pressure to get a higher % power when still LOP.   I run my -SB at 29.5/2300 and 10.1 GPH for 63 - 64% power.

For a Continental, Mike Busch (and others) recommend keeping CHTs under 380.

[Falcon Man]

Previous experience with my K models operated in cruise at peak or 25F LOP, TIT ~ 1550F:

1) 231 w/ LB engine, intercooler and auto wastegate, cruise 2200 rpm and MP to keep CHT's less than 350F, lean test showed < 0.5 gph spread, cylinders lasted ~1500 hrs. The cowl flaps had to be at least partially open almost all the time in cruise.

2) 262 w/ MB engine operated same parameters, cylinders never > 350F, lean test showed 0.3 gph spread, I had to modify the cylinder baffles on # 3 & 5 to lower CHT on # 3 (5 was very cool and 3 was too hot w/ stock baffling). In the cooler times the cowl flap was usually closed, sometimes trailing open in the hotter times. This engine went ~ 700 hrs past TBO.

The speed loss due to open cowl in cruise was just a couple of knots TAS.

My goal was to keep the engine out of the Red zone.

 

 

 

 

 

 

 

 

o keep the engine out of the RED box.

[Ragsf15e]

13 hours ago, Red Leader said:

Good morning guys! I have a 1981 M20k with the GB engine. We all know this model engine is prone to getting hot fast if you don't watch the temps, even with the intercooler and Merlyn (which I have)  Usually I run LOP and watch my pressures and temperatures - running about 29" and keeping it under 400 degrees. At these settings, I don't get the of speed our model is known for but I do get good fuel burn (9 - 9.5gps). On my last trip, I was running a little late on the return leg so I ran it ROP. I was burning around 11gph, going noticeably faster while not seeing the high cylinder temperatures I notice on my typical flights. Other than fuel burn (and higher speed) is there anything I am doing wrong by running it ROP? 

It’s interesting to me that you say you were slower and hotter LOP… if you ran the same power lop vs rop, you should see roughly the same speed, but lower CHTs when lop.  If you’re running higher power ROP (to get the more speed), then you’d likely see higher CHTs.  Exactly what mp, rpm and ff do you use lop vs your power setting rop?

[PeteMc]

15 hours ago, Red Leader said:

Usually I run LOP and watch my pressures and temperatures - running about 29" and keeping it under 400 degrees.

I had a -GB engine for a few years when I first got my plane and then went to the -LB.  But from what I remember of the -GB, if you're only running at 29" and you're having problems keeping the cylinder below 400 degrees, something seems off.  Is this from the single factory cylinder temp probe or do you have a JPI or some other engine monitor?  With a stock -GB at 9.6g/hr, 31.8" and 2200 RPM, you'd be at 65% power (Best Economy).  You're below that and the Intercooler should be bring your temps down below the stock engine.  An even at a higher RPM with that fuel flow, you're even lower than 65%, so again, the 400 degrees seems strange.   

And if you've been going to the same mechanic for years, maybe you want to have someone one else take a look.  For starters, I'd be checking to see if the temperature probe(s) are giving you the correct reading.  Also, is the baffling sealing correctly or is there just no air going through the cooling fins.  And I guess it could also be your Tach or MP gauge that are off.  I'd say to check the Fuel Flow too, but you'd probably notice when you filled up if the 9.6g/hr was off.  

 

 

[Red Leader]

Good points, all. My engine monitor is a new Insight G4 installed a couple of years ago. I took a picture of the instruments on that flight and just looked again - ROP I was at 11k feet, 30" and 2350rpm. The engine just loved that setting and I was faster by about 5 knots indicated. Still running close to 400 on 5 & 6 but lower on 3 & 4 and lowest on 5 & 6. The temperature spread was about 100-degrees (between #1 and #5) and my fuel flow indicated a solid 10.2gph. Not a big difference but noticeable. I don't remember things running that smoothly when operating LOP. In fact, usually when lean, the engine would stumble several times during cruise. On shorter trips when I lean, it might not stumble at all, but the fact that I feel it when LOP and can also feel the reduction in speed when leaning suggests I may not be doing something right. I am taking another short trip tomorrow and will play with it a little more.

[Falcon Man]

It is common for the GB, LB and some MB engines to run slightly rough when LOP. My LB engine didn't stumble at peak or slightly below. I tried all sorts of RPM/MP settings and it didn't make any difference.

[Ragsf15e]

2 hours ago, Red Leader said:

Good points, all. My engine monitor is a new Insight G4 installed a couple of years ago. I took a picture of the instruments on that flight and just looked again - ROP I was at 11k feet, 30" and 2350rpm. The engine just loved that setting and I was faster by about 5 knots indicated. Still running close to 400 on 5 & 6 but lower on 3 & 4 and lowest on 5 & 6. The temperature spread was about 100-degrees (between #1 and #5) and my fuel flow indicated a solid 10.2gph. Not a big difference but noticeable. I don't remember things running that smoothly when operating LOP. In fact, usually when lean, the engine would stumble several times during cruise. On shorter trips when I lean, it might not stumble at all, but the fact that I feel it when LOP and can also feel the reduction in speed when leaning suggests I may not be doing something right. I am taking another short trip tomorrow and will play with it a little more.

I suspect you’re a little too much LOP which is causing the stumbling.  Id recommend starting LOP to try a setting that will get you 65% so you can’t hurt anything and then play with it from there.  You just need the FF for your engine.  As long as you’re on the lean side of peak,  FF is the only determinant of power.  So your engine, 65% is 210hp x .65% = 136.5.  Divided by 13.7 gives you a ff of ~10gph.  Try that ff at say 27/2300 or so maybe 28/2300 if it will run smoothly.  set the mp/rpm, then big mix pull to 10, then reset the others back to 27/2300 or 28/2300 as required.
Then slowly richen until peak to see how far lean of peak you were (and then set back to 10gph).  At 65%, you can shoot for closer to peak, maybe 20-40degrees.  If it’s too lean, pull throttle back just a little as that’s what is controlling the air into the cylinders.

you should see much cooler cylinders.

[Falcon Man]

Have you performed a GAMI lean test to see what your FF difference is for the injectors? 100 degree CHT spread between cylinders is very high.

[Red Leader]

Don't have Gami's but have been considering them. At my last annual I had several fuel flow tests performed and the output of the injectors was almost exactly the same.

[Pinecone]

The goal is not to have the injector outputs exactly the same, but to have each one matched to the airflow of THAT cylinder.

But, do the GAMI lean test.   They won't sell me a set of GAMijectors.  They say my engine/injectors won't benefit from them.

[jlunseth]

I have an LB and a JPI930 but in all other respects about the same as yours. I don't think you are running ROP at all at 11 GPH and 29". In my engine, ROP at that MP and 2450 RPM would be 13+ something GPH to get to about 125 degrees ROP.

Maybe the problem is, what makes you think you are running LOP at that setting? 

Here is what I suspect. I don't know your engine monitor, but using my JPI the standard instructions are to start by running the engine well on the ROP side, put the monitor in lean of peak mode for leaning, and then lean out until the monitor tells you that you are LOP. Select your degrees LOP from the monitor and there you are. The problem is that this method is completely wrong for the 231. Lean of Peak and Rich of Peak are air/fuel ratio settings. They are not power settings although they can be used to make a power setting. The leaning mechanism in the monitors I am familiar with are all algorithms, in other words, our aircraft have no sensors that directly measure the air/fuel ratio the engine is running at.

The algorithms I am familiar with all make the same assumptions, (1) that you start on the Rich of Peak side, and (2) that while you are leaning the fuel all other things remain equal, most importantly, that the manifold pressure remains where you set it when you started your leaning process. This works in the 252 and other turbo Mooneys that have truly automatic wastegate controllers that maintain MP where you set it while you lean out the fuel. The problem with the 231, even with the Merlin, is that MP does not remain where you set it if you start reducing the fuel flow. The Merlin is a differential controller, not a density controller. There are several things going on. One, there is an interlink in the TSIO360 engine so that if you set a power setting of, let's say, 30" and 13.3 GPH, and then decide you want to slow to approach speed, you can pull the MP back and the interlink automatically reduces the fuel flow to maintain roughly the same air/fuel ratio that you had at 30/13.3. Notice that when you do this the air/fuel ratio is not changing, you are still running ROP even though the EGTs have all dropped from their starting point. This interlink in my experience does not work as effectively when you pull the fuel back as it does when you pull the MP back, but it does work. So the MP is dropping as you lean the fuel. Second, as the engine begins to produce less power the turbo produces less compression, further causing the MP to drop as you pull the fuel back. These things are causing you to violate the basic assumption of the leaning algorithm - that MP stays steady while fuel flow is reduced, thus changing the air/fuel ratio from a rich setting to a lean setting. In short, if you do your leaning this way all you have done is reduce power, but you have done little to change the air/fuel ratio. The engine monitor will most likely detect a drop in EGT in all the cylinders, which is their signal to tell you that you are running lean of peak when the monitor really has no idea where you are running any longer.

We just did a lean test on my new engine. The proscribed method is to start at an approximately 65% Rich of Peak setting, then slowly lean in increments of .3 GPH and record the EGT's on each cylinder. We chose 24.8 MP, 2400 RPMs, and 9.5 GPH, which I know from experience is a ROP setting. A move of .3 GPH did not change the MP much but a move of .6 changed it by a few tenths, enough that for about every two moves of .3 GPH we had to bump the MP back up a little. If you make a relatively big fuel move, like 11 GPH to 9.5 GPH you will get a fairly large MP decrease - on the order of 1" or more - and that invalidates the lean test you think you are running.

If it helps - if I want to fly at least 125 degrees ROP in my engine at an MP of 29", I need a fuel flow of 13.3 GPH to 13.5 GPH. A setting of 11 GPH is going to be a LOP setting most likely, but not a very LOP setting at 29". At 29" you are probably just a few degrees LOP, pretty close to peak. Anything in the 12's is going to be a Peak setting or worse, around 50 degrees rich of peak.

There is more to it, but based on what I would see in my engine at those power settings your 9.5 setting is probably a 60-65% power LOP setting and your 11 GPH setting is probably not lean of peak at all or if it is, it is just barely lean of peak, but it is not rich of peak. You are running too close to peak.

[Fix]

On 2/25/2025 at 2:23 AM, Ragsf15e said:

So your engine, 65% is 210hp x .65% = 136.5.  Divided by 13.7 gives you a ff of ~10gph.

Been thinking about this.

If I want to fly 65% LOP on a 210HP Engine (GB/LB/MB) we can use this formula above.

But I have an -SB with 220HP, and the same formula says that 65% is 10.5gph.
And 10.5gph on the 210HP would be 68.5%

Owners with -SB engines do you calculate fuel flow / LOP for 210HP or 220HP?

 

[jlunseth]

I don’t have an SB but I can tell you that the LOP formula divides by rated horsepower of the engine, in your case 220. That is what “percent power” means, it is percent of the engine’s rated horsepower.

But just remember that LOP is an air/fuel ratio, not a power setting. In other words, just because you are running the engine at 10 GPH or whatever number you come up with from applying the formula to your engine does not necessarily mean you are running lean of peak. You need sufficient manifold pressure to make the air/fuel mixture lean. It probably does not make much difference in this instance since you can run the engine LOP, ROP or at peak if you are operating at or under 65% HP, which you probably would be. In other words, it does not matter much whether you are lean of peak or not at a 65% power setting, it is safe for the engine. But it is important to know that you are actually operating lean of peak because the formula only applies to lean of peak operations, it definitely does not apply to rich of peak operations. %HP for rich of peak operations must come from your POH table.

[Pinecone]

4 hours ago, Fix said:

Been thinking about this.

If I want to fly 65% LOP on a 210HP Engine (GB/LB/MB) we can use this formula above.

But I have an -SB with 220HP, and the same formula says that 65% is 10.5gph.
And 10.5gph on the 210HP would be 68.5%

Owners with -SB engines do you calculate fuel flow / LOP for 210HP or 220HP?
 

220, that is what it is rated at.

[Pinecone]

Easy way to tell of you are LOP or ROP.

While watching the EGT, lean or richen the mixture.  If you lean and the EGT goes down, you are LOP.  If you lean and the EGT goes up, you are ROP.  If the EGT goes down both ways, you are at Peak.

Richening would be the opposite.  If richer is higher EGT you are LOP.  If richer is lower EGT, you are ROP.

The problem comes, if you think you are LOP, but you are not.  That puts you in the slightly ROP range, which is not good.  If you are at 65% power or less, not a big deal.

Once you figure out an LOP power setting, you can pull back the throttle, pull the prop back, and set your proper fuel flow.  I run at (-SB) 29.5 inches, 2300 RPM, 10.1 GPH.  And that is 65% power and about 25 degrees LOP.

[Marc_B]

You have to use the HP of your engine as well as the proper coefficient for your compression ratio. Even then it’s probably not perfect but close. I scribbled these for a 252 buddy talking about FF on the LOP side. One is for 252 (210hp) and the other is for Encore (220hp).