Big gap between LOP and ROP

[AndreiC]

I went flying yesterday to do some more testing of my plane, and I am wondering if what I see is typical, or if I could improve things perhaps by moving my injectors around. 1970 M20E with IO-360-A1A, 600 SMOH, 350 since new cylinders. 

The difference between what fuel flow I need to run to get to 50 ROP and 20 LOP is quite big. To be specific, yesterday at 6500 feet, 25” MP, 2500 RPM I could fly 20 degrees LOP (on richest cylinder) for 9.3 gph, while to get to 50 ROP on the leanest cylinder I had to push her up to 11.7 gph. 

Does this spread look high to you guys? Is there any way to check that this is correct? My fuel flow is well calibrated, and I can’t see anything unusual about my CHT/EGT probes. (EDM-700).

[redbaron1982]

If you have the data from the engine monitor, you can calculate your GAMI spread (the FF at which the leanest cylinder peaks - the richest cylinder peaks). Anything 0.5 gal or lower is ok and will let you run LOP smoothly.

GAMI spread is what really matters to know how well "tuned" the injectors are.

Edited February 10 by redbaron1982

[Ragsf15e]

4 minutes ago, AndreiC said:

I went flying yesterday to do some more testing of my plane, and I am wondering if what I see is typical, or if I could improve things perhaps by moving my injectors around. 1970 M20E with IO-360-A1A, 600 SMOH, 350 since new cylinders. 

The difference between what fuel flow I need to run to get to 50 ROP and 20 LOP is quite big. To be specific, yesterday at 6500 feet, 25” MP, 2500 RPM I could fly 20 degrees LOP (on richest cylinder) for 9.3 gph, while to get to 50 ROP on the leanest cylinder I had to push her up to 11.7 gph. 

Does this spread look high to you guys? Is there any way to check that this is correct? My fuel flow is well calibrated, and I can’t see anything unusual about my CHT/EGT probes. (EDM-700).

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.

 

[varlajo]

Sounds about right. I don't have exactly the same data set as yours (I don't love either 20 LOP or 50 ROP setting at 6500 ft), but I go from 8.7 gph running 50 LOP at 11500 ft to 10.7 gph 100 ROP. This is running at WOT and 2450 RPM. 1971 M20E, A1A.

[bigmo]

I also have the A1A and almost always cruise between 6K - 10K. Your numbers look pretty close to mine (your ROP are a hair higher, but nothing weird). I do not have GAMI injectors, but have a pretty tight spread. I tend to run 50 LOP as it seems to yield the best bang for the buck. I have notes scribbled down, but think at 6K I was right at 9 even. For each 1K higher in altitude, that number drops about .1 gph. I know that at 12K, I'm right at 8.5 gph.

I honestly don't obsess to much now as I know what the plane likes. I get settled in a cruise, then pull back to a bit above what I know the numbers will be, watch my 'icicles' and lock in around 50. 

[PT20J]

2 hours ago, AndreiC said:

I went flying yesterday to do some more testing of my plane, and I am wondering if what I see is typical, or if I could improve things perhaps by moving my injectors around. 1970 M20E with IO-360-A1A, 600 SMOH, 350 since new cylinders. 

The difference between what fuel flow I need to run to get to 50 ROP and 20 LOP is quite big. To be specific, yesterday at 6500 feet, 25” MP, 2500 RPM I could fly 20 degrees LOP (on richest cylinder) for 9.3 gph, while to get to 50 ROP on the leanest cylinder I had to push her up to 11.7 gph. 

Does this spread look high to you guys? Is there any way to check that this is correct? My fuel flow is well calibrated, and I can’t see anything unusual about my CHT/EGT probes. (EDM-700).

That sounds about right. The FF drops off when you go LOP if you keep MAP and rpm constant due to improved brake specific fuel consumption (gph/hp) because all the fuel is being burned as opposed to some being unburned ROP, and also because the engine is producing less power.

[AndreiC]

25 minutes ago, PT20J said:

That sounds about right. The FF drops off when you go LOP if you keep MAP and rpm constant due to improved brake specific fuel consumption (gph/hp) because all the fuel is being burned as opposed to some being unburned ROP, and also because the engine is producing less power.

Sorry, maybe I was not clear about what I was asking about. To me 11.7 gph to get only 50 ROP seems very high fuel consumption. I was expecting more like 10.5-10.8. But no one seems to be fazed out by this number, so it’s probably ok I guess. 

[Hank]

2 minutes ago, AndreiC said:

Sorry, maybe I was not clear about what I was asking about. To me 11.7 gph to get only 50 ROP seems very high fuel consumption. I was expecting more like 10.5-10.8. But no one seems to be fazed out by this number, so it’s probably ok I guess. 

What do your Performance Tables show for fuel flow at 6500 msl and this power setting?

[PT20J]

7 minutes ago, AndreiC said:

Sorry, maybe I was not clear about what I was asking about. To me 11.7 gph to get only 50 ROP seems very high fuel consumption. I was expecting more like 10.5-10.8. But no one seems to be fazed out by this number, so it’s probably ok I guess. 

25 squared at 6500' is somewhere around 75% power. According to the IO-360 Operator's Manual Figure 3-6 it should burn about 73pph at 75% and 2500 rpm best power mixture which is about 100 dF ROP. This is 12.1 gph. So 11.7 at 50 ROP is in the ballpark. My M20J POH lists 10.4 gph for 75% at 2500 rpm at 25 dF ROP. So, again 11.7 gph at 50 ROP is in the ballpark. If it's running well, I wouldn't fret over it. Another check is that you should be getting around 18 gph +/- at sea level 2700 rpm and full throttle.

[MikeOH]

Chiming in another, "looks about right to me".  Fuel burn is a LOT less, cooler CHTs, and I can live with around 5 kts less....more time in the logbook

[AndreiC]

Fair enough. I am a bit more concerned about something else now. According to what I looked up, 25 squared at that altitude is closer to 85% power. So I assume running at 50 rop is not at all a great idea, as I think this is pretty well inside the “red box”.

I wonder though how accurate these numbers are. My MP gauge was calibrated about a year ago, so I believe it should be showing correctly. But how can you get 25” MP at 6500 feet? Even without induction losses, you would expect at best 29.92-6.5 = 23.42” best MP. 

Could I be having a leak in my MP tubing? Or is there something else going on?

[MikeOH]

If you are really at 25"MP, then 50 ROP would be bad at 85% power.  25"MP does seem high for 6500 feet for non-turbo.  If you are WOT, I don't think a 'leak' in the MP line is going to matter; at WOT I'd think you'll read pretty close to ambient pressure anyway.

[PT20J]

Sounds like you are looking at the Lycoming altitude performance curves or the Aircraft Power app based on them. If so, those curves will overestimate power because they are based on test cell measurements with no exhaust system or induction system losses. You need to reference the POH power tables that are based on the airframe installation. Very doubtful you could get 85% at 6500’ (BTW, was this indicated or density altitude? What was the OAT?) Also, the fuel flow doesn’t sound high enough for that much power. Comparing the TAS to the POH should give you a check on power. 

[Hank]

My C shows 24.5" at 5000 msl, and 22.5" at 7500, so then WOT at 6500 would be 23.3". Not sure how similar the intakes in an E are to my C. Fallout with altitude should be the same, but the starting point is likely different (28.0" at Sea Level for me). 

Was Ram Air open? That would boost MP some, but not to 25".

[AndreiC]

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.

 

Certainly more than 5kts. Closer to 9kts. (152 kts -> 143 kts, calculated by the GPS 3 cardinal points method).

[AndreiC]

How do you guys use the performance tables when the OAT is not standard? My temp gauge is unreliable, but the ground temp (1000 ft) was close to freezing, so I think at 6500 it was probably around -10 C. DA is then 5000ft. Should I use the performance tables for 5000 feet, even though these are calibrated for standard ISA? 

[Ragsf15e]

40 minutes ago, AndreiC said:

Certainly more than 5kts. Closer to 9kts. (152 kts -> 143 kts, calculated by the GPS 3 cardinal points method).

When you are LOP, power falls off quickly the leaner you go.  Much more noticeable than ROP.  You want to be pretty close to peak, obviously leaner than the red box if that’s a factor, but as close to peak as possible.  So at higher altitudes, less than 65% power, you can pretty much run right at peak.  At ~70%, 10-20 lop is fine.  If you’re well lean of peak, speed will fall off quickly.  You can ensure you’re not hurting anything by checking your CHTs.  They should be cooler than ROP.

[MikeOH]

If I'm cruising at or above 8,000 feet, I'm running at peak EGT.

[varlajo]

1 hour ago, Ragsf15e said:

If you’re well lean of peak, speed will fall off quickly. 

But the range will increase.

[Bartman]

9.3 GPH sounds about right for that power setting at that altitude.  I typically aim for 10-15 LOP at that altitude and the higher I go the closer to peak EGT.  As posted above, I lose more than 5 kts at the power setting you described.  I'm typically 145 kts LOP, but others may see more.  I can do 155kts under optimal conditions while ROP, but the juice is not worth the squeeze.

[Bartman]

If cruising ROP, I am always at least 125 ROP.

[0TreeLemur]

15 hours ago, Ragsf15e said:

When you are LOP, power falls off quickly the leaner you go.  Much more noticeable than ROP.  You want to be pretty close to peak, obviously leaner than the red box if that’s a factor, but as close to peak as possible.  So at higher altitudes, less than 65% power, you can pretty much run right at peak.  At ~70%, 10-20 lop is fine.  If you’re well lean of peak, speed will fall off quickly.  You can ensure you’re not hurting anything by checking your CHTs.  They should be cooler than ROP.

For some reason in my J  CHT #2 is always about 20F cooler than the other three.  Its the coolest cylinder.  Always.  One challenge I sometimes have in winter LOP ops is keeping CHT #2 above 300F.   This happens up higher where it is colder and max power WOT is less than 65%.   When that happens I'll run a little ROP just to keep that CHT up.

[Pinecone]

Below 65%, no harm in running a peak.  If I was flying normally aspirated, once I got high enough that WOT was less than 65% I would probably run peak or nearly peak for the speed.

[bigmo]

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).

[jlunseth]

My fuel ranges are 13.3 ROP or slightly higher (depending on temps) at 125 ROP and 11.1 or slightly lower when LOP for the same airspeed. Airspeed for my aircraft varies quite a bit with altitude, usually around 155 kias when down low. I would definitely not recommend running at 50 ROP unless below 65% power. The “normal operations” low temp for the 231 is 240 dF although it can be difficult to keep all cylinders above that number when it is really cold (like today). 231 operators need to keep oil temp above 100 dF. The only consequence I know of for running with low CHTs is poor lead scavenging, which would affect the plugs and the valves.