LOP vs ROP in real-life

[Max Clark]

For the record I'm a WOT+LOP guy and do most flying in the 9-12k MSL range.

Ended up lower this weekend to avoid a gnarly headwind and plenty of fuel in the tank which lead to a little experimentation:

6500' MSL
WOT
2400 RPM

147 KIAS LOP 11.1 GPH
178 KIAS ROP 17.2 GPH

Not sure that this will change how I fly/engine management, but I found it interesting.

[Fly Boomer]

2 hours ago, Max Clark said:

147 KIAS LOP 11.1 GPH
178 KIAS ROP 17.2 GPH

Do you have the degrees off peak for both?

[Max Clark]

ROP used the 1400-1450 power band - which oddly matches the LOP range exactly exactly. LOP stopped using Garmin's EIS and just to the big pull and enrichen. FWIW I'm usually in the 40-60° range according to the EIS. But for simplicity now I have a good feel for where GPH should land after the pull so I'm watching that and listening to the engine.

[Deb]

5 hours ago, Max Clark said:

For the record I'm a WOT+LOP guy and do most flying in the 9-12k MSL range.

Ended up lower this weekend to avoid a gnarly headwind and plenty of fuel in the tank which lead to a little experimentation:

6500' MSL
WOT
2400 RPM

147 KIAS LOP 11.1 GPH
178 KIAS ROP 17.2 GPH

Not sure that this will change how I fly/engine management, but I found it interesting.

11.1 gph LOP yields a power setting of approximately 59% (14.1 gph is ~75% power).

17.2 gph ROP at a pressure altitude of 6234’, 2400 RPM yields a power setting slightly greater than 75%.

A more relevant experiment might be to compare airspeeds at 65% LOP (~12.3 gph) at 2500 RPM and 65% ROP (18.7 MP, 2500 RPM, 14.1 gph, 50° ROP according to the table). At 10,000’ LOP we see ~175 kts TAS at 2500 RPM, 12.3 gph and WOT. ROP fuel flow at 10,000’ WOT, 2500 RPM and 75° ROP might be close to 15 gph; that would probably also get ~175 kts.

[Max Clark]

13 hours ago, Deb said:

A more relevant experiment might be to compare airspeeds at 65% LOP (~12.3 gph) at 2500 RPM and 65% ROP (18.7 MP, 2500 RPM, 14.1 gph, 50° ROP according to the table). At 10,000’ LOP we see ~175 kts TAS at 2500 RPM, 12.3 gph and WOT. ROP fuel flow at 10,000’ WOT, 2500 RPM and 75° ROP might be close to 15 gph; that would probably also get ~175 kts.

I doubt I'd ever go through the hassle of figuring out 65% LOP vs 65% ROP. For me I was curious what's the difference LOP vs ROP at the same throttle and rpm.

[hypertech]

Something seems off.  If you try that again higher (10-11k), what do you get?  I lose 5-10 knots LOP not 30.  At 10-11k and 30 LOP I'm getting just under 170 true.

[Rick Junkin]

35 minutes ago, Max Clark said:

I doubt I'd ever go through the hassle of figuring out 65% LOP vs 65% ROP. For me I was curious what's the difference LOP vs ROP at the same throttle and rpm.

The thing is, power output is determined by fuel flow when you're LOP. The RPM and MP are irrelevant from a power output calculation perspective when you're LOP, although they must be managed to keep the EGTs reasonable. There is a spreadsheet in the downloads section here (link below) that allows you to input your specific engine parameters and it will give you the HP output of your engine at any given fuel flow when LOP.

ROP power output, on the other hand, is a more complicated calculation involving RPM, MP, fuel flow, and atmospherics. Using the charts in the POH and the engine operating manual is the easiest way to determine the power output at any given combination of parameters. 

This is borne out by the CHTs in your examples. Your LOP CHT is 274 (which is extremely low) at 59% where your ROP CHT is 339 at 75%. This is because of the extreme difference in the power output (59% vs 75%) and of course the airspeeds will be very different.

40º-60º LOP is unnecessary at the power output you are running, and results in much lower power output (airspeed) with no additional benefit to your engine. The fuel savings may be a false economy based on the airspeed differences but I haven't done the math on that. At 65% power you can run AT peak EGT and your engine will be happy. If you were running at 75% power LOP then you would want to be about 40º LOP to stay out of the "red box".

There's a little more to it than running as lean as you can, but you're on the right track.

Cheers,
Rick

 

[Chocol8]

1 hour ago, Rick Junkin said:

The thing is, power output is determined by fuel flow when you're LOP. The RPM and MP are irrelevant from a power output calculation perspective when you're LOP, although they must be managed to keep the EGTs reasonable.

 

That's a simplifying assumption that is good enough for back of the envelope calculations, but not entirely true all the time.  You hit on it with the RPM.  If the timing and RPM is such that combustion is occurring too late in the cycle, the fuel burn is not all going towards turning the crank and power is going out the exhaust just like unburnt fuel when RoP.  This shows up on EGT/TiT numbers so the "must be managed" part is critical.  

[hazek]

1 hour ago, Rick Junkin said:

The RPM and MP are irrelevant from a power output calculation perspective when you're LOP

MP matters when one isn’t WOT, no? More air changes how far LOP one is for a given FF, right?

[Rick Junkin]

32 minutes ago, hazek said:

MP matters when one isn’t WOT, no? More air changes how far LOP one is for a given FF, right?

If you change MP, the FF will change with it. For example if you are WOT running 30º lean of peak and you reduce the throttle (MP), the fuel flow will also decrease and the mixture will remain lean of peak. It probably won't be the same number of degrees lean of peak, and the peak EGT itself will be lower at the lower MP. If you haven't reduced MP pressure too much you can increase the fuel flow (mixture) back to where it was at WOT and still be lean of peak but closer to peak EGT, and the power output will be exactly the same for that fuel flow. So yes, MP affects where you are with reference to peak EGT (use RPM and MP to manage EGT) but regardless of how far lean of peak the engine is running, it will still put out the same amount of power for a given fuel flow when lean of peak, regardless of RPM or MP.

[Rick Junkin]

The Masters Class in how all of this works, particularly in turbocharged engines but also applicable to normally aspirated engines, is the series of articles written by John Deakin in his AvWeb column "Pelican's Perch". Written many years ago, they are timeless in their detailed technical explanations and methods for practical application. Google "Pelican's Perch" and you'll find them.

Unfortunately the conglomerate that now owns AvWeb has co-opted John's byline and republished his articles with an "Editorial Staff" byline. Be that as it may, the information is there. It's quite voluminous but well worth the time to read and study if you want to gain an expert understanding of engine operation and management.

Here's an example of John's writing, still rightly attributed to him.

https://avweb.com/features/pelicans-perch-84-dont-set-mixture-with-cht/

 

[buddy]

On 5/4/2026 at 1:53 PM, Max Clark said:

For the record I'm a WOT+LOP guy and do most flying in the 9-12k MSL range.

Ended up lower this weekend to avoid a gnarly headwind and plenty of fuel in the tank which lead to a little experimentation:

6500' MSL
WOT
2400 RPM

147 KIAS LOP 11.1 GPH
178 KIAS ROP 17.2 GPH

Not sure that this will change how I fly/engine management, but I found it interesting.

 

[buddy]

178 KIAS at 6500’ and 2400 RPM??? Are you sure it’s not 178 True Airspeed???

[Max Clark]

8 minutes ago, buddy said:

178 KIAS at 6500’ and 2400 RPM??? Are you sure it’s not 178 True Airspeed???

Correct that was a typo.

[dkkim73]

21 hours ago, Rick Junkin said:

The Masters Class in how all of this works, particularly in turbocharged engines but also applicable to normally aspirated engines, is the series of articles written by John Deakin in his AvWeb column "Pelican's Perch". Written many years ago, they are timeless in their detailed technical explanations and methods for practical application. Google "Pelican's Perch" and you'll find them.

A primary reference IMHO. I also paid for the Advanced Pilot Seminars online course a couple years ago. While dated, it is still good information. A bit pricey but I was new to the scene and in a hurry prior to my initial transition training, so it was worth it at the time. But all the info is available for free and Deakin (who was involved in the course) outlined his thoughts well in that article series. Read all of them, IMHO. 

[GeeBee]

On 5/4/2026 at 1:53 PM, Max Clark said:

For the record I'm a WOT+LOP guy and do most flying in the 9-12k MSL range.

Ended up lower this weekend to avoid a gnarly headwind and plenty of fuel in the tank which lead to a little experimentation:

6500' MSL
WOT
2400 RPM

147 KIAS LOP 11.1 GPH
178 KIAS ROP 17.2 GPH

Not sure that this will change how I fly/engine management, but I found it interesting.

You are at 6700 ft DA. If you set 22.2/2400 and fuel flow at 12.3 you are at 65% power which will yield conservatively 167 knots TAS and be at 50 LOP. Going to 11.1 is yielding 147 KTAS which yields 13.22432 nm/gal. If you go to 12.3 LOP you get 13.5772 nm/gal. If you go LOP you want to be at 50 LOP/65%. It is the sweet spot which is why it is in your AFM because it gets the most miles per volume of fuel. Your LOP operation at 11.1 is requiring a higher angle of attack increasing your induced drag. Your ROP is yielding 10.3488 nm/gal

 

[hazek]

On 5/5/2026 at 7:41 PM, Rick Junkin said:

If you haven't reduced MP pressure too much you can increase the fuel flow (mixture) back to where it was at WOT and still be lean of peak but closer to peak EGT, and the power output will be exactly the same for that fuel flow.

This doesn't track for me.

If MP is reduced (which reduces FF proportionally, even if not at the exactly the same ratio) and then FF is increased to the previous FF, the new FF per air available (which is now reduced from before) will put the combustion closer to peak EGT at from position 1 to position 2 on the graph. It's not like the amount of air available on the LOP side suddenly doesn't matter anymore, right? The combustion temperatures and pressures are the result of both the amount of air and fuel?

Isn't it only that the combustion is more efficient and only constrained by the efficiency of the engine and the amount of fuel supplied rather than the amount of air when LOP when the amount of air is fixed e.g. at WOT? But the ratio still matters evidenced by the fact that the Percent Power curve is not linear nor is the Specific fuel consumption curve. Too far LOP, i.e. too much air for a given fuel, will actually reduce efficiency and eventually starve the engine, also evidenced by the specific fuel consumption curve at the bottom?

So it matters equally how far LOP one is for a given FF. If too far LOP for the same FF, one is producing less power because the combustion is now less efficient. If less LOP for the same FF, one is producing more power. Or am I wrong?

I was careful to say that if one is at WOT, then yes, MP is a fixed variable. You can only change FF and so once LOP, whatever power output you get is what your engine can produce for a given set FF based on the graph below. But if you have a turboed engine or are not at WOT, then MP also becomes a variable, no?

[Rick Junkin]

2 hours ago, hazek said:

This doesn't track for me.

You can learn all about it here. https://www.advancedpilot.com/onlinecourse.html

And here. https://avweb.com/topics/flight-safety/fbo/pelicans-perch/