Is cruising at lower power bad for the engine?

[RescueMunchkin]

Have the IO-360-A1A in an M20F and am more concerned about getting the most flight time at the lowest cost in my plane.  I can cruise at around 105MPH with 6.5 gph burn on my fuel flow meter at around 16" of MP/2450 RPM, but am wondering if that's bad for the engine at all.  I thought that low power is "nicer" on the engine, but then I was reading the wikipedia of the Cessna 175 having a bad reputation because of pilots running at 2500 rpm instead of the recommended 3200 rpm cruise power setting causing engine issues - that made me wonder if what I was doing was bad at all.

Of course I'd like to fly in a way that prolongs the engine life since an OH is one of the largest lump sum costs of maintenance.

[Fly Boomer]

1 minute ago, RescueMunchkin said:

Have the IO-360-A1A in an M20F and am more concerned about getting the most flight time at the lowest cost in my plane.  I can cruise at around 105MPH with 6.5 gph burn on my fuel flow meter at around 16" of MP/2450 RPM, but am wondering if that's bad for the engine at all.  I thought that low power is "nicer" on the engine, but then I was reading the wikipedia of the Cessna 175 having a bad reputation because of pilots running at 2500 rpm instead of the recommended 3200 rpm cruise power setting causing engine issues - that made me wonder if what I was doing was bad at all.

Of course I'd like to fly in a way that prolongs the engine life since an OH is one of the largest lump sum costs of maintenance.

Completely different engine.  That engine was geared, and too low RPM was a killer.

[RescueMunchkin]

3 minutes ago, Fly Boomer said:

Completely different engine.  That engine was geared, and too low RPM was a killer.

Ok, so it's not so much that running low power was the cause - it's the harmonics in the gearing mechanism at that RPM and the reduced air flow for cooling from the low airspeed.

Other than not using the plane for the speed it was made for, is there anything bad about my flying with low power settings as long as I see CHTs below 350?  My takeoffs are WOT and prop full forward then i reduce to 25/25 after about 800 AGL, have to stay under a Class C 1700 ft shelf when I depart so I throttle back and slow cruise pretty early.

[Fly Boomer]

2 minutes ago, RescueMunchkin said:

Ok, so it's not so much that running low power was the cause - it's the harmonics in the gearing mechanism at that RPM and the reduced air flow for cooling from the low airspeed.

Other than not using the plane for the speed it was made for, is there anything bad about my flying with low power settings as long as I see CHTs below 350?  My takeoffs are WOT and prop full forward then i reduce to 25/25 after about 800 AGL, have to stay under a Class C 1700 ft shelf when I depart so I throttle back and slow cruise pretty early.

Certainly nothing wrong with 25/25.  Take a look at your POH for other combinations.  Failing that, look at Lycoming docs for that engine.  You don't necessarily have to stick with only power setting specifically named in the POH, but you might find some admonition against prolonged power settings.  Some engines have power settings that are verboten for continuous running.  Continental, after some issues with the 520, now recommends not going below 2300 for continuous operations.  Look in your documentation.

[philiplane]

seriously, who buys a plane to go slow? 

Low power settings might save some gas. But the engine is built to run at full power all day long. Running at low power, such as below 55 percent, results in more gunk built up inside the engine. Which in turn will shorten the life of the engine.

 

[McMooney]

I hope not, i just found 19/19,  such a nice setting for hamburger runs

[RescueMunchkin]

17 minutes ago, philiplane said:

results in more gunk built up inside the engine. Which in turn will shorten the life of the engine.

 

Is this related to colder engine temps?  And is this gunk permanent, or will the gunk "burn off" with higher power settings?

[RoundTwo]

Just now, RescueMunchkin said:

Is this related to colder engine temps?  And is this gunk permanent, or will the gunk "burn off" with higher power settings?

This week, I pulled off a rocker cover and was presented with a thick coagulated goo of suspended oil and water. This was after a 3 hour flight, right at freezing level last week. Only the cover had it as the cylinder stays warm enough to cook off the moisture.

[jetdriven]

What was your oil temperature? The usual 192?

[philiplane]

1 hour ago, RescueMunchkin said:

Is this related to colder engine temps?  And is this gunk permanent, or will the gunk "burn off" with higher power settings?

Yes, cooler engine temps allow liquid water to combine with the lead sulfates from burnt avgas, producing sludge, and gumming up oil control rings. You want 190-210 dF oil temps, and CHT's in the mid 300's as much as possible for a clean engine. 

Running at lower power and lower MP settings also causes reversion in the induction system. You'll find exhaust deposits and oil in the intake runners. Also not good for the engine. 

Fly fast!

Your engine will be happier.

[PT20J]

Generally, lower power is easier on the engine. The airlines used to cruise the big radials at around 55%. But, as others have pointed out, there is a point where the temperatures are too low. Low oil temperature will allow moisture to collect and cause corrosion, and Ed Kollin (Camguard creator) argues that low CHTs cause poor lead scavenging and increased deposits in the top end leading to valve sticking. The oil cooling and CHT cooling is designed by Mooney to handle high cruise powers because most people buy airplanes to go fast. Bottom line is that running at 65% is probably the best for economy and longevity.

[DCarlton]

I’ll slow down and cruise at 120 sometimes in my F but 105 would seem slow to me unless you are in the pattern.  Are you clear of any low rpm / MP restrictions on your prop ?  

[RescueMunchkin]

1 hour ago, DCarlton said:

I’ll slow down and cruise at 120 sometimes in my F but 105 would seem slow to me unless you are in the pattern.  Are you clear of any low rpm / MP restrictions on your prop ?  

I'm staying clear or any RPM restriction on my prop (I think that's at 2000-2350).

What do you mean by "105 would seem slow"? I could cruise even slower if I wanted to pull more power, but the gear warning alarm would trigger and make the flight unpleasant.  I'm trying to burn as little fuel as possible so I can prolong my flights for time building.

If I wanted to get somewhere faster, I could cruise at 65% or 75%, but that's not my "mission" all the time.  I purchased the plane so I could get the best of most worlds and get to choose to burn less fuel for time flown when flying locally and take the 600 mile trips in 4 hours when I need.

I just wanted to make sure that low power wouldn't cause other issues, but it does sound like it will unfortunately.

Edited February 3 by RescueMunchkin

[DCarlton]

26 minutes ago, RescueMunchkin said:

I'm staying clear or any RPM restriction on my prop (I think that's at 2000-2350).

What do you mean by "105 would seem slow"? I could cruise even slower if I wanted to pull more power, but the gear warning alarm would trigger and make the flight unpleasant.  I'm trying to burn as little fuel as possible so I can prolong my flights for time building.

If I wanted to get somewhere faster, I could cruise at 65% or 75%, but that's not my "mission" all the time.  I purchased the plane so I could get the best of most worlds and get to choose to burn less fuel for time flown when flying locally and take the 600 mile trips in 4 hours when I need.

I just wanted to make sure that low power wouldn't cause other issues, but it does sound like it will unfortunately.

I like the way the plane feels at higher speeds.  That’s all.  105 seems slow to me unless your in the pattern preparing to drop gear and flaps.  It’s a pleasant cruise around 120.  Just personal preference.  I’m not a CFI.  

[Will.iam]

3 hours ago, RescueMunchkin said:

I'm staying clear or any RPM restriction on my prop (I think that's at 2000-2350).

What do you mean by "105 would seem slow"? I could cruise even slower if I wanted to pull more power, but the gear warning alarm would trigger and make the flight unpleasant.  I'm trying to burn as little fuel as possible so I can prolong my flights for time building.

If I wanted to get somewhere faster, I could cruise at 65% or 75%, but that's not my "mission" all the time.  I purchased the plane so I could get the best of most worlds and get to choose to burn less fuel for time flown when flying locally and take the 600 mile trips in 4 hours when I need.

I just wanted to make sure that low power wouldn't cause other issues, but it does sound like it will unfortunately.

Since you are below 65% power you can set the mixture where ever you want and not harm the engine, just set the mixture to peak egt let the cht temps settle and then enriched until your cht’s peak, hopefully they will be above 300 degrees if not then apply more power until they are or if you are using more fuel than you want to keep the cht’s up just practice some slow flight I guarantee you will being up your cht’s and your oil temperature due to lack of airflow into the cowl if the cowl flaps are closed. 
in the summer I can keep the cht’s up on 6 gal/hr in the winter doing some slow flight every now and then keeps them up or I just fly at a higher fuel flow.. I also do not have the rpm limitation and turning 2200 rpm I can apply more power at the same 6 gal/hr and keep the cht warmer. Also with lower rpm more power goes to the prop due to less friction losses.

[ArtVandelay]

seriously, who buys a plane to go slow? 
Low power settings might save some gas. But the engine is built to run at full power all day long. Running at low power, such as below 55 percent, results in more gunk built up inside the engine. Which in turn will shorten the life of the engine.
 

First, if he runs LOP that should minimize build up. No point in running ROP unless you want maximum speed.
I would try to keep cylinder temps above 300°, other than that I wouldn’t sweat it.

But full power all day long, not exactly, not if you mean 100%. The certification tests only requires the engine to run 50 hours at 100% power, that doesn’t mean it will reach TBO when doing that. The other tests are done at 75% power. The engine only needs to run 150 hours to complete the tests, so they don’t really test longevity of the engines.

[Pinecone]

IMO you would be better served by running 60 - 65% power and as lean of peak as you can stand. 

If you want to build time by flying slow, you picked the wrong plane.

 

[N201MKTurbo]

I would run it at 19/19 and lean it as lean as you can get it without it running rough. If you are concerned about dehydrating your oil, change it more often or fly at high power every now and then. 
 

I would say any time spent flying is better for the engine than time spent sitting on the ground. No matter what power setting. 

[philiplane]

the cam in these engines is optimized for 2450 RPM. 

At slower speeds, you get more reversion, and more deposits in the intake and on the valve stems.

[Shadrach]

On 2/3/2024 at 2:06 PM, philiplane said:

the cam in these engines is optimized for 2450 RPM. 

At slower speeds, you get more reversion, and more deposits in the intake and on the valve stems.

I’m not doubting your statement, but I am curious where you found this information?

[skykrawler]

9 hours ago, Pinecone said:

IMO you would be better served by running 60 - 65% power and as lean of peak as you can stand. 

If you want to build time by flying slow, you picked the wrong plane.

 

I think he wants to fly more time with low fuel consumption.   The goal is not slow, slow is more of a result.

As someone pointed out, slow flight tends to keep CHTs up due to less airflow.  I've found that to be true with level flight and gear extended.  Not sure about clean.

 

[Mooney in Oz]

@philiplane or others in the know - Does constantly flying around at low power settings risk cylinder glazing?

[Will.iam]

9 minutes ago, Mooney in Oz said:

@philiplane or others in the know - Does constantly flying around at low power settings risk cylinder glazing?

I thought that was only during breakin but you raise an interesting point. 

[A64Pilot]

Of course some will tell you that Lycoming doesn’t know anything about their engines, but below is their recommendation for maximum engine life, personally I wouldn’t run higher RPM and low manifold pressure, I’d pull the RPM back, but not so low that it vibrates. Of course keep things in the green, if you can’t keep cyl head temp in the green, increase power or enrichen the mixture. I always cruise LOP, probably more LOP than ideal, I’ve found adding 1/2 GPH in mixture will bring the cyl head temp right back up. Even in Fl on a Fl cold day I can’t keep cyl head temp in the green at low power and LOP sometimes.

With any engine there are people that will tell you that they are designed or meant to be run hard and if you don’t you will kill the engine. I don’t know where this idea came from or why it’s so persuasive, but it is, even sail boat engines. The geared motors were hurt by high MP and excessively low RPM, by lugging if you will, people just couldn’t get it into their head that they needed higher RPM than direct drive motors. One engine design even drove the prop off of the Cam so of course the prop turned at 1/2 engine RPM. Good idea because it didn’t need a gear box, but it had lots of unrelated problems.

I personally very rarely cruise above 65% power myself, I’m usually around 60 ish. 8 GPH gives me 135 kts indicated down low, plenty fast for a 20 min trip to eat Breakfast. So long as you keep everything in the green the motor will be fine, Once broken in its very difficult to glaze cylinders, what usually causes them to glaze even in break in is high cyl temps, not necessarily high cyl head temps, but cylinder. Best way to overheat the cylinders is extensive ground ops, and new, tight cylinders due to friction get hotter faster. Glazing is exactly what it sounds like and it’s high temps not cold temps that will cook on a glaze layer. It’s literally a varnish type of coating that clogs the cross hatching.

Lycoming has this stated in more than one of their publications.

• For maximum service life, maintain the following recommended limits for continuous cruise operation:
• Engine power setting – 65% of rated or less.
• Cylinder head temperatures – 400˚ F. or below.
• Oil temperature – 165˚ F. – 220˚ F.
•  
• ‘This is actually the full quote, interesting that they give numbers that apparently don’t match the book for max engine life. I’m sure they are much lower numbers.
•  

• During normal operation, maintain the following recommended temperature limits:
  • Cylinder head temperature – limit listed in the Lycoming Operator’s Manual.
  • Oil temperature – limit listed in the Lycoming Operator’s Manual.
  • For maximum service life, maintain the following recommended limits for continuous cruise operation:
  • Engine power setting – 65% of rated or less.
  • Cylinder head temperatures – 400˚ F. or below.
  • Oil temperature – 165˚ F. – 220˚ F.

 

[DCarlton]

8 minutes ago, A64Pilot said:

Of course some will tell you that Lycoming doesn’t know anything about their engines, but below is their recommendation for maximum engine life, personally I wouldn’t run higher RPM and low manifold pressure, I’d pull the RPM back, but not so low that it vibrates. Of course keep things in the green, if you can’t keep cyl head temp in the green, increase power or enrichen the mixture. I always cruise LOP, probably more LOP than ideal, I’ve found adding 1/2 GPH in mixture will bring the cyl head temp right back up. Even in Fl on a Fl cold day I can’t keep cyl head temp in the green at low power and LOP sometimes.

With any engine there are people that will tell you that they are designed or meant to be run hard and if you don’t you will kill the engine. I don’t know where this idea came from or why it’s so persuasive, but it is, even sail boat engines. The geared motors were hurt by high MP and excessively low RPM, by lugging if you will, people just couldn’t get it into their head that they needed higher RPM than direct drive motors. One engine design even drove the prop off of the Cam so of course the prop turned at 1/2 engine RPM. Good idea because it didn’t need a gear box, but it had lots of unrelated problems.

I personally very rarely cruise above 65% power myself, I’m usually around 60 ish. 8 GPH gives me 135 kts indicated down low, plenty fast for a 20 min trip to eat Breakfast. So long as you keep everything in the green the motor will be fine, Once broken in its very difficult to glaze cylinders, what usually causes them to glaze even in break in is high cyl temps, not necessarily high cyl head temps, but cylinder. Best way to overheat the cylinders is extensive ground ops, and new, tight cylinders due to friction get hotter faster. Glazing is exactly what it sounds like and it’s high temps not cold temps that will cook on a glaze layer. It’s literally a varnish type of coating that clogs the cross hatching.

Lycoming has this stated in more than one of their publications.

• For maximum service life, maintain the following recommended limits for continuous cruise operation:
• Engine power setting – 65% of rated or less.
• Cylinder head temperatures – 400˚ F. or below.
• Oil temperature – 165˚ F. – 220˚ F.

 

I've gotten lazy over the years referencing my POH.  After a while, you just fly.  I've gotta look again and see what altitude 65% becomes the best you can do.  Curious now.