Is cruising at lower power bad for the engine?

[A64Pilot]

4 minutes ago, DCarlton said:

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.  

Unless you’re willing to run max governed RPM it’s likely lower than we think. I like to pick an RPM that’s close to the middle of the green myself but one she is smoothest, you can feel what smooth is by holding the glareshield. Smooth is gentler on everything avionics included.

I too am lazy and as I don’t have to cross mountains and don’t do O2 anymore I always fly LOP so being lazy I use the 1GPH = 15 HP. It may not be exact, but for me it’s close enough.

8x15=120 HP and 200 div by 120 = 60% power. So I fly at 60% power almost always

[A64Pilot]

To continue 8 GPH at 23 squared gives me 135 kts which is 16.9 MPG

6 GPH at 21 squared gives me 120 kts, which is 20 MPG, but 120 is usually too slow for me, and 3MPG just isn’t a big enough number to matter to me. Maybe if I had to push range though.

Climb higher and run 155 kts at 10.5 GPH gives you 14.8 MPG, but I’ve found that I burn more climbing than I realize so unless it’s a several hour flight I don’t.

Just for grins a Legend Cub’s book cruise speed at 75% power is 82 kts, an O-200 will burn about 6 GPH which gives the Cub 13.6 MPG, the Mooney J model gets better fuel milage than a Legend Cub at any cruise speed.

My C-140 flies at 91 kts burning 5 GPH which gives it 18 MPG. I’ve not checked any lower speeds and the fuel burn is a SWAG as it doesn’t have fuel flow, but I’m sure it could match the Mooney at 20 MPG.

[Shadrach]

@DCarlton it’s not as low as some would think according to the POH.

[DCarlton]

3 hours ago, Shadrach said:

@DCarlton it’s not as low as some would think according to the POH.

Yep, that’s a bit higher than I remembered.  I usually cruise between 8 and 10k at 2400 full throttle.  Might be good to slow the rpm’s down a tad more.  Will have to watch manifold pressure more closely.  

[Shadrach]

5 hours ago, DCarlton said:

Yep, that’s a bit higher than I remembered.  I usually cruise between 8 and 10k at 2400 full throttle.  Might be good to slow the rpm’s down a tad more.  Will have to watch manifold pressure more closely.  

I’ve always felt like 2500 was optimal for noise, speed and N, V & H. 

[Echo]

Of course you can fly it however you want, but if I wanted slow hamburger runs I would of bought a Luscomb or a Citabria.  I bought the Mooney as a means of traveling fast from point A to point B.  No worries about other drivers and stop lights.  Just covering a lot of ground quickly to arrive at the destination of choice.  My prop has an RPM limitation regarding MP for continuous operations.  I work around that.

[A64Pilot]

Those charts are at best power? 

LOP is significantly lower power output, one reason to cruise lower power is reduced fuel burn, so surely your LOP. I believe it’s pretty simple if I understand. Forget the charts, to be at 75% power for our angle valve motors LOP you have to have a fuel burn of 10 GPH. If you turn middle of the green RPM that’s going to require more than 25” MP I believe, and the general rule of thumb is 1” MP loss per 1,000 ft, so the highest you can pull 25” on an average day is 5,000 ft.

Do LOP cruise charts exist? Is there even a need?

10,000 ft you can only get roughly 20” MP so to keep power up requires both higher RPM and best power mixture.

That rule of thumb isn’t linear of course or otherwise you would be in space at 30,000 ft, but it’s close enough for us low altitude guys.

As we are talking engine longevity RPM is tied to wear, actually it’s piston speed and that’s why one motor can run higher RPM and not have high wear because of its stroke, where a longer stroke motor can’t.

But anyway within limits reducing RPM will reduce wear. But within limits, as a general statement higher RPM wears on the valve train while low RPM with corresponding higher load to maintain HP wears on the pistons, cylinders and bottom end. If you keep MP the same and accept the power loss then lower RPM increases engine longevity.

The Green ranges are of course the points where the engine manufacturer expects if obeyed an engine will make it to TBO, those of us that are working for an even longer life if we run closer to the middle of the green ranges it’s likely the engine will last longer.

The IO-540 line is a very good example, those motors range from 235 to 300 HP, there are differences of course but the 235 is to a great extent mostly a de-tuned if you will 540, de-tuned primarily by lowering the RPM limit, it’s max RPM for example is 2400 RPM. I prop struck mine years ago and as it was right at TBO and as I’m an A&P/IA I chose to overhaul it for resell reasons. I sent everything off of course, but every single wear item I sent off was still within NEW limits, not serviceable but new. That motor would likely have gone to twice TBO is my guess just by following the Lycoming mandatory SB replacement items required on major disassembly.

I’m not saying don’t cruise at higher RPM if you choose to, but primarily by turning the RPM limit down to 2400 RPM max Lycoming hugely increased the life of the 235 HP IO-540. Why didn’t they increase the TBO beyond the 2200 hour current limit? Who knows, I don’t. Its green range was 2000 to 2400 RPM. I usually cruised at 2200 being in the middle of the green.

For those that worry about being oversquare, the IO-540W1A5D engine I’m talking about at every sea level takeoff pulled 30” MP at 2400 RPM, that’s pretty seriously oversquare, but it was the factory setting. So being oversquare within limits won’t kill your motor.

Everything has the within limits disclaimer of course. It’s my opinion that cruising within the middle of the green will give you the longest life, avoid continuous operation at the extremes, either high or low. Higher altitude will most likely require higher RPM or the engine just can’t make the power, sometimes yiu just gotta do what you gotta do, best power mixture and high RPM to not hit the ground in places.

[A64Pilot]

8 hours ago, DCarlton said:

Yep, that’s a bit higher than I remembered.  I usually cruise between 8 and 10k at 2400 full throttle.  Might be good to slow the rpm’s down a tad more.  Will have to watch manifold pressure more closely.  

Up to you of course but up there I’ve been at full throttle, heck by 10K I’m abandoning LOP too. 10K at LOP mine is a dog.

Excepting for cloud clearance I don’t fly that high anymore. For most people as we age our tolerance for altitude decreases. I think by 10K to keep from getting fatigued etc at 65 I probably ought to be on O2, and that’s a PIA. I used to spend hours higher than I was supposed to and it didn’t bother me, but then I could run 2 miles in 12:30 too, where now I couldn’t run out of a burning building  

[Echo]

At 7500' My max throttle MP is 2400.  I love 24x24 power settIng.  Adjust red handle to suit desire.

Edited February 4 by Echo

[philiplane]

34 minutes ago, A64Pilot said:

 

The IO-540 line is a very good example, those motors range from 235 to 300 HP, there are differences of course but the 235 is to a great extent mostly a de-tuned if you will 540, de-tuned primarily by lowering the RPM limit, it’s max RPM for example is 2400 RPM. I prop struck mine years ago and as it was right at TBO and as I’m an A&P/IA I chose to overhaul it for resell reasons. I sent everything off of course, but every single wear item I sent off was still within NEW limits, not serviceable but new. That motor would likely have gone to twice TBO is my guess just by following the Lycoming mandatory SB replacement items required on major disassembly.

 

The reason Lycoming made those changes has nothing to do with extending the life of the engine. It has to do with reducing the number of engines in the catalog to increase profitability. It's an accounting department move.

A 235 HP, 2575 rpm 540 can be made by lowering the compression via a piston change in a 250 hp 2575 rpm 540, or by simply limiting the max rpm of a  2700 rpm 260 HP engine. Operate a 260 hp engine at 2575 instead of 2700, and you get 250 hp. Operate the 260 at 2400 rpm, and you get 235 HP. Now you get three engine models out of one, with no difference in parts. Just a governor adjustment. 

[A64Pilot]

I understand why the engine was derated. The newer Cessna 182 that used to have the 230 HP Conti 470 now has a 230 HP IO-540, to increase the HP would be a whole new Cert program that may have required changes, but one 230 for another 230 is easy.

The reason why hasn’t got anything to do with its life. There are 2 235 HP 540’s. One is low compression and carbureted, it turns 2700 maybe RPM to make 235 HP.

The IO 235 is both high compression and fuel injected and therefore only has to turn 2400 RPM to make 235 HP. Take that motor and turn the RPM up to 2700 I think it is and you have the 260 HP motor, the 260 motor doesn’t have the reputation of extreme long life that the IO 235 does and the IO 235 motor lasts longer than the carbureted 235 one. The 300 HP motor is significantly different, you can draw correlations with it but as it’s not identical it’s not an apples to apples comparison

The point is that by reducing power output and RPM, the life of the engine is increased. Don’t get caught up in the weeds, these are generalizations.

De-rating motors is extremely common in everything from Outboards to Diesels to turbines, without exception that I can think of the de-rated motors last longer.

Your right it’s to only make a couple of engines cover a range of power, Mercury with its Verado engine covers from 150 HP to 350 I think it is with only two motors, the difference being the motors computer.

 

[DCarlton]

52 minutes ago, A64Pilot said:

I understand why the engine was derated. The newer Cessna 182 that used to have the 230 HP Conti 470 now has a 230 HP IO-540, to increase the HP would be a whole new Cert program that may have required changes, but one 230 for another 230 is easy.

The reason why hasn’t got anything to do with its life. There are 2 235 HP 540’s. One is low compression and carbureted, it turns 2700 maybe RPM to make 235 HP.

The IO 235 is both high compression and fuel injected and therefore only has to turn 2400 RPM to make 235 HP. Take that motor and turn the RPM up to 2700 I think it is and you have the 260 HP motor, the 260 motor doesn’t have the reputation of extreme long life that the IO 235 does and the IO 235 motor lasts longer than the carbureted 235 one. The 300 HP motor is significantly different, you can draw correlations with it but as it’s not identical it’s not an apples to apples comparison

The point is that by reducing power output and RPM, the life of the engine is increased. Don’t get caught up in the weeds, these are generalizations.

De-rating motors is extremely common in everything from Outboards to Diesels to turbines, without exception that I can think of the de-rated motors last longer.

Your right it’s to only make a couple of engines cover a range of power, Mercury with its Verado engine covers from 150 HP to 350 I think it is with only two motors, the difference being the motors computer.

 

I think my next boat is going to have that small 250 hp Mercury V8 OB.  Seems like it would be a great engine.  And a mythical mid body Mooney with a NA IO540 would have been my choice.  

[Ibra]

Give it a good 2700rpm & WOT until it screams and burn all the moisture, lead and sulfure, you need to warm up your oil/cylinders past 200F/300F and fly that for while, then you can bring it back to 20/20 LOP cruise 

What you can’t do is fly it 20/20 all the time, this gets you the same result as people who are doing ground runs instead of flying, that does not get cylinders and oil hot enough and likely more bad for the engine…

In any case, flying at 20/20 is better than no flying 

Edited February 4 by Ibra

[A64Pilot]

1 hour ago, DCarlton said:

I think my next boat is going to have that small 250 hp Mercury V8 OB.  Seems like it would be a great engine.  And a mythical mid body Mooney with a NA IO540 would have been my choice.  

It’s been years since I had my CC with the Verado, back then they only had inline 4’s which I had and an inline 6. The 6 was smooth as glass as an inline 6 is better balanced than a V8, it reminded me of the old Tower of Power way back when. The Verado was a much better engine than a Yamaha in my opinion.

My boat couldn’t handle the weight of the inline 6, I was slightly underpowered with only 175 HP because it was outfitted as a dive boat so we always carried at least 6 dive tanks, usually 10.

I’ve been happy with my 4 cyl IO-360, it’s smoother than I expected, four cylinder swinging a two blade prop I expected more vibration than I’m getting, and it’s frugal which I like. From memory I think between 10 and 12 thousand running WOT at 2200 50 ROP I burned 12 GPH with the 235 IO-540. It just wouldn’t run LOP very well, actually it would run like -25 LOP but if I ran the exact same speed  ROP it just wasn’t much fuel savings.

I’ve not bothered trying that test with the Mooney, I’m convinced the engine I have is the poster child for LOP, where the 540 just wasn’t.

I was stationed in Germany for a tour, hence the name.

 

[Shadrach]

On 2/4/2024 at 10:29 AM, A64Pilot said:

Those charts are at best power? 

LOP is significantly lower power output, one reason to cruise lower power is reduced fuel burn, so surely your LOP. I believe it’s pretty simple if I understand. Forget the charts, to be at 75% power for our angle valve motors LOP you have to have a fuel burn of 10 GPH. If you turn middle of the green RPM that’s going to require more than 25” MP I believe, and the general rule of thumb is 1” MP loss per 1,000 ft, so the highest you can pull 25” on an average day is 5,000 ft.

Do LOP cruise charts exist? Is there even a need?

10,000 ft you can only get roughly 20” MP so to keep power up requires both higher RPM and best power mixture.

That rule of thumb isn’t linear of course or otherwise you would be in space at 30,000 ft, but it’s close enough for us low altitude guys.

As we are talking engine longevity RPM is tied to wear, actually it’s piston speed and that’s why one motor can run higher RPM and not have high wear because of its stroke, where a longer stroke motor can’t.

But anyway within limits reducing RPM will reduce wear. But within limits, as a general statement higher RPM wears on the valve train while low RPM with corresponding higher load to maintain HP wears on the pistons, cylinders and bottom end. If you keep MP the same and accept the power loss then lower RPM increases engine longevity.

The Green ranges are of course the points where the engine manufacturer expects if obeyed an engine will make it to TBO, those of us that are working for an even longer life if we run closer to the middle of the green ranges it’s likely the engine will last longer.

The IO-540 line is a very good example, those motors range from 235 to 300 HP, there are differences of course but the 235 is to a great extent mostly a de-tuned if you will 540, de-tuned primarily by lowering the RPM limit, it’s max RPM for example is 2400 RPM. I prop struck mine years ago and as it was right at TBO and as I’m an A&P/IA I chose to overhaul it for resell reasons. I sent everything off of course, but every single wear item I sent off was still within NEW limits, not serviceable but new. That motor would likely have gone to twice TBO is my guess just by following the Lycoming mandatory SB replacement items required on major disassembly.

I’m not saying don’t cruise at higher RPM if you choose to, but primarily by turning the RPM limit down to 2400 RPM max Lycoming hugely increased the life of the 235 HP IO-540. Why didn’t they increase the TBO beyond the 2200 hour current limit? Who knows, I don’t. Its green range was 2000 to 2400 RPM. I usually cruised at 2200 being in the middle of the green.

For those that worry about being oversquare, the IO-540W1A5D engine I’m talking about at every sea level takeoff pulled 30” MP at 2400 RPM, that’s pretty seriously oversquare, but it was the factory setting. So being oversquare within limits won’t kill your motor.

Everything has the within limits disclaimer of course. It’s my opinion that cruising within the middle of the green will give you the longest life, avoid continuous operation at the extremes, either high or low. Higher altitude will most likely require higher RPM or the engine just can’t make the power, sometimes yiu just gotta do what you gotta do, best power mixture and high RPM to not hit the ground in places.

Those charts state that the numbers are for full rich above 75% and 100ROP below 75%.  There is no operational advantage to operating an NA engine LOP at the altitudes in question. Of course, you’ve seen me say many times that LOP operations are better suited to altitudes where there is a surplus of air. Above 7500, operating at peak EGT (richest cylinder) makes the most sense. It puts you in the best BSFC curve, sacrifices little in the way of power while offering good temperature margins, excellent efficiency and smooth operation. 

It’s interesting that you feel that your airplane gets doggy at higher altitudes. I’ve always thought that the aircraft had a unique aptitude in its class for for cruising at the least busy GA altitudes (10-13K). I prefer flying high unless I am west bound. I have been fortunate enough to enjoy 200kt+ GS on many, many occasions over the years, and nearly 250kts a few times. Typically you have to fly high to catch those kinds of winds.

 

 

[Justin Schmidt]

What if your CHTs are only mid to high 200s, max 320s in a climb with cowl flap closed. Been keeping power in 70%s.

[Shadrach]

1 hour ago, Justin Schmidt said:

What if your CHTs are only mid to high 200s, max 320s in a climb with cowl flap closed. Been keeping power in 70%s.

That’s me all winter. I don’t close cowl flaps in climb though.

[Will.iam]

1 hour ago, Justin Schmidt said:

What if your CHTs are only mid to high 200s, max 320s in a climb with cowl flap closed. Been keeping power in 70%s.

Don’t run so rich in the climb the cht’s will rise never seen an engine not. 

[Bolter]

On 2/2/2024 at 4:31 PM, 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.

If the goal is best economy (miles per gallon), then you are a little slow at 105 MPH.  You want Carson speed, which is about 120 MPH for your F.  At 105 MPH, you are burning a little more fuel as well as going slow.  Note that this is about aerodynamic efficiency.  You then want to run the engine at its most economical for this airspeed, which is probably at or just below Peak EGT at the low power settings to get only 120 MPH in a Mooney.  

I think this all applies to IAS, so climbing higher will also help.  

@201er can expand more on Carson number, as he has posted about it quite a bit, and I think even used it for maximum range in the past. 

Or read this: https://www.aopa.org/news-and-media/all-news/2017/august/pilot/efficiency-slow-burn

[Yetti]

I have run my A1A 24/24 for the 6 years that I owned it.   It always gets 79 or 80 on the compression tests.  It is fun to do 21/21 for a bit as the plane feels completely different but this is the more like the landing speed and teaches you how it feels when landing.

[mhrivnak]

I'll also endorse using lower RPM to achieve low power settings. It'll reduce the wear on your engine, which is one of your goals, and significantly reduce the amount of friction loss, leading to better efficiency.

It's also quieter, which is nice for quality of life!

This article includes a good discussion of using lower RPM settings for best fuel efficiency, but I think it fits your goals as well: https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2012-10_flying-efficiently.pdf

[Pinecone]

10 hours ago, Bolter said:

If the goal is best economy (miles per gallon), then you are a little slow at 105 MPH.  You want Carson speed, which is about 120 MPH for your F.  At 105 MPH, you are burning a little more fuel as well as going slow.  Note that this is about aerodynamic efficiency.  You then want to run the engine at its most economical for this airspeed, which is probably at or just below Peak EGT at the low power settings to get only 120 MPH in a Mooney. 

Carson speed is not the best economy.  It is the least bad economy.  Or it is the fastest, where the fuel burn is not excessive over best economy.

Best economy is least drag, which is the same as best glide.  But no one wants to cruise at 80 knots in a Mooney.

[Shadrach]

10 hours ago, Bolter said:

If the goal is best economy (miles per gallon), then you are a little slow at 105 MPH.  You want Carson speed, which is about 120 MPH for your F.  At 105 MPH, you are burning a little more fuel as well as going slow.  Note that this is about aerodynamic efficiency.  You then want to run the engine at its most economical for this airspeed, which is probably at or just below Peak EGT at the low power settings to get only 120 MPH in a Mooney.  

I think this all applies to IAS, so climbing higher will also help.  

@201er can expand more on Carson number, as he has posted about it quite a bit, and I think even used it for maximum range in the past. 

Or read this: https://www.aopa.org/news-and-media/all-news/2017/august/pilot/efficiency-slow-burn

His stated goal is maximizing flight time not distance traveled.  He is theoretically seeking a power setting that yields the lowest fuel flow with smooth operation  and adequate temps for L//D Max without causing harm to the powerplant.

[Adverseyaw0317]

I am doing a similar activity and finding in these cold winter months that a minimum of 55% power is required to keep everything warm. 

[A64Pilot]

6 minutes ago, Adverseyaw0317 said:

I am doing a similar activity and finding in these cold winter months that a minimum of 55% power is required to keep everything warm. 

I think that’s key, staying in the green temp wise, you can run one too cold, too cold may not be as bad as too hot, but it’s not good.

Even in Central Fl mine can get to the bottom of the green, and the difference in being at the bottom and near the middle for me is only .5 GPH.

I’m all about being a CB, but saving .5 GPH but running at the bottom of the green is I believe being penny wise and pound foolish.