Learning a Turbo

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JustD

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JustD
New pilot. New owner. Lost… :)

I’m flying a 2006 Cessna T182T with a turbocharged Lycoming TIO-540-AK1A out of Denver. I did all of my training in a normally aspirated 172 so the Prop control and turbo are freaking me out a bit - I suspect I'm overthinking it. The CFI who helped me with the High Performance didn't explain (or, more likely, I didn't ask the right questions) this enough for me.

My goal is to reduce wear and tear on the engine, spend as little as possible on fuel and I don’t care about going fast. I believe I’ve got a good grasp on the concepts of engine management (leaning, CHT, TIT, RoP vs LoP, EGT, etc.) but I haven’t been able to nail down the practical application.

As I’m still getting used to the airplane I’m mostly spending time in the pattern and local practice areas doing maneuvers and I find that my temperatures all seem to be running very low (CHTs peaking at 300 after takeoff but steady at about 275 in short cruise areas).

A few questions:

1 - It seems most of the leaning settings are for cruise. If I’m practicing in the pattern or basic maneuvers in the practice area would I still lean or just leave it at the full forward/~24gph takeoff setting and control power with the throttle? Not to go too far into the rabbit hole yet, but what about the Prop setting when puttering around?

2 - There are lots of references to “Percent or Power” in the POH but obviously no dial labeled that in the plane. What is “65% power”? Manifold pressure? I get about 33” manifold pressure on takeoff so would 21.5” MAP (65% of 33) be my 65% power setting?

The Lycoming Power chart for the TIO540 doesn't even show a 65%, it stops at 80% in the "Best Economy Range" (which is lean of Peak) yet the POH says to cruise between 55 and 88%, and then further says that "Lean of Peak TIT is not approved"

THANKS

-David
 
I won't comment on the POH stuff but I feel like you are using a hammer instead of a screwdriver with regard to aircraft type, lol. A T182T is a traveling machine, great at running up high (Denver does help this somewhat) and hauling a load. Running back and forth to the practice area doing laps around the pattern aren't its forte and it's just costing you a lot of money in fuel burn and hourly rental cost.

You should be leaned out for your density altitude, but I wouldn't be worrying about rich of peak/lean of peak issues when you're constantly changing throttle doing maneuvers and such. I'd probably look in the POH for the engine table to find some thing simple like an squared combination (24" manifold/2400 rpm) for running back and forth to the practice area. When actually traveling/cruising use the POH table for establishing an appropriate throttle/prop setting.
 
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Nice plane!! Show some pics if you get a chance. I'd get another CFI who has experience with turbos and get better orientation. Anyone else at your airport also have a turbo Cessna? Maybe look them up and ask questions. Buy them lunch if they'd go up with you.
 
DSC_0247 2.jpg

Mine has a turbo (note the single exhaust and the extra side cowl flaps)....and I run it no different than the others. Use the vernier controls to adjust power....smooth constant changes. No jamming the power. And power reductions....same. Smooth gradual reductions. I don't worry with the 1" per min thingy...just make gradual reductions.

Run up....make sure the oil temp is warm. You won't want to run things up with thick oil. Mine requires 75 deg F and is placarded.

As for cool down....doesn't need it. If you look at the TIT, they will be the coolest during landing. Just don't do a prolonged taxi and the turbo will stay cool. Shuts down like all the others.

Cowl flaps.... use them on taxi and climb out....

Easy peasy....no biggie.;)
 
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Turbos. They simplify life a LOT.

Do you have the Lycoming operations manual for the AK1A? If not google/get it. All my advice below is general/based on my ops , and overridden by the ops manual if there's a conflict. I fly an S1AD.

Yes, depart and climb full rich.
The ops manual should have settings for 75/65% power.
No, you don't calculate 65% the way you did. You're close, but not on it.
I putter around at 2,200 or 2,300 RPM. 55% in my bird is 2,200 at 22" if my memory serves.

Just as an FYI, I'm attaching a power chart for my S1AD. Not for you to use on your engine, but simply for illustration of what you should find for your engine. This came out of the POH from Piper.


IMG_2627.jpg
 
David,

I suggest you train with a CFI who specializes in engine management, and ideally someone who owns (or has owned) an aircraft and engine similar to yours. The added complexity introduces additional failure modes and operating procedures which go beyond a normally aspirated engine. Interestingly, some things are easier in a turbo (e.g. leaning in the climb - no need to do that in a turbo), but a good understanding of why you do what you are doing is essential.

- Martin
 
JustD said:
New pilot. New owner. Lost… :)

I’m flying a 2006 Cessna T182T with a turbocharged Lycoming TIO-540-AK1A out of Denver. I did all of my training in a normally aspirated 172 so the Prop control and turbo are freaking me out a bit - I suspect I'm overthinking it. The CFI who helped me with the High Performance didn't explain (or, more likely, I didn't ask the right questions) this enough for me.

My goal is to reduce wear and tear on the engine, spend as little as possible on fuel and I don’t care about going fast. I believe I’ve got a good grasp on the concepts of engine management (leaning, CHT, TIT, RoP vs LoP, EGT, etc.) but I haven’t been able to nail down the practical application.

As I’m still getting used to the airplane I’m mostly spending time in the pattern and local practice areas doing maneuvers and I find that my temperatures all seem to be running very low (CHTs peaking at 300 after takeoff but steady at about 275 in short cruise areas).

A few questions:

1 - It seems most of the leaning settings are for cruise. If I’m practicing in the pattern or basic maneuvers in the practice area would I still lean or just leave it at the full forward/~24gph takeoff setting and control power with the throttle? Not to go too far into the rabbit hole yet, but what about the Prop setting when puttering around?

2 - There are lots of references to “Percent or Power” in the POH but obviously no dial labeled that in the plane. What is “65% power”? Manifold pressure? I get about 33” manifold pressure on takeoff so would 21.5” MAP (65% of 33) be my 65% power setting?

The Lycoming Power chart for the TIO540 doesn't even show a 65%, it stops at 80% in the "Best Economy Range" (which is lean of Peak) yet the POH says to cruise between 55 and 88%, and then further says that "Lean of Peak TIT is not approved"

THANKS

-David
Click to expand...

Cruise performance numbers start on page 174 at T182TPHBUS-01_Revision_Cover_Sheet.fm (stpeteair.org)

Sounds like your POH is missing some pages.
 
interesting chart. Which is "better" at a specific power percentage, higher RPM and lower manifold pressure or lower RPM and higher manifold pressure?
 
UngaWunga said:
interesting chart. Which is "better" at a specific power percentage, higher RPM and lower manifold pressure or lower RPM and higher manifold pressure?
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Lower RPM = lower wear, all else equal.
 
Rgbeard said:
oh - and anytime someone talks about "square" anything - hit the mental DELETE button and put them on MUTE.
Click to expand...

Some of us enjoy all that "Injin neer'in mumbo-jumbo".

All relative, "Square" means is Manifold pressure compared to RPM/100. It makes a difference in the performance charts for a turbo.

In Cruise lots of people run engines at lower MP and higher RPM like 22/2500 (under-square) or square 23/2300. A turbo-normalized engine can safely be run with higher MP and lower RPM (over-square). Running over-square gives the same performance at a lower fuel flow & less friction wear, and lower cabin noise.

The lower RPM SLOWS the combustion process for a more compete fuel burn with the net effect of synthetically advancing timing - a net result of the engine operating slower is improved volumetric efficiency.

Pilots that have turbos should look at the power/performance envelope from the engine manufacturer and the POH.

Running at a higher Manifold pressure and lower RPM we get essentially the same performance with less engine wear with 1 GPH reduction.

Example from OP's Cessna T182T at 10,000 MSL:

OverSquareEx-T182T.png
 
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Thanks all, especially @Martin Pauly I feel honored after watching your exceedingly educational videos, thank you! How does one find a CFI who really knows engine management? I've been working with two guys. One young guy on the airline track for basic flight training and then another (much older) King Air pilot who is great on the flying but not so much on the theory. I suspect I'm just getting bogged down on wanting to know the "why" and "how" versus just the "push this, pull that" mentality...

This is my plane (not a rental) which I'm trying to become more familiar with and I agree it's not the best plane for training in the pattern, but I'm trying to unlearn some 172 habits and re-learn the 182.

I've got a solid grasp of the concepts of engine management and leaning for cruise (I think) but I'm still fuzzy on what to do for what I'll call "non-cruise" flight. Pattern work, maneuvers in a training area, etc.

For example, today I did some laps in the pattern and basically flew the plane like the 172. I didn't touch the mixture or prop at all (other than leaning on the ground after she'd warmed up). Full forward Mixture and Prop while flying and just played with the Throttle to control altitude.

CHT never got above 300º and TIT never above 1440º. Am I to understand that this is fine? Just fly it like a 172 until I'm in real "cruise" and then lean as desired? I just don't want to break my pretty toy...

upload_2023-1-26_17-13-3.png

upload_2023-1-26_17-14-39.png
 
I don't see any issues flying it like you do. It's not optimal....because that turbo wants to run...but, I don't see it hurting anything.

nice plane.... ;)
 
Checkout_my_Six said:
I don't see any issues flying it like you do. It's not optimal....because that turbo wants to run...but, I don't see it hurting anything.

nice plane.... ;)
Click to expand...

Thanks!

I guess that's my point of this thread. What would be optimal for flying a turbo around the pattern to 1) not hurt the plane and 2) not burn unnecessary gas?
 
I’ve flown the non-turbo version. I remember the POh had pretty good suggestions on settings for climbout/cruise.
 
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I would say to bring back the mixture slightly, to avoid running full rich all of the time. Full rich can foul your spark plugs too. This would become your “default” position then you can lean when you reduced throttle according to your POH charts. For taxing you can further lean the mixture as well. I typically do full rich for takeoff but at higher DA you can adjust for best performance.
 
mandm said:
I would say to bring back the mixture slightly, to avoid running full rich all of the time. Full rich can foul your spark plugs too. This would become your “default” position then you can lean when you reduced throttle according to your POH charts. For taxing you can further lean the mixture as well. I typically do full rich for takeoff but at higher DA you can adjust for best performance.
Click to expand...

what you are posting is good for taxi but that’s it. You takeoff and climb it full rich.
 
yes but....depending on density altitude, he may need to lean to get full power. Mine runs overly rich and I need to pull it back for take off....once the gear comes up I crank in a few turns to richen the mixture during the climb.

If the TIT's are good and the CHTs are under 380....everything is happy.
 
*disclaimer - as previously said, my plane and my plane’s books/procedures

Not so on the S1AD. Regardless of altitude, Lycoming ops manual says to take off full rich.

I follow the book, lean a little for taxi, take off between 34-36” MP, and don’t foul plugs.

This could be a TN vs. TC discussion we are getting into.
 
masloki said:
Lower RPM = lower wear, all else equal.
Click to expand...

I’m not sure that’s a safe assumption.

My understanding is that if an aircraft engine is operating anywhere in its approved continuous power range, there is virtually no metal-to-metal contact - all the important parts have a thin layer of oil separating them. The importance of this is made very clear in how little running time you have left if oil pressure is lost and that oil film breaks down.

Most engine wear occurs before on startup, before oil pressure has built up to keep the bearings and rings and whatever apart. In addition, excessively low engine rpm can increase cylinder head pressures, causing more wear and an increased likelihood of preignition and/or detonation.

I’m not sure if anyone has data indicating less wear at reduced rpm, within the limits prescribed in the POH or by the manufacturer. If they do, I’d like to see it and I’ll change my position on this.
 
FastEddieB said:
I’m not sure that’s a safe assumption.

My understanding is that if an aircraft engine is operating anywhere in its approved continuous power range, there is virtually no metal-to-metal contact - all the important parts have a thin layer of oil separating them. The importance of this is made very clear in how little running time you have left if oil pressure is lost and that oil film breaks down.

Most engine wear occurs before on startup, before oil pressure has built up to keep the bearings and rings and whatever apart. In addition, excessively low engine rpm can increase cylinder head pressures, causing more wear and an increased likelihood of preignition and/or detonation.

I’m not sure if anyone has data indicating less wear at reduced rpm, within the limits prescribed in the POH or by the manufacturer. If they do, I’d like to see it and I’ll change my position on this.
Click to expand...
I've been surrounded by engine engineers a lot of my career. When I've pressed this question to some, I've normally had them telling me that higher pressures are worse than RPMs. But to be fair, they acted like it was more of a push and not a clear winner.
 
ChatGPT*:

Lower RPM (revolutions per minute) in an aircraft engine can mean less wear on certain components, such as the gears and bearings. However, it also depends on the specific design of the engine and the operating conditions. For example, if the engine is being operated at a low RPM but at a high power output, this can still lead to significant wear. Additionally, low RPM operation can also result in higher fuel consumption and reduced performance. Overall, it is important to operate an aircraft engine within the manufacturer's recommended RPM range to ensure optimal performance and longevity.


* I have a feeling we’re going to see more such ChatGPT references going forward. A LOT more!
 
An example: I flew Cirrus planes for a company for a short while. Cirrus recommends 2,700 rpm in the climb, so in my own plane that’s what I used. This company’s checklist had a reduction to 2,500 rpm at 1,000’. So that’s what I did. Their planes, their rules.

Probably made no discernible difference, except maybe noise reduction for those on the ground. I suspect it was a relatively benign carryover from planes that restrict maximum power to 5 minutes or whatever, but that’s not the case for the IO550N in the SR22.
 
I also own and fly a T182T...a 2002 model. I now have almost 300 hours in it. My CFI (who had a lot of turbo 182 time, and who I still fly with a lot) taught me to take off and climb with full throttle, and the mixture and prop full forward. I like to cruise climb at 95 knots, and I keep everything firewalled until I'm at cruise altitude. The POH suggests a cruise climb setting of 25" MP, 2400 RPM, 16 GPH fuel flow...but my CFI suggested that it is better for the engine to get to cruise altitude as quickly as possible.

Once at cruise, I pull power back to 23-24" MP, set RPM's around 2300, and then lean the mixture to 14.5-15 GPH (watching CHT's and EGT's). Those are the settings my plane seems to like the best. I've experimented with more "oversquared" settings...21-2200 rpms, at 23-25" mp...but my engine just sounds happier and seems to run smoother at 2300-2350 RPM's.

Regarding flight in the pattern or around the airport, I keep throttle and mixture full forward. I'll only lean it out and adjust the prop if I'm going to be in that configuration for a while.

About the suggestions of leaning on takeoff...that seems like a quick way to cause a problem on a turbo'd engine. All the takeoffs I've done in it (including Leadville and other high CO airports) have been full rich mixture (but leaned out on the ground for taxi). I've never had a problem with fowling plugs operating this way.
 
NealRomeoGolf said:
I've been surrounded by engine engineers a lot of my career. When I've pressed this question to some, I've normally had them telling me that higher pressures are worse than RPMs. But to be fair, they acted like it was more of a push and not a clear winner.
Click to expand...

Some years ago I read an article (I haven't been able to find it again) about research done using HALT (highly accelerated life testing) on engines. The bottom line was that lower RPM and higher pressures were not as healthy for engine longevity as slightly higher RPM and lower cylinder pressure. Let it spin and let it breathe ... :)
 
Yup....heat is a dominate stressor. So, get it running cool and run it at a higher power setting and everything is happy.
Daleandee said:
Some years ago I read an article (I haven't been able to find it again) about research done using HALT (highly accelerated life testing) on engines. The bottom line was that lower RPM and higher pressures were not as healthy for engine longevity as slightly higher RPM and lower cylinder pressure. Let it spin and let it breathe ... :)
Click to expand...
 
NordicDave said:
@JustD I'm a turbo owner. This video from Mike Busch is terrific. He also goes into the beauty of running over square with a turbo, meaning 2-4 inches of MP over RPM, and within the engine manufacturers opening envelope.
Click to expand...

To be clear, over square is a myth.

2-4 inches over is barely turbo normalized. I run 10 inches over. And yes, this is well within the POH limits.
 
Josey Wales said:
I also own and fly a T182T...a 2002 model. I now have almost 300 hours in it. My CFI (who had a lot of turbo 182 time, and who I still fly with a lot) taught me to take off and climb with full throttle, and the mixture and prop full forward. I like to cruise climb at 95 knots, and I keep everything firewalled until I'm at cruise altitude. The POH suggests a cruise climb setting of 25" MP, 2400 RPM, 16 GPH fuel flow...but my CFI suggested that it is better for the engine to get to cruise altitude as quickly as possible.

Once at cruise, I pull power back to 23-24" MP, set RPM's around 2300, and then lean the mixture to 14.5-15 GPH (watching CHT's and EGT's). Those are the settings my plane seems to like the best. I've experimented with more "oversquared" settings...21-2200 rpms, at 23-25" mp...but my engine just sounds happier and seems to run smoother at 2300-2350 RPM's.

Regarding flight in the pattern or around the airport, I keep throttle and mixture full forward. I'll only lean it out and adjust the prop if I'm going to be in that configuration for a while.

About the suggestions of leaning on takeoff...that seems like a quick way to cause a problem on a turbo'd engine. All the takeoffs I've done in it (including Leadville and other high CO airports) have been full rich mixture (but leaned out on the ground for taxi). I've never had a problem with fowling plugs operating this way.
Click to expand...

I agree with this.

You do NOT lean a turbo engine for takeoff, as every takeoff is at full manifold pressure. Leaning one is a VERY bad thing.

Not a 182, but I also climb at full throttle, full RPM, and full rich.

For cruise, I am in the LOP camp. So I normally pull the throttle back to 32", 2300 RPM, then mixture to 10.3 GPM. This is 65%.
 
My advice is to find a GOOD instructor and do a proper high performance/turbo checkout.

IMO, your previous instructor did not do a great job.

Percent power is from the power chart in the POH. It will show you various power settings at various altitudes and temperatures with combinations of RPM and MP. Pages 5-22 through 5-32 in the manual I found online.

I don't know what the recommended Lean is. But for a turbo engine, lean of peak, the HP is 13.7 times the fuel flow. Your engine is rated at 235 HP, so 65% is 152.75 HP. So a LOP setting of 11.1 - 11.2 would give you 65% power.

Based on the charts, the Cessna recommended lean is probably right in the red box/fin. I would not run their at over 65% power. At 12,000 feet with standard temp, you are looking at 2400 RPM at 21" or 2300/22", or 2200/22", or 2100/23" or 2000/26"
 
JustD said:
New pilot. New owner. Lost… :)

I’m flying a 2006 Cessna T182T with a turbocharged Lycoming TIO-540-AK1A out of Denver. I did all of my training in a normally aspirated 172 so the Prop control and turbo are freaking me out a bit - I suspect I'm overthinking it. The CFI who helped me with the High Performance didn't explain (or, more likely, I didn't ask the right questions) this enough for me.

My goal is to reduce wear and tear on the engine, spend as little as possible on fuel and I don’t care about going fast. I believe I’ve got a good grasp on the concepts of engine management (leaning, CHT, TIT, RoP vs LoP, EGT, etc.) but I haven’t been able to nail down the practical application.

As I’m still getting used to the airplane I’m mostly spending time in the pattern and local practice areas doing maneuvers and I find that my temperatures all seem to be running very low (CHTs peaking at 300 after takeoff but steady at about 275 in short cruise areas).

A few questions:

1 - It seems most of the leaning settings are for cruise. If I’m practicing in the pattern or basic maneuvers in the practice area would I still lean or just leave it at the full forward/~24gph takeoff setting and control power with the throttle? Not to go too far into the rabbit hole yet, but what about the Prop setting when puttering around?

2 - There are lots of references to “Percent or Power” in the POH but obviously no dial labeled that in the plane. What is “65% power”? Manifold pressure? I get about 33” manifold pressure on takeoff so would 21.5” MAP (65% of 33) be my 65% power setting?

The Lycoming Power chart for the TIO540 doesn't even show a 65%, it stops at 80% in the "Best Economy Range" (which is lean of Peak) yet the POH says to cruise between 55 and 88%, and then further says that "Lean of Peak TIT is not approved"

THANKS

-David
Click to expand...

I have 1600+ hours in the T182T in all the lower 48 states. It's been ten years, but remember most of it.

1) The aircraft is leaned with the TIT gauge.

2) The power tables in the restarts are all very good - you set the MP and RPM and then use the book fuel flow to estimate the correct leaning. I think the book talks about 125 ROP on the TIT and 50 ROP on the TIT as best power and economy cruise. Mine almost always peaked at redline (1650F) but not 100% of the time. The percentage of power in the table is what power you are making compared to full power or 235 HP. The aircraft can make full power all the way up to its certificated altitude limit (FL200).

3) You will always run a turbo over square. It's normal and expected. I used 25-27" Hg KP and 2300 rpm for cruise almost universally.

4) I didn't even know it was possible to get 275 CHT. You may be drowning that engine in fuel. On a Lycoming I don't even get excited until around 400F; for the maneuvers I'd expect them to be executed around 21-23" and maybe 340 CHT.

5) It was meant to be a traveling machine and run hard. The only weak spot I found was the almost 90 degree bend in the exhaust system right before the exhaust pipe itself. The hot gases would toast that corner in about 500-700 hours.

It's a great machine and still regret selling mine! Feel free to PM if you have other questions. It does not sound like the CFI knows turbos or the airplane in detail.
 
JustD said:
I guess that's my point of this thread. What would be optimal for flying a turbo around the pattern to 1) not hurt the plane and 2) not burn unnecessary gas?
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My Seneca has turbos with the Continental L/TSIO-360EB engines and Merlyn Automatic Wastegates. I fly it in the pattern. If you want details, feel free to ask specifics. Not sure how much would be relevant to your aircraft.
 
Made this hot start guide at one point.
 

Attachments

  • TIO540-AK1A Hot Start Guide.pdf
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OP here, still trying to wrap my head around some of the theory…

In looking through the Lycoming manual for the TIO 540 I found this chart which seems to be showing that Best Economy Cruise is lean of peak, but I have two questions with respect to this graph.

upload_2023-1-30_13-19-23.png

  1. Why is the “Zero” line on CHT not actually at the peak CHT? (noted by “1” on the photo)
  2. What does “Percent of Best Power” mean? At this lean setting and best economy I can’t imagine that the engine is putting out 93% power (note 2)

Finally, in the POH it looks like I could run at 2200 RPM and 20” MAP for 57% power and 10.5GPH for 127KTS.
upload_2023-1-30_13-19-51.png


Practically, would I just lean back to 10.5GPH according to the Fuel Flow indictor? or “Big mixture pull” back to very lean and then add fuel back until hitting Peak TIT (which theoretically should be close to 10.5gph)?

Again, I appreciate all of the advise and help.
 
"would I just lean back to 10.5GPH according to the Fuel Flow indictor?"
Yes.

"“Big mixture pull” back to very lean and then add fuel back until hitting Peak TIT"
No.
 
Two things:

I have found the POH numbers in my T182T to be optimistic about fuel flow. Mine burns more fuel than what they publish.

I would not be comfortable operating a turbo Lycoming lean of peak based on fuel flow or EGT/TIT without a good engine monitor (and probably GAMI injectors). But I'm interested to see the discussion on this.
 
In my experience I almost never ran lean of peak as in this airplane it's a pretty big speed hit. That said, it was balanced enough to pass the GAMI test from the factory.

Lycoming's "best economy" is at peak EGT, which in my experience is also redline (1650 F). I never liked to run right there, and tended to rely on the Cessna manual (which I believe is technically governing) of 50 to 125 ROP. Most of the time ran about 100 ROP without issue.

For pattern work would takeoff as stipulated (32.5/2400/24 gph) and at level off would reduce power to about 20 "Hg MP as that's a typical approach configuration anyhow. Then prop to 2300 and mixture to maybe 1600 TIT. It's a fair amount of levers and knobs and the first few times would recommend a quiet airport where you extend if needed on downwind.

I never flew at the low MP setting circled in the chart except attached at altitude - always 24-27" MP and 2300 rpm. I'm not a "if you baby your engine it will last longer" disciple and tend to have much better luck running my engines fairly hard, generally always at or above 65% power.

Power up and power down flows are essential in a turbo. Power up is from the floor up - cowl flaps, mixture, prop, then throttle. Power down is the opposite.
 
masloki said:
Lower RPM = lower wear, all else equal.
Click to expand...
I don't think that is true. I would say that all else equal, the lower the temperatures the lower the wear (assuming the engine and oil are above minimum operating temps.
 
jbehler said:
I don't think that is true. I would say that all else equal, the lower the temperatures the lower the wear (assuming the engine and oil are above minimum operating temps.
Click to expand...

I think one has to ask "lower than what"? The machine is designed to run thousands of hours. The stresses and lubrication needs have all been worked out - for running at maximum power! (Most of our engines can make rated power to TBO, some have 5 minute limits).

One example - running at low speeds and power settings may not be optimal due to insufficient heat transfer across the valves leading to more carbon deposits and sticking. Obviated by running "harder."

Overall the only sure way to get to TBO and often much beyond it is to just run the darn thing. Most GA engines sit much too much and corrosion, not cylinder pressures or tribological concerns is the issue. One large operator of TSIO-520s had permission to run to 2x TBO - they ran their engines something like 500-700 hours per year. They rarely even so much as changed a jug on the way.

Back to one aspect of OP's question - was taught to severely lean the TIO540 on the ground to keep the plugs cleaner - but must be lean enough that attempts to add power on the takeoff roll without enriching cause the engine to die as opposed to run way way too lean.
 
Agree completely....frequent utilization trumps manner of utilization in all but the most extreme examples. With utilization as a constant, I would guess the longevity sweet spot for a properly equipped big bore engine would be 65-70%-ish power and LOP. Of course in some planes that results in a major performance degradation vs. book max performance and it's up to you as an owner or pilot how much you value extra performance when weighed against the potential of shorter engine life.
 
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