[Treekiwi]

Howdy all,
Looking for feedback from folks who actually overhaul engines for a living.
My engine has just had a full overhaul, and the shop says never run LOP.
This is based on their experience......LOP causes deformation of the cylinder walls, and they lose their choke and become bell shaped. It doesn't become a valve issue, according to them, but a ring issue.
He said while CHT's may be lower, the other areas can experience higher temperatures which causes the damage.
He said LOP makes him a lot of money, and recommends 100° ROP.
Now, I have always been an LOP guy, but maybe the savings are not really worth it in the long run. I've also had to do 2 top end overhauls on engines I've run LOP. Hmmmm?
I realize many so called gurus recommend LOP, but is it possible they've had it wrong all this time?
Tangible evidence is encouraged.
Cheers
Stephen

 

[rocketman1988]

Howdy all,
Looking for feedback from folks who actually overhaul engines for a living.
My engine has just had a full overhaul, and the shop says never run LOP.
This is based on their experience......LOP causes deformation of the cylinder walls, and they lose their choke and become bell shaped. It doesn't become a valve issue, according to them, but a ring issue.
He said while CHT's may be lower, the other areas can experience higher temperatures which causes the damage.
He said LOP makes him a lot of money, and recommends 100° ROP.
Now, I have always been an LOP guy, but maybe the savings are not really worth it in the long run. I've also had to do 2 top end overhauls on engines I've run LOP. Hmmmm?
I realize many so called gurus recommend LOP, but is it possible they've had it wrong all this time?
Tangible evidence is encouraged.
Cheers
Stephen

Look at the data. I would love to see the supporting data on "cylinder wall deformation" and "losing their choke". What other areas are going to be "hotter" and what is going to make those areas "hotter"? Running LOP at power levels below ~65% actually reduces internal temperatures as well as CHTs and fuel flow.

Is it possible that your mechanic is wrong and the "so called gurus" are correct?

What was the specific reason for the top overhauls? (Running LOP isn't a valid answer)

 

[Ironflight]

I don’t build engines for a living, but I have built them for my own (and a few for others) use, so take this for what its worth.

Tangible Evidence…..

RV-8: 2280 hours - all of it (except take-offs) LOP
RV-6: 2,000+ hours - all of it LOP (except Take-offs)
RV-3 : 980 hours - all of it LOP (except Take-offs)
Dream Tundra: 450 hours - all of ti LOP (except Take-offs)


There are many, many old wives tales out there, and many folks will tell you things that they think are true but can’t back up with data. Most engine shop employees, builders, and owners don’t fly….they just work on engines. They’re not dumb - they just have no first-hand experience how engines are actually operated in aircraft. Many will say “Lycoming says don’t do it!” But they are actually refering to the boosted Lycomings not our normally-aspirated motors.

Good reason not to run LOP? You’ll get a biut mnore power and speed - and if your coolign system and wallet are happy there, then run at peak or ROP. But we find with our tightly-cowled motors in the hot high desert, we go LOP as soon as we reduce power below 70% (which is generally right after take-off) and leave it there until shutdown.

By all means, fly the way you are comfortable - but don’t be afraid of LOP because one shop told you to be….

 

[lr172]

I have built my own and a couple others, so not necessarily what you are looking for. As paul said, you should just ignore that. I am sure they see all sorts of issues but have basically no data or info on what led to those outcomes. Most of these guys understand the mechanics of engine assembly and machining. They are not engineers working on dynos and running engines to destruction in various different conditions to understand cause and effect. I would far prefer the opinions of local mechanics that see an engine every year and understand how it is run and how that behavior results in issues. Overhaulers only see the carnage, with absolutely NO appreciation for how it got there. Be sure you accept opinions from those with the credentials to support them.

LOP operation are not damaging. Every auto engine built in the last 35 years runs lop in light load conditions, even the air cooled porsche boxer engines, which are very close to a lyc. I have one.

You will find many engines that don’t make it to tbo with messed up cylinders; some will have run lop and others always rop. There are countless variables in play beyond mixture and absolutely no way to isolate just mixture as root cause in these cases. Cylinder wall deformation can cone from various different issues, but mixture is not one of them.

My 320 has 1600 hours and i never run ROP beyond take off or climb and sometimes climbs are done lop. Cylinders look great and make good compression.

 

[blaplante]

He said while CHT's may be lower, the other areas can experience higher temperatures which causes the damage.

So,
Seems simple enough. Lets instrument the barrel of a cylinder at multiple points, then run it ROP and LOP, and get the data showing where some part of the barrel becomes hot/hotter. In fact have the shop tell us where they want the probes.

 

[plehrke]

The reality is that there are very few people/shops/companies that have enough data to make definitive claims. This includes most gurus and local or internet experts that are on both sides of the LOP/ROP discussion. Of those with enough data there are probably only one or two that need to be right or face financial liabilities therefore are very cautious in what they say.
You need to make up your own mind between those opinions with little or no data and the few that are experts but have financial risk that maybe too conservative.

 

[Treekiwi]

Thanks for the comments. Mostly pretty much echo what I have learned over the years.
Paul's real world example numbers are quite persuasive.
I agree there probably isn't great data to back up the shops claim. They have been in business for years, and have rebuilt many 1000's of engines. I can only presume they have asked owners how they run their engines, and then observe what the issues are. That is not very good data, of course. How many owners with cylinder problems routinely operated in the red box due to lack of training? That right there could very well cause the issues observed by the shop. And, they most certainly are very conservative and have a financial incentive to be so.
LOP was probably misunderstood by pilots for awhile until the red box and big pull concept came into the light.

If any engine shops want to comment on their real life experiences, that would be great. Absent that, we are left to make our own decision based on figures and experience.

Tangible Evidence…..

RV-8: 2280 hours - all of it (except take-offs) LOP
RV-6: 2,000+ hours - all of it LOP (except Take-offs)
RV-3 : 980 hours - all of it LOP (except Take-offs)
Dream Tundra: 450 hours - all of ti LOP (except Take-offs)

These numbers are a very good start.

 

[rv8ch]

I hope the Savvy guys expand the amount of data that they gather, like fuel type, oil type, spark plugs, oil filter type, ignition type and timing, when consumables are changed, when cylinders are changed/honed, sticking valve cleanups, regular maintenance, etc. If they do, in about another 10-20 years, they will be able to tell us definitively if LOP is bad or good. Until then, we have to process all the data we have consumed, and make a decision. Personally, I run as lean as I can depending on the speed I want to fly. If not in a hurry, I lean it to just before roughness.

 

[Bicyclops]

I was told by the guy selling me my kit engine, with many years of experience and working for a respected shop and who should have known better, that if I ran too LOP the exhaust gases "would go toxic" and burn my valves. When I asked what he meant by going toxic, he gave the example of too much oxygen in an oxy/acetylene flame. Utter BS. ~3000* vs ~6000*F. Granted, 3K* would not be great for the lip of an open valve, but that's not what it experiences. The gas has cooled significantly as the moving piston allows expansion. By the time it makes its way past the valve, it's cooled a lot more and then the valve slams shut and begins transferring heat to the seat. Valves burn when the contact on the seat is not concentric and localized lack of cooling results. Running slightly ROP and at higher CHT that causes will make it more difficult for the valve to cool enough between exposures making guide wear more rapid and leading to loss of concentricity.

As to the OP's mechanic's statements, I'd like to understand the physics of how running LOP deforms cylinder walls. Bell shaped? Does the mechanic mean that the bore is worn more at the bottom than the top? Pretty typical. Cylinder bores also typically wear slightly oval in cross section. The crank and con rod put side forces on the piston. Reduction of power and heat by LOP ops isn't going to increase side forces and heat stress on the cylinder, and somehow this is going to localize stresses and/or wear somewhere in the bore?

Choke is the result of the head shrinking onto the barrel as it cools during manufacturing. Before mating, the bore is straight. Choke doesn't improve the operation of the engine or somehow completely disappear at operating temperature. In fact, piston to cylinder clearance needs to be looser to accommodate it. Ring gaps, too. Show me an automotive engine manufacturer that bores their cylinders with choke. You'll hear that choke isn't required on liquid cooled cylinders, only air cooled. How about all those Volkswagens with straight bores?

Ed

 

[clarenceoveur]

This is a pretty good video on LOP ops by George Braly. Starts at 15:24 if you don't want to watch the whole thing.

 

[paulsmeds]

Maybe folks who are having more issues with their engines not making TBO or the need for top overhauls running LOP might accidentally not be following all the best practices? Running LOP at too high a HP is an example. I assume engines running higher than suggested HP LOP could greatly shorten the time to top overhaul. Personally I set my limit at 65% HP or lower to be extra safe I can't screw it up - engines are costing more than the entire plane did 10 years ago.

I know there are exceptions to rule so not trying to paint all shorted engine life with a broad brush or that incorrect LOP operations causes all premature engine overhauls. I knew someone who would lean at sea level on take-off because he said he had way more power. Six months later he was getting a top overhaul. Just a thought, not trying to offend or accuse anyone of not following the LOP best practices.

 

[Capt]

Each to their own re LOP, I own an RV to go fast, fuel is the cheap known part of flying, you know, dead a long time, can’t take it with you, being old school ROP I’ll go to my grave knowing I did it fast

 

[skirting_virga]

Each to their own re LOP, I own an RV to go fast, fuel is the cheap known part of flying, you know, dead a long time, can’t take it with you, being old school ROP I’ll go to my grave knowing I did it fast

I sort of agree. LOP costs me... maybe 10ktas? I want to see how much I can gain back with timing advance when I get the EI Commander installed, but I for sure do not like to go slow.

 

[FlyinTiger]

I was skeptical at first about LOP. Then I tested it out over the years on dozens of different normally aspirated Lycoming and Continentals.

After in cruise and LOP for about 10 minutes at 60-65% power I consistently I see the CHTs even out at a lower temperature, the oil temp comes down 10-15*F, the engines look happier to me. All at WOT cruising between 8000-11000 feet when the winds are favorable.

Running ROP, even 50*F, is faster, but the oil and CHTs are 15*F hotter.

My RV-4 had 2500 hours on the O-320 when I sold it, perfect health. Had to use some carb heat to smooth out the induction flow but throttle with altitude and left it WOT.

I’ll continue running LOP in cruise until
I see evidence to change my mind.

 

[skirting_virga]

I was skeptical at first about LOP. After testing it out over the years on dozens of different normally aspirated Lycoming and Continentals.

After in cruise and LOP for about 10 minutes at 60-65% power I consistently I see the CHTs even out at a lower temperature, the oil temp comes down 10-15*F, the engines look happier to me. All at WOT cruising between 8000-11000 feet when the winds are favorable.

Running ROP, even 50*F, is faster, but the oil and CHTs are 15*F hotter.

My RV-4 had 2500 hours on the O-320 when I sold it, perfect health. Had to use some carb heat to smooth out the induction flow but throttle with altitude and left it WOT.

I’ll continue running LOP in cruise until
I see evidence to change my mind.

Angle valves cool like crazy, especially with a big oil cooler. I have no trouble getting the CHTs super low at (admittedly-high) cruise altitudes ROP. But I wouldn't mind less carbon/lead deposits of LOP, it's just a bit slower.

 

[newt]

My engine has just had a full overhaul, and the shop says never run LOP.

If you fly a Cirrus SR22 in accordance with POH numbers, you're LOP. Is their claim that LOP is bad unless you're in a Cirrus?

(Your engine buiilders might need a "correlation is not causation" tutorial)

- mark

 

[SkyDweller]

It is known that a ton of guys fly LOP and a ton of guys fly ROP. Some fly faster and a little bit hotter, some fly slower and a little bit cooler (CHT). Some are much cooler or much hotter regardless of mixture setting. Every one of these aircraft is a little bit different. Is it a guarantee that your engine will run longer between overhauls by running LOP? Nope. Is it a guarantee it’ll run less time by flying ROP? Nope.

When going by “seat of the pants” feel, my engine feels happier ROP. I was a 60-63% LOP every flight kind of guy with my last RV. I was so focused on trying to prolong engine life that I was staring at my instruments more than I’d like to admit. I can assure you I’m having more fun flying and spending more time “outside” the cockpit than before.

You’re going to get about half LOP recommendations and half ROP. If your CHT are within Lycoming’s safe operating range, do what feels best for your aircraft and your engine. As others have said, there are so many other factors which can contribute to needing an overhaul. Best to not dwell on just one of them.

 

[Ironflight]

Folks that talk about running LOP (and the resultant lower speed) as a cost-saving measure are missing the point - I don’t run LOP to save gas, I do it to lower temperatures. We have very tightly cowled engines, and some folks spend lots of hours working on their cooling, others get lucky, others get unlucky…. Running LOP drastically reduces CHT’s. Of course, back in the days before digital engine monitoring, none of us knew what our CHT’s were, so we were fat, dumb, and happy….and no one made it to TBO…..

 

[FlyinTiger]

Another reason I run LOP is that for a couple knots I can trade having more reserve fuel when I arrive at my destination airport. Sometimes I fly 3 hours IFR to go places in an RV. This isn’t all the time, so I suppose when flying formation or doing other lower altitude stuff I’m ROP by 100*F or so to make sure I don’t wander into the red fin.

I wonder where the ROTAX fuel mapping is? Mostly LOP or does it change based on RPM? I’m not familiar, but the four cycle higher RPM engines people are running now on larger planes (915/916) seem to be modern engines that are FADEC.

 

[skirting_virga]

I don’t run LOP to save gas, I do it to lower temperatures. We have very tightly cowled engines, and some folks spend lots of hours working on their cooling, others get lucky, others get unlucky…. Running LOP drastically reduces CHT’s.

I think I understand this, but I have an angle valve and it runs super cool all the time. I'm talking like 300° CHTs. Rare to get over 340° on the hottest cylinder, even in climb on a hot day. I probably have too much exit area and the resulting cooling drag, but it's not an easy fix...

Edit: at some point I worry about overcooling. My #3 was running 260-270° in cruise the other day. 2500rpm, max MP , ROP at 15.5k feet.

 

[lr172]

Edit: at some point I worry about overcooling. My #3 was running 260-270° in cruise the other day. 2500rpm, max MP , ROP at 15.5k feet.

Water cooled auto engines run at a very consistent 190-210* and can go for half a million miles in some cases. Many / most have steel barrels and aluminum heads, just like ours. I think you will struggle to find evidence that CHTs below 200 are damaging to your engine cylinders or heads. Lead deposits can increase at low temps, but that is an annoyance, not wear or damage.

 

[blaplante]

Two things:
1) to the OP, any of those engines operated at LOP and had issues... have engine monitor data? I wonder if there's a correlation between those that run LOP and running insufficiently ROP when operating ROP?

2) 50F ROP is taught by a lot of flight instructors, but isn't always sufficient to protect the engine, see https://www.savvyaviation.com/red-box-red-fin/#tbofallacy

 

[lr172]

The Lyc operating manual tells you for economy cruise to lean to roughness then richen until smooth. For a carb, that will be around peak to 10 or so LOP. For an injected engine, that is going to be LOP. That is straight from the mfr. Manual is also clear that when using EGT that peak is good. Greatest economy is around 20* LOP. How different can that be from peak to be causing all of this supposed damage?

 

[skirting_virga]

Water cooled auto engines run at a very consistent 190-210* and can go for half a million miles in some cases. Many / most have steel barrels and aluminum heads, just like ours. I think you will struggle to find evidence that CHTs below 200 are damaging to your engine cylinders or heads. Lead deposits can increase at low temps, but that is an annoyance, not wear or damage.

They have tighter tolerances because they aren't designed to run at 400°+. An air cooled engine has to tolerate that indefinitely without the pistons jamming in the bore. It could easily be that at 200° and high power, the blow by becomes excessive. I don't know for sure, but I assume there's a sweet spot.

 

[Freemasm]

They have tighter tolerances because they aren't designed to run at 400°+. An air cooled engine has to tolerate that indefinitely without the pistons jamming in the bore. It could easily be that at 200° and high power, the blow by becomes excessive. I don't know for sure, but I assume there's a sweet spot.

and don't forget barrel choke. You want to try and be in the middle of the green for CHT. Too cool will wear a step near TDC and thus you'll "fail" the leak-down test.

 

[skirting_virga]

and don't forget barrel choke. You want to try and be in the middle of the green for CHT. Too cool will wear a step near TDC and thus you'll "fail" the leak-down test.

I don't know what this is, but it seems to be confirmation that too cool is possible. Have more reading I can do on it?

 

[RV8JD]

Regarding “too cool” CHTs. From the Lycoming (I)O-360 Operator’s Manual:

 

[tom_AZ]

Another reason I run LOP is that for a couple knots I can trade having more reserve fuel when I arrive at my destination airport. Sometimes I fly 3 hours IFR to go places in an RV. This isn’t all the time, so I suppose when flying formation or doing other lower altitude stuff I’m ROP by 100*F or so to make sure I don’t wander into the red fin.

I wonder where the ROTAX fuel mapping is? Mostly LOP or does it change based on RPM? I’m not familiar, but the four cycle higher RPM engines people are running now on larger planes (915/916) seem to be modern engines that are FADEC.

I run ROP for takeoff and climb. I lean modestly during climb, targeting EGTs of 1300 or so, but still well ROP. RVs climb so well, that I'm at cruise altitude (10K +/- 2K) in just a few minutes, so any fuel savings from additional leaning would be small. At cruise, I run LOP, but do so crudely--WOT, 2350 or so RPM, and 10.5 gph--rather than watching EGTs. Based on GAMI lean tests, EGTs tend to peak around 11.2 gph +/-, depending on atmospheric conditions, e.g. DA. Onset of engine roughness tends to be around 8.9 gph. At those cruise power settings, TAS is 160 knots or so. i should experiment more with different RPM settings to see if I can extract a few additional knots. If anything, both CHTs and oil temps are tend to be lower than I would have expected, perhaps 320-ish and 170 (with ~1/3 of the scat hose to the oil cooler blocked off), respectively on warm days. Lower for what passes as a "cold day" in Arizona.

My decision to run LOP at cruise is based on two things, one presumed, the other definitive: The presumed part is engine longevity. What data I've seen from GAMI (intracylinder pressures, etc...) seems pretty solid and presumably those translate to engine longevity--but I can't really prove it. The definitive part is fuel savings...but not for the sake of "saving fuel" in and of itself. Rather, my objective is overall time to destination. My minimum fuel at destination is 14 gallons usable (conservatively, I consider full tanks as 58 gallons usable with a 60 gallon capacity). That's an hour plus, even if I needed to be at lower altitudes and higher power, e.g. needing to climb/divert for some reason after descending. From Tucson on a day without really adverse winds aloft, I can fly to Monterey without a fuel stop even dealing with a moderate amount of SUA. If I were running ROP, at perhaps 12.5-13 gph, my TAS would be higher, but a fuel stop (KWJF) would be needed to stay comfortably within my admittedly conservative reserve minimums. Going the other direction, for a trip to Ames, IA again taking SUA into account as well as flyover terrain, I can make the trip with one fuel stop (generally KDUX). ROP, would necessitate a second stop. I generally think of each fuel stop as adding an hour--slowing down for the pattern, landing, taxi, fueling, "sumping" pilot and PAX tanks as well as those in the aircraft, more taxi, runup, T.O. and climb, etc... For me at least, the time saved by skipping a (or an extra) fuel stop more than offsets a small decrease in TAS.

 

[RV10inOz]

Howdy all,
Looking for feedback from folks who actually overhaul engines for a living.
My engine has just had a full overhaul, and the shop says never run LOP.
This is based on their experience......LOP causes deformation of the cylinder walls, and they lose their choke and become bell shaped. It doesn't become a valve issue, according to them, but a ring issue.
He said while CHT's may be lower, the other areas can experience higher temperatures which causes the damage.
He said LOP makes him a lot of money, and recommends 100° ROP.
Now, I have always been an LOP guy, but maybe the savings are not really worth it in the long run. I've also had to do 2 top end overhauls on engines I've run LOP. Hmmmm?
I realize many so called gurus recommend LOP, but is it possible they've had it wrong all this time?
Tangible evidence is encouraged.
Cheers
Stephen

With all due respect he is talking 1,000,000,000,000 pure export grade BS.

If you would like to learn more, reach out and contact me.

A concert pianist does not take music lessons from the piano tuner.

 

[Freemasm]

Choke is the result of the head shrinking onto the barrel as it cools during manufacturing. Before mating, the bore is straight. Choke doesn't improve the operation of the engine or somehow completely disappear at operating temperature. In fact, piston to cylinder clearance needs to be looser to accommodate it. Ring gaps, too. Show me an automotive engine manufacturer that bores their cylinders with choke. You'll hear that choke isn't required on liquid cooled cylinders, only air cooled. How about all those Volkswagens with straight bores?

Ed

I don't know what this is, but it seems to be confirmation that too cool is possible. Have more reading I can do on it?

Going to disagree, Sir. The manufacturing process may create the choke but it’s engineered in for a reason.

Comparing a Volkswagen engine to an aviation engine is really baseless when the latter’s objective was primarily affordability, made less HP than a single cylinder of an O360, and didn’t operate in the temperature extremes that an aircraft can experience in a single mission. I’m not a car guy but I’d bet find a different barrel config. in the Porsche air cooled engines.

@chris. Can’t find my copy but I believe there was a good description in the Sky Ranch Engineering manual way back. Below is a description shamelessly taken from an internet advanced search engine.


2. Cylinder Bore Choke (Mechanical Taper)In engine manufacturing, a "cylinder choke" refers to a precise tapering of the cylinder bore, where the diameter is slightly smaller at the top of the cylinder than it is at the bottom.Counteracting Thermal Expansion: Air-cooled engines experience extreme temperature variations. The top of the cylinder (near the combustion chamber) gets much hotter and expands more than the bottom.The Importance: By making the top of the bore slightly smaller when the engine is cold, the cylinder achieves a perfectly straight, parallel bore when the engine reaches its normal operating temperature. This ensures that the piston rings maintain a tight, uniform seal under heat, preventing compression loss and blow-by.

And salutations to the Republic of Boulder. I’ll be up in Summit County in a couple of months.