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Temperature, Temperature, Temperature

I’ve had wonderful luck with piston aircraft engines throughout my nearly 50 years as an aircraft owner. All the engines on my airplanes have made TBO with minimal maintenance along the way, and in recent years they’ve gone far, far beyond TBO.

For decades, I was convinced that the secret of my success was the fact that I “babied” my engines, typically limiting my cruise power settings to no more than 60 or 65 percent power. I felt that sacrificing a little airspeed in exchange for long engine life and reduced maintenance cost was a good tradeoff.

I’ve come to learn, however, that such “babying” is one way to achieve long engine life, but it’s not the only way. That’s because it’s not POWER that damages out engines—it’s TEMPERATURE. It turns out you can run these engines as hard as you like so long as you are obsessive about keeping temperatures under control.

Or as my late friend, powerplant guru and former Continental Motors tech rep Bob “Mose” Moseley used to say, “There are three things that affect how long your engine will last: (1) temperature, (2) temperature, and (3) temperature!”

It’s all about the heat

CHTOur piston aircraft engines are heat engines. They have moving parts—notably exhaust valves and valve guides—that are continually exposed to extremely high temperatures in the vicinity of 1,500°F and sometimes hotter. Since engine oil cannot survive temperatures above about 400°F, these moving parts must function with no lubrication. They depend on extremely hard metals operating at extremely close tolerances at extremely high temperatures with no lubrication. Aluminum pistons and cylinder heads are also exposed to these very hot temperatures, despite the fact that aluminum melts at about 1,200°F. It’s nothing short of miraculous, and a testament to outstanding engineering, that these “hot section” components last as long as they do.

The key to making these critical parts last is temperature control, and the most important temperature is cylinder head temperature (CHT). Mose monitored and overhauled these engines for nearly four decades, and he swore to me that an engine that is operated at CHTs above 400°F on a regular basis will show up to five times as much wear metal in oil analysis as an identical engine that is consistently limited to CHTs of 350°F or less. “It’s amazing how much a small increase in CHT can accelerate engine wear,” Mose said.

As critical as CHT is, many owners don’t have a clue whether their CHTs are 400°F+ or 350°F-. That’s because the engine instrumentation provided by most aircraft manufacturers is pathetically inadequate. The typical factory CHT gauge looks at only one cylinder, and it’s not necessarily the hottest one. Further, the typical factory CHT gauge often isn’t even calibrated, and its green arc extends up to a ridiculously hot 460°F (for Continentals) or 500°F (for Lycomings). Those numbers may be okay as emergency red lines, but they’re horribly abusive for continuous operation. If all you have is factory gauges, you could easily be cooking your cylinders to death while blissfully thinking that all is okay because the CHT gauge is well within the green arc.

To know what’s really going on in front of the firewall, you have to have a modern multiprobe engine analyzer with a digital readout. Such instrumentation isn’t cheap—figure $2,500 for a single or $5,000 for a twin, installed—but if it saves you from having to replace a couple of jugs en route to TBO, it has more than paid for itself. Installing a digital engine analyzer is probably the best money you can spend on your airplane.

Fuel system setup

MaintenanceFor takeoff and initial climb, we normally are at wide-open throttle, full-rich mixture, maximum RPM (if we have a constant-speed prop), and wide-open cowl flaps (if we have those). So there’s not much we can do from the cockpit to affect CHT during these phases of flight.

What does affect CHT is how our full-power fuel flows are adjusted. Unfortunately, it is shockingly common to see damagingly high CHTs due to improperly adjusted fuel flows, particularly in fuel-injected engines. It is not unusual for the fuel flows to be set wrong from the day an engine is installed, and never to be checked or adjusted all the way to TBO. The owner winds up going through cylinders every 500 hours and never knowing why (or blaming the manufacturer).

In part, the problem lies with mechanics who don’t fully understand how critical it is to test and adjust the fuel system setup on a regular basis. Continental recommends that the fuel system setup on its fuel-injected engines be checked and adjusted several times a year to account for seasonal changes. I’ll grant that’s a bit anal, but most Continental-powered airplanes go year after year without this ever being done, and many shops that maintain these airplanes don’t even have the necessary test equipment to do it.

Even when mechanics do test and adjust the fuel system, they often adjust it wrong. For example, Continental Manual M-0 (formerly SID97-3G) contains a lengthy table that specifies full-power fuel flow as a range (minimum and maximum). The “fine print” instructs mechanics to adjust the full-power fuel flow to the high end of the specified range, but many mechanics miss this subtlety and adjust it to the middle of the range. Experience shows that this is simply not enough fuel flow to keep CHTs cool during hot-weather takeoffs.

Lycoming engines with RSA fuel injection have no field adjustments for takeoff fuel flow. If it’s inadequate, the fuel servo has to be sent in to a specialty shop for bench checking and adjustment.

Then there’s the problem of aftermarket engine modifications. For example, engines that have been retrofitted with Superior’s Millennium cylinders often run higher CHTs than they did with their original factory cylinders. That’s because Millennium cylinders have substantially better “volumetric efficiency” than factory cylinders—in other words, they breathe better. Since they breathe more air during every combustion cycle, they need more fuel to maintain the same fuel/air mixture. The full-power fuel flow marked on your fuel-flow gauge may simply not be high enough if you have Millennium cylinders installed.

Even worse are turbocharged engines with aftermarket intercoolers installed. The intercooler reduces the temperature of the air that the cylinder breathes, making it denser. Denser air demands more fuel to maintain the desired fuel/air mixture, so full-power fuel flow must be increased significantly above original factory specifications. Too often this is not done, and the result is fried cylinders.

Many A&Ps are reluctant to adjust takeoff fuel flow above red-line. However, if you have Millennium cylinders, an aftermarket intercooler, or some other “mod” that allows your engine to produce more power than it did when it left the factory, that’s exactly what must be done to keep your CHTs cool and avoid premature cylinder failure.

Enough fuel flow?

MP and FF guage comboHow can you tell if your full-power fuel flow is adequate? If you’re limited to factory gauges, you probably can’t, at least with any precision. About the best you can to is to watch your fuel flow gauge (if you have one).

A good rule of thumb is to multiply your engine’s maximum rated horsepower by 0.1 to obtain the minimum required fuel flow in gallons-per-hour, or by 0.6 for pounds-per-hour. For example, if your engine is rated at 285 horsepower, your takeoff fuel flow should be at least 28.5 GPH; if it’s rated 310 horsepower, the minimum should be 31.0 GPH. If your takeoff fuel flow is significantly less than this, have your mechanic crank it up. And don’t forget that if you have Millennium cylinders or an aftermarket intercooler, your engine might be producing a few percent more horsepower than what the book says, so it might need a few percent more fuel flow.

Now if you have a digital multiprobe engine analyzer, it’s easy to tell if your fuel flow is adjusted high enough. Just make sure none of your CHTs exceed 380°F during takeoff and climb for Continentals or 400°F for Lycomings. Lower is even better.

What about cruise?

Cruise flight represents the lion’s share of our flying time. Just as in takeoff and climb, it’s essential to keep all our CHTs at or below 380°F (for Continentals) or 400°F for Lycomings during cruise to achieve good cylinder longevity, and lower is even better. There are basically three different strategies for keeping CHTs low during cruise:

  • Baby the engine
  • Operate very rich
  • Operate lean-of-peak

All three strategies work, and conscientious use of any of them will give you a good shot at making TBO with minimum cylinder problems. But each has its pros and cons. Let’s take a closer look.

Engine graph

The mixture that many POHs refer to as “recommended lean mixture” is 50°F rich of peak EGT. As this graph shows, using that mixture results in very nearly the highest possible CHT. To reduce CHTs to the level required for good cylinder longevity, you need to do one of three things: (1) reduce power, (2) operate very rich, or (3) operate lean-of-peak.

Baby the engine

Many POHs talk about operating at three alternative mixture settings: “best power mixture” (~125°F rich-of-peak), “recommended lean mixture” (~50°F rich of peak, and “best economy mixture” (~peak EGT). It turns out that “recommended lean mixture” (~50°F ROP) is just about the worst possible mixture setting for keeping CHT low.  If you look at Figure 1, you’ll see that CHT reaches a maximum very close to 50°F ROP. So if you want to operate at “recommended lean mixture” and simultaneously keep CHT low, there’s only one way to get there: reduce power dramatically (generally 65% power or less). In other words, baby the engine.

Both “best power mixture” (~125°F ROP) and “best economy mixture” (~peak EGT) result in somewhat lower CHTs than does “recommended lean mixture.” At either of these mixture settings, you can usually operate at 70% power or so and still keep CHTs in the acceptable range.

In any of these cases, you’re trading power and airspeed for reduced temperatures and increased longevity. For most of us, that’s a reasonable tradeoff to make.

Operate very rich

But what if you are unwilling to sacrifice power and airspeed? Is it possible to go fast and still keep CHTs low?

Sure it is. We already talked about one way to do this in our discussion of takeoff and initial climb: pour lots of 100LL on the problem. In other words, operate very rich.

How rich? Figure 1 suggests that to reduce CHTs by 25°F, you need to enrich the mixture to about 160°F ROP. For each additional 10°F of CHT reduction, you need to enrich an additional 50°F ROP. Using such very rich mixtures, you can go fast and still stay cool. (This is how Reno racers usually operate.) But before you decide to go this route, consider the downsides.

The most obvious downside is that this strategy is very fuel-inefficient. Compared to “best economy mixture,” the very-rich strategy consumes about 25% more fuel, and reduces range by a similar amount. Advocates of very rich mixtures will tell you that “fuel is cheaper than engines,” but don’t be so sure. At today’s avgas prices, using 25% more fuel in a 300 horsepower engine can cost more than $40,000 over the engine’s TBO, and that’s enough to change out quite a few cylinders.

A second and less obvious downside is that very rich mixtures result in “dirty” combustion with lots of unburned byproducts in the exhaust gas. Operating this way for long periods of time tends to cause deposit buildup on piston crowns, ring grooves, spark plugs and exhaust valve stems. Do it long enough and you could wind up with stuck rings, stuck valves, worn valve guides, and fouled plugs.

Operate lean-of-peak

The third way to reduce CHTs is to lean even more aggressively than the POH recommends and operate on the lean side of peak EGT. Figure 1 shows that you can reduce CHTs by 25°F by leaning to about 10°F LOP. For each additional 10°F of CHT reduction, you need to lean an additional 15°F LOP. Using these very lean mixtures, you can go fast, stay cool, and obtain outstanding fuel economy, all at the same time.

What’s the downside of the LOP approach? The only major downside is that if your engine has uneven mixture distribution among its cylinders, it will usually run unacceptably rough at LOP mixture settings.

Uneven mixture distribution can usually be corrected in fuel-injected engines by “tuning” the fuel injector nozzles to eliminate the mixture imbalances. GAMIjectors are tuned nozzles that are STC’d for the majority of fuel-injected Continentals and Lycomings. Continental now offers its own version of tuned injectors on some of its premium engines.

If your engine is carbureted, you have no injector nozzles to tweak. Most carbureted Lycomings have pretty decent mixture distribution and can be run at least mildly LOP without running rough. Some carbureted Continentals (notably the O-470 used in the Cessna 182) have miserable mixture distribution, making it difficult to run those engines LOP without uncomfortable roughness.

Stay cool!

Whatever strategy you prefer, the important thing is to keep a close watch on your CHTs and ensure that they remain cool. The best way to do this is to install a multiprobe digital engine monitor and program its CHT alarm to go off at 390°F (Continentals) or 410°F (Lycomings). If the alarm goes off during takeoff or initial climb, you’re going to have to get your mechanic to turn up the full-power fuel flow.  If it goes off during cruise, either enrich (if ROP) or lean (if LOP) to bring the CHT down to acceptable levels.

If you don’t have a multiprobe digital engine monitor, install one. The cost of such instrumentation (including installation) is usually less than the cost of replacing one cylinder. Failure to install such instrumentation is a classic case of “penny wise, pound foolish.”

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from Amazon.com in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from Amazon.com in paperback and Kindle versions. (508 pages).

10 Comments

  1. Thanks for the article, as usual, Mike! I have a question about the fuel flow numbers you have – I have a T210 with intercooler, known to run hot. I have a JPI930 and on climb-out, I can usually keep it in the 380-390 range on warm days (360-370 on cold ones), and my fuel flow is 20-22gph or so (that’s at 75% power). On hot days (ground temps >100F) I will hit 400 which is where my alarm goes off, but it’s doable to keep it lower even then, just by not climbing as aggressively.

    From this description, and I know it’s hard to say for sure… in your opinion does it sound like I should increase the fuel flow even more? I have experimented once in running the boost pump on ‘low’ to get a higher fuel flow on climb-out, and of course, the CHTs were a good 15 F lower, at an increase of 4-5gph on top of the usual. I have read in some places that people simply use the boost pump on climb-out on 210’s but it might be better to increase fuel flow (and I can always lean it with the mixture)… I guess now that we’re getting into the cold time of year this is more a question for next spring/summer, I’m curious to hear your opinion.

    • Peter, what is the HP rating of your T210 and what is the full-power takeoff fuel flow and MAP? I can’t really tell from your 75% figures whether or not your FF is adjusted appropriately. –Mike

    • I fly a Continental IO520 equipped Bellanca Super Viking with a JPI EDM 900 and am experiencing the same issue as Peter in the T210. I’ve tried plugging obvious holes in engine baffling, turning the fuel up, etc… but am still experiencing 385+ temps on takeoff. I do my best to get them down as quickly as I can after takeoff, but this still concerns me. My fuel flow at takeoff is about 25gph. During cruise flight, the temps are good, usually in the 330-360 range running LOP. I do have the after-market Globe cowling. Any suggestions would be most appreciated.

      • Shane, Your fuel flow sounds very low for your engine. Using Mike’s comment above, you should see a fuel flow that is roughly one tenth of your max horsepower. I believe your engine is a 300HP max engine and thus, your “full power” fuel flow should be roughly 30 gph. I would have your mechanic check your fuel flow settings. Running leaner than the engine manufacturer recommends during full power operations is particularly dangerous due to higher risk of detonation and pre-ignition. Since you have an engine monitor, take a look at your EGTs. During full power operations, the EGTs should be roughly 200 to 250 degrees F below your cruise EGTs assuming you operate 50 ROP or 50 LOP. The absolute EGT number is not very relevant, just the relative EGT when comparing full power operation to properly leaned for cruise flight.

        • Thanks for the info Frank. I’ll do some fact gathering on the EGTs and look at turning up the fuel more. Appreciate the feedback!

    • Peter, Your fuel flow sounds like you are running ~50 ROP. For a Turbocharged 310HP engine running 50 LOP, your fuel burn at 75% power should be roughly 17 gph versus 20 to 22 gph. If you are running 50 ROP, you are creating the highest CHTs in your engine and 75% power will definitely keep you operating at or above the 400 CHT range, which is not good for the life of the engine. Adding more fuel flow will certainly bring down the CHT, but at the cost of loading up your spark plugs and exhaust valves with lead deposits. Overtime, running 50 ROP, you will slowly get less performance out of your engine until you are forced to pay maintenance to clean up things. I would recommend backing down to 65% power whether you choose to run ROP or LOP. However, running LOP will dramatically lower your CHTs. Yes your EGTs will go up, but that is a good thing as it means that all of the fuel is burning in the cylinder and not on the way out of the cylinder. Those blue flames you have seen on engines at full power is fuel still burning well outside of the cylinders. It is not only very wasteful, but also causes a number of problems in exhaust systems and your turbocharger. I have been running LOP for 5+ years on GAMI injectors and using an MVP-50 engine monitor. My CHTs are typically <330 in cruise, although I only run at 65% power as I am not in a rush to burn up my engine. LOP is how the airlines always ran engines until they went to jet engines and yes, even jet engines run LOP in cruise too. Your car is also running LOP now. No business would willingly through un-burned fuel out the exhaust for 90% of the time. Its fine and necessary for takeoff and most climbs, but not in cruise flight. I would recommend you attend Advanced Pilot Seminars to learn more about LOP operations and its inherent benefits.

      • Thanks for the response. No, on climb-out I’m running very ROP, at least >100 ROP, sometimes full rich which is even more rich-of-peak (EGTs in 1300s). I usually take-off at full power, on this TSIO-520-H it means 285hp as that’s what the engine is rated for (32.5″ MP in POH, but since I have an intercooler, usually I stop at 30″ MP as I get a 30degC cooling from the intercooler, the flight supplement calls for 2″ reduction, this is for standard temps). Anyway, pretty quickly (pattern altitude) I reduce throttle even further to 75% power which is around 27-27.5″ MP.

        Anyway, I always cruise at 65% power after that, and I have only had this plane for a couple of months, but have mostly ran it 50-100 ROP, which I realize isn’t the most efficient. Yesterday I did an hour long flight at 9500′ and experimented, 60-65% power and LoP, I was burning 11-12gph and I think the engine ran smooth (a little rough at 10.5-11gph but enrichening a bit smoothed it out), and I was still getting 155-160kt TAS, where as ROP burning 16gph in cruise I will get 165-170kts at similar 65%ish power setting. However, I realize I need to run some more tests, and I wonder if I will need GAMI injectors to run LOP based on my CHT spread. Note that if I run 100 ROP, my CHTs are fairly uniform – lowest is 340F, highest 360-365F, and EGTs are between 1450-1550 for all cylinders, which is way different than what I’m used to flying a carburated engine before. When I ran LOP yesterday, I did experience much better fuel economy but am not sure my CHT spread is OK – my hottest cylinder was 360F, but the coolest was 280F (280, 300, 310, 320, 340, 360). EGTs were fairly uniform 1450-1550 so nothing crazy, TIT was also 1550 or so. https://www.savvyanalysis.com/flight/1603533/d60c011f-6b74-48df-93b7-347463d1dc7e for my LOP experiment (in the middle portion), https://www.savvyanalysis.com/flight/1603528/bf1d4404-be43-4c0c-bbf1-3da2fae1784d is how i usually have been operating. Note that if you look at these logs, ignore EGT#6, that probe is defective and I haven’t replaced it yet.

        I am definitely trying to learn more and also considering paying for the savvy analysis. I used to own a O-470 C-182, so LOP operations weren’t possible with the carburated engine and super-uneven fuel distribution between cylinders (we did have a JPI830 on it) – this is a new world to me. Thanks for any advice, I’ll definitely be reading and learning more, like I said, I’m a newbie at this…

        • Hello Peter. Congratulations on your new airplane. Good news and bad news for you. The good news is that you have an engine monitor and that puts you on the path to awesome engine management. The bad news is that your fuel injectors are all over the place and that you currently can’t run LOP safely.

          Let me explain. I went to your Savvy Analysis page and checked out the flight you made. There were two instances that you did a mixture pull that allowed me to assess your GAMI Spread. Although you went a little fast from rich to lean (actually the right speed normally, but fast for the GAMI test), there was enough data to determine that your GAMI spread is roughly 2.5 gph. That means your leanest fuel injector (Cylinder 2) peaks in EGT about 2.5 gph before your richest fuel injector (Cylinder 5). When you thought you were fully LOP, in reality cylinder 5 was running at peak EGT and that is why it is running around 360 deg F CHT and cylinder 2 was running 80 deg LOP and thus why it was so much cooler at around 270 deg F CHT. It is normal to have the cylinders at different temperatures, but that should strictly be due to their position in relation to their cooling airflow. The spread in temperature you were seeing was more due to the fact that each cylinder was operating at a different point relative to peak EGT. Also, that means when you are running ROP, that cylinder 5 is running way ROP and that means it will be the cylinder with bad plugs first and then eventually leaded up valves and valve stems. It will be followed shortly by cylinder 3. It could also mean that cylinder 2, 1 or 4 is running at peak EGT, which would be dangerous from a detonation standpoint if you go above 65% power in cruise.

          So my recommendation is that you can’t safely run LOP until you get GAMI fuel injectors and you should get them anyway even if you elect to go ROP. GAMI will work with you to get all of your cylinders peaking within 0.5 gph of each other and they can give you technical advice too. The Savvy Analysis folks are fantastic too, so definitely use them as a resource also. I would first look at the Advanced Pilot Seminar as a great way to learn about proper engine management and you can take the course while you are waiting for your GAMI injectors to arrive.

          You can also home school yourself using John Deakin’s work on the AVWEB site. He had a great series of articles called the Pelican’s Perch. Go through all of those of interest and especially his work on Those Fire Breathing Turbo’s and the links are here for all 6 parts:
          http://www.avweb.com/news/pelican/182102-1.html
          http://www.avweb.com/news/pelican/182103-1.html
          http://www.avweb.com/news/pelican/182104-1.html
          http://www.avweb.com/news/pelican/182105-1.html
          http://www.avweb.com/news/pelican/182106-1.html
          http://www.avweb.com/news/pelican/182107-1.html

          When you get to the 5th part, you will learn that the approximate ratio of fuel energy to horsepower in a typical turbo engine is 13.7 horsepower per gph of fuel. Thus, when you are running at 11.5 gph (when LOP, every drop of fuel is theoretically being burned in the cylinder), then you are generating roughly 157.6 hp (11.5 * 13.7). Since your engine is rated for 285 HP, that means you were using just 55% of your available hp (simply 157.6 divided by 285 HP) and that is why you noticed the reduced airspeed. You will learn all about that in either the APS course or by reading Deakin’s Pelican Perch articles.

          It is interesting how much energy we spend to learn how to safely fly an airplane, but how little energy is spent truly understanding how the engine works in our piston powered airplanes. Someday, either engine operation will be properly taught or the FAA will finally allow digital engine control. I prefer the later (as we are actually not being safe by preventing modern digital engine controls), but until that happens I hope all piston pilots learn more about their engines and how to operate them and extend the life of the engine.

          Happy flying and learning!!!

  2. Right Mike, I’m ringing up and getting one ….damn the 180’s only got 140 hours on the O470 engine … shame the old owner didn’t put an IO 520/550 with games in it would have paid him way more $$.

  3. Buford T. Justice

    May 4, 2018 at 12:29 pm

    I fly a 2000 182s model. 1450 hours on the engine. I have over 3500 hours in 182’s of various types. I run this model 75% and burn 14.5 GPH most of the time 100 ROP. I have had 2 go to overhaul running this way. Recently had #6 cylinder changed and purchased an EDM 700. Just had a fresh annual and baffling, nozzles, manifold, bore scoped all cylinders, and plugs changed and or cleaned as needed. #1,2,5, and 6 still run from 400-430. 3 and 4 are around 385ish. Called Lycoming and they said this was okay and acceptable. Since I just had the EDM installed, I am just wondering if there should be any concern?? This plane is for my business use( my time machine) so I fly it as fast as the charts allow. Great article! Advice appreciated??

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