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Old 08-08-2011, 02:18 AM
nucleus nucleus is offline
 
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Post Understanding LOP Operations - A Summary of John Deakin's Knowledge

This summary of my understanding of LOP operations. It is all based on John Deakin's Pelican's Perch column on AVweb. His LOP know-how is so spread around in those columns that I find it difficult put it all together, so hence this attempt to summarize. He now works at GAMI, and I am grateful to him for all the knowledge and graphs. I will try to put quotes around his words and not mine.

I am big advocate of LOP operations, and even do it at low altitude, like 700 ft MSL, and I hope this summary will clearly illustrate why this is safe and desirable.

Let's start with a graph demonstrating the relationship between mixture, EGT, CHT, ICP and HP:



You can see that the Intra Cylinder Pressure tracks very well with the Cylinder Head Temperature. There is also a good correlation between those lines and the Exhaust Gas Temperature.

Key point from Pelican's Perch #65:

"On the rich side of peak, leaner is hotter, but on the lean side, leaner is cooler." That's a crucial concept! Repeat that to yourself, until you "get it.""

Temperature and Detonation:

Pelican's Perch #43:
"We know that combustion temperatures are in the 3,000ºF to 4,000ºF range, EGT "only" run around 1,600ºF, and CHTs down around 400ºF. How can this be? 4,000ºF is more than enough to melt steel, so how does the interior lining of the cylinder survive? Why don't we see hotter temperatures on our instruments? Why doesn't the aluminum piston melt down, when aluminum melts at less than 1,000ºF?
There is a thermal boundary layer, on the order of a millimeter thin or so, that acts as a buffer to protect the steel cylinder walls and the surface of the aluminum piston. Think of it as the thermal equivalent of the aerodynamic boundary layer out on your wing. The metal and the molecules right next to it will be at roughly the CHT reading or a bit higher, the next layers will be hotter and hotter, until the layer next to the combustion event will be at the combustion temperatures. That very thin thermal boundary layer acts as a nice insulation barrier, limiting the rate at which heat can be transferred from the bulk combustion gases into the interior walls of the cylinder head, cylinder barrel, and piston.
The heat transfer is continuous, as the heat moves first through the boundary layer, and then the cylinder wall and is finally carried away by the cooling airflow around the fins on the cylinders. Each intake stroke brings in a cool new charge, which starts the process all over again. There is also a matter of time of exposure. The high-pressure part of the combustion event takes up only about 40 degrees or so of crankshaft rotation, and the very hottest part of that only about 20 degrees, so during the other 700 degrees of crank rotation, cooler temperatures prevail. EGT shows only a number that represents a momentary flash of heat during a small portion of the combustion cycle (when the exhaust valve opens and exhaust gas flows across the EGT probe), and a rapidly dropping temperature at that.
This is NOT the major factor that determines how hot their exhaust valve is during operation. The events that happen a few degrees of crankshaft rotation earlier are much more significant because the temperatures are MUCH hotter than the piddling little 'ol 1500ºF measured by the EGT probe."

"We have nice cool induction air and fuel entering a cylinder;

The cylinder happens to have very hot walls. Those hot walls cause some of that nice cool induction air to start to heat up. And it doesn't all happen uniformly.

Further, shortly after the sparks go off, we have a couple of flame fronts, giving off lots of infrared heat, adding to the continuing heat load being absorbed by some of those little remote pockets of fuel and air that are waiting for the flame front to arrive and consume them;

The unburned mixture is experiencing a very rapid increase in pressure, because of two things: A) The piston is rising rapidly during the compression stroke; and B) the flame front combustion products are creating a huge increase in released energy and resulting bulk gas pressure, all of which is neatly measured on the pressure traces you see in the accompanying graphics.

At least some of those little "local pockets" of unburned combustion mixtures have exactly the right mixture of fuel and air to be just a hair-trigger away from exploding.

And … if the fuel is the wrong octane, or the spark advance was set too soon, or the manifold pressure was too high, or the cylinder head temperature was too high ... then one or more of those little "local pockets" of unburned fuel do just that ... they "explode."
That is what we call "detonation".

Each explosion creates a shock wave that travels at the speed of sound (remember, the speed of sound inside the cylinder, at somewhere near 4000 degrees, is very much faster than at a standard day!) and bounces off the walls of the combustion chamber every 1/5th of a millisecond or so (giving off a 5KHz "ping" that you will not hear in the cockpit). Each of those explosions creates a very sharp rise in pressure and sets off a shock wave, which vibrates back and forth across the cylinder. This shock wave can be just the right amount of additional pressure to cause some other little remote local pocket of fuel and air to, in turn, explode, adding to the problem.

As detonation grows more serious, it will become audible, and this is the pinging you'll hear from that old auto engine on the uphill grade. Remember, you will NOT hear it on an aircraft engine."

Mild Detonation:


Moderate Detonation:
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Last edited by nucleus : 08-08-2011 at 03:38 AM. Reason: clarity
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Old 08-08-2011, 03:08 AM
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Post Understanding LOP Operations - A Summary of John Deakin's Knowledge #2

Heavy Detonation:


"How Damaging Is Detonation?
There are newly proposed "standards" that define "light," "medium," and "heavy" detonation. How those are arrived at is far too complex to go into here (which means "I don't know"), but suffice it to say that a little light detonation, even for hours at a time may not be harmful, and in fact, can be beneficial. It does a marvelous job of cleaning deposits off the top of pistons, for example!
The truth of the matter is, most of these engines can operate in the light detonation condition as shown in the graphics for several hundred hours with no detectable damage, PROVIDED the CHTs remain cool and you do not experience a runaway cylinder head temperature during the process.
The problem is how to detect it, and prevent it from becoming worse, because "light" can progress rather quickly into "medium" and worse. It is a "positive feedback" process, with a very negative result! The mechanism that causes it to be self-feeding is that the shock waves from the light detonation tend to begin to "scrub" the thermal boundary layer inside the cylinder. As that happens, the rate of heat transfer increases from the bulk combustion gases into the cylinder. That starts the CHT rising. When the CHT rises, it tends to heat up the incoming charge of new air and fuel a bit faster than the previous crank rotation, and that increases the likelihood of there being more light detonation in the next combustion cycle, which increases the disruption of the thermal boundary layer even more, which heats up ... well, you get the picture. If the cylinder is not really well-cooled, with some cooling reserve, the whole process can snowball to **** in a hurry and you end up in deep detonation trouble."

"I think we can all agree it's better to just stay away from detonation entirely. Much better!"


Pre-Ignition

"Pre-ignition is the ugly cousin of detonation. It is never beneficial, and can cause failure in seconds. As the name implies, preignition occurs before the normally anticipated spark event. Pre-ignition starts a fairly normal flame front, but early, often very early. This forces the pressure higher before top dead center, and that rapid rise in pressure instantly drives the temperature up. At top dead center, the pressure from pre-ignition can reach 1500 to 2000 PSI, rather than a normal maximum of 900 to 1100 PSI. That extremely excessive peak pressure causes all kinds of havoc. Among other things, it can damage internal components, like valves, spark plugs and rings, and it will quickly eat its way through the aluminum piston head.
Pre-ignition is most often caused by some projection, some sliver of metal, or something inside the combustion chamber that can be heated white hot during the combustion event, and that spot must remain at very high temperature all the way through the subsequent power, exhaust, intake, and compression strokes. This projection acts just exactly like the glow plug in a model engine.
One cause is a "helicoil tang." Cylinders are made of soft aluminum. Helicoils are steel inserts, into which the spark plugs can be screwed. After inserting the helicoil it is not unusual to have small, sharp slivers of metal, which must be cleaned off carefully.
A damaged spark plug can lose its capacity to transfer heat from the ceramic and then the ceramic insulator and tip can become unacceptably hot inside the cylinder (somewhere up around 1600F, rather than a normal 1100 to 1200F).
When this happens, the spark plug itself can become the source of the pre-ignition event.

For completeness here, I'll mention that carbon deposits in the combustion chamber can heat up and cause preignition, but bluntly, if the engine is run properly, there should not be any carbon deposits. These come from operating exclusively on the rich side for many hours."

One thing I take from this is that as long as my CHTs are low, I am very unlikely to cause any damage to my engine.

Pelican's Perch #65:

The Dangerous Red Box

Just where is that "red box" I keep talking about? Some rough numbers, good (that is to say, BAD) for most of these engines -- these are "no fly zones," DO NOT set the mixture between them:

Red Box = No Fly Zone

At and below about 60% power, there is no red box. Put the mixture wherever you want it.
At about 65% power or so, 100ºF ROP to Peak.
At about 70%, 125ºF ROP to 25ºF LOP.
At about 75%, 180ºF ROP to 40ºF LOP.
At about 80%, 200ºF ROP to 60ºF LOP.
All those numbers are approximate! Please don't start splitting hairs, here!

You probably don't want to run your engine between those mixture settings. If you do, you are running very high peak pressures inside the combustion chambers, and that peak pressure is occurring too close to top dead center."

Okay, now you have Deakin's take on what we want to avoid, let's look at normal ops:

"Outside the Box

At 65% power, use richer than 100 ROP, or leaner than peak EGT.
At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.
At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.
At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP."


Summarized in Graph Form:
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Last edited by nucleus : 08-08-2011 at 03:36 AM. Reason: Insomnia
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Old 08-08-2011, 03:33 AM
nucleus nucleus is offline
 
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Post Understanding LOP Operations - A Summary of John Deakin's Knowledge #3



Pelican's Perch #66:
We use these charts in our seminars, and they are a variation of those showing "the red box." Ok, so these are triangles, so sue me!

Both charts assume WOT ("Wide Open Throttle"), and a fairly high (or full) RPM.

The left edge represents sea level, and note that to stay outside the red box at sea level, you must be somewhere around 250 degrees ROP (slightly off the chart), or you must be very close to 100 LOP. Both are completely safe settings for the engine, with the LOP setting running much cooler, but producing less power.

Reminder, this assumes full throttle, full RPM (or nearly so)! We are fooling around with the mixture only, here.

Move across the chart to about the 5,000-foot level, and note the red box has gotten smaller, because altitude has taken away a lot of MP, and there's no recovery from that in a normally aspirated engine. Now our red box is around 30 LOP to about 90 ROP. At roughly 8,000 feet, the red box goes away, and that's the area where you can't get 65% due to altitude.

Again, let me remind you, these are approximations; don't be a slave to precise numbers! Don't yell at me because I say "80" in one place, and "90" in another. That's "close enough"!

The same chart can be used for illustrating the climb, if you climb as I suggested a couple of columns ago. Leaning to that target EGT as you climb produces pretty much what you see on the second chart, just rich of the red box. Once you get to your altitude (the example is at 4,500 feet), do the "big mixture pull," and set LOP. The green lines show a good area to be in when running LOP, up to about 9,000 feet altitude. At and above that point, you probably want to switch over to ROP to keep the power up as much as possible." [I don't do this until above 11,000 depending on temp]


Pelican's Perch #64:

"Parking the Engine

It is worth noting at this point another crucial concept we teach in the seminars. We call it "parking the engine." Any time we adjust the engine controls, we want to leave the engine in a situation where it will not, if left alone or neglected, change by itself into a "bad" setting. That way, if we get busy, or have to concentrate on ATC, another airplane system, a radio setup, or a baby barfing, the engine will be safe. In the climb I have just described, the worst that can happen is that you forget to lean, the engine slowly goes richer and gets cooler. The engine and its controls are "parked."

A reminder here, "On the rich side of peak, leaner is hotter, but on the lean side, leaner is cooler." That's a crucial concept! Repeat that to yourself, until you "get it."

Some like climbing LOP. This works, as the engine doesn't know what the airplane is doing; all it cares about is fuel, air, spark, and cooling airflow. But if you set up a LOP climb in a normally aspirated engine and forget it, the mixture will gradually go richer and richer, perhaps getting too hot. LOP climbs violate our general principle of "parking" the engine safely. It doesn't mean you can't do it, or shouldn't do it, it just takes a little more care. The alarms on the JPI [or something modern!] make this a much more feasible operation, and it will save a little fuel, perhaps enough to give you just a bit more range to make that long non-stop.

Don’t be anal about adjusting the climb mixture, please? Many people fiddle it to death, and that’s not necessary. Once you make an adjustment, it will be just fine for a thousand feet of climb, or more. Tweak it again, and it’s good for another thousand feet or two. There's a subtle (and graceful) distinction between tortuously trying to set it within a degree or two, and just setting it so that it drifts into what you want. It doesn't matter if the climb CHT runs 320, 340, 360, or even 380. If it's cooler than that, and you're still ROP, lean it a bit. If the CHT is sneaking up slowly, wait a bit, leave it alone, and altitude will take care of it.

Note: There are some notoriously bad engine installations in which the cooling air flow baffling is so poorly designed, installed, and maintained that holding CHTs under 380 to 400°F on a normally aspirated engine in cruise is a problem. If yours is one of these, you should spend some serious time and effort to get that corrected. There are people that understand these issues and who know how to get it done properly."


IntraCylinder Pressure Graph Comparing LOP vs ROP at same HP:

While both ROP and LOP slow the flame front, at the same HP LOP reduces both peak cylinder pressure and pre-TDC pressure.

Pelican's Perch #65:

"Set RPM for Smoothness

I have one more consideration, smoothness. I'll go for a smooth engine every time, and some airplanes have specific RPM settings where they seem smoothest. You also don't need to be at some exact multiple of 50 or 100! 2534.202 RPM is a perfectly valid setting! It amazes me how some pilots will work so hard to get an RPM at exactly some "even" setting, especially those with digital tachs, accurate to 1 RPM. If it helps, put a piece of masking tape over that final digit! Heck, if you're LOP, cover up the whole tach!

A few engines just won't run smoothly LOP, no matter what you do. Bearing in mind that "smoothness" and "roughness" are VERY subjective terms; a little roughness from small cylinder-to-cylinder power variations won't hurt the engine at all. It MAY have a long-term effect on the airframe, accessories, instruments and hemorrhoids.

If you find that this roughness hurts your ears and your head (from your spouse beating you upside the head, or screeching in your nearest ear, "DON'T DO THAT!"), then perhaps you need your subjectiveness adjusted to more nearly match your spouse's.

While you're fiddling around, trying to find a power setting that will give you the range/speed you want, be aware that small changes in RPM do not affect the mixture ratio very much (fuel pump RPM changes with engine RPM), so you don't need to twiddle the mixture at every RPM change. Once you're where you want, if you want, do one final check for peak, and set the mixture to the appropriate setting.

Sound complicated? It's a lot harder to write and read about it, than to just do it! It's complicated here, because I'm trying to give you the reasons, the logic, and the science behind it, along with a few bad jokes. With a few practice runs, while thinking about it, these procedures become second nature, are very easy, and you're using science for power settings, rather than witchcraft, old wives' tales, and the "knowledge" of a 300-hour CFI. The single most common comment I get from folks who ride along with me is, "But you're not doing anything!" As in anything worthwhile, it takes a bit of practice to get used to it. Take along a safety pilot, the first few times, to watch for traffic, or even have him fly while you learn your engine."


So what I do at low altitude with a fully warm engine is keep my mixture between the green lines of the "Red Triangle" chart. This means the lower you are the more important to run further LOP, at sea level this means at least 90 degrees LOP, I usually go about 100.
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Last edited by nucleus : 08-08-2011 at 03:49 AM.
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Old 08-08-2011, 04:05 AM
nucleus nucleus is offline
 
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Smile Understanding LOP Operations - A Summary of John Deakin's Knowledge #4

"Run It the Factory Way?

We get a few comments from people who report they used to run as the factory says, and usually enjoyed good engine life, some even making TBO. That's true, but there's more to the story.

Most pilots have used some variation of the 65% power setting for decades. All the magazines show the data at 65%, the power charts all seem to suggest 65% is "the way to go." Everyone is comfortable with "good old 65%."

Folks, note that's right on the edge of our "red box"! We suggest that you can safely set the mixture anywhere you please at 60% power, and 65% is probably "close enough." Remember, just because we teach the concept of the "red box" doesn't mean the engine will instantly explode if you get near it, and we cannot begin to state with assurance EXACTLY where the red box begins and ends. There's room for error here, and we're trying to be conservative in this. We DON'T want pilots to look at our "red box," and say too themselves, "Well, if 125 ROP is the edge of the 'red box,' I'll add a margin and use 150."

These are "fuzzy numbers," folks! You may be able to operate well within the "red box" in some cases (65%), but if you do, be aware you may not be treating your engine with quite the care it deserves.

So, what does "The Factory Way" (65%, 50 ROP) get you? At 50 ROP, we suggest you run at 60%, and certainly not more that 65%. We can say, then, that at 50 ROP, 65% is your LIMIT on power. At that setting, you will probably be burning up to 3 GPH more fuel than you need to. You will be running "dirty" enough to eventually foul your valve guides and encrust the tops of your pistons with deposits. Finally, you will be making enough carbon monoxide to be lethal if you develop an exhaust leak into the cabin.

Until recently, speaking in broad general terms, we operated the whole fleet that way, industry-wide. Have you ever looked at the "for sale" ads in Trade A Plane, and the engine numbers in them? How often have you seen "1,200 since new, 600 since top overhaul." If TBO is 1800 or so, WHY did they do that "top overhaul" at 600? Ever think about that? Now, how many of those ads that don't mention "top overhaul" are about engines where the mechanics/owners/operators just sorta forgot to report the removal of one or more jugs during that run, for valve work, or a damaged piston? How many of those "events" just get quietly forgotten when it comes time to make that required entry in the logs? Certainly, no one would EVER omit such work intentionally! I would be aghast at such dishonesty! Sure I would. Riiiiiight.

So, if most of the ads have that verbiage in one form or another, and there are significant numbers that have had the same problems without recording them, I can only leave it to the reader to make the judgment. Have we been operating these engines properly? I think not.

Do we really NEED to do more of this, to prove a point? No, that experiment has been ongoing since World War II, and the results have been dismal at best, even with "conforming" engines.

We have become so accustomed to "early top overhauls," or frequent jug work, that we forget these engines should make TBO and beyond, assuming they are "conforming" engines to start with, and they are operated in any reasonable manner. We overhaul a jug here, and another there, maybe one per annual (when the compression test is done), and we shrug it off as the normal cost of doing business. It isn't.

In spite of all that, we still think ROP operation is a viable part of any pilot's bag of tricks, provided it is rich enough, and used properly. Climbs are probably best done well ROP, for example. For normally aspirated engines cruising at altitudes above about 9,000 feet msl, 80 ROP will produce the most possible power, and 50 ROP will produce almost as much power, but on less fuel.

But for normally aspirated engines cruising below about 9,000 feet (and for turbos cruising at all altitudes), LOP operations, properly done, will give the needed power, will burn less fuel at the same power, will operate with lower peak combustion pressures and temperatures, and therefore run cooler (in all parts of the engine, valves included). They will run cleaner, making no deposits on the pistons or the valves, and without making measurable carbon monoxide, with less stress on the engine for any given power. It will remove that 60% or 65% limit, and allow normal (LOP) cruise at higher power settings, even up to 85%.

It seems intuitively obvious that all that will extend the service life of the engine, but we'll probably never see hard data for that. For everything else, there is hard data, already."

"Summary

Use full power on all takeoffs, with a "rich enough" mixture. FORCE your mechanic to set that fuel flow high enough to get roughly 1300 degrees EGT or a bit less, and CHTs in the low to middle 300s.

If you have that full-power mixture set properly, determine your "target" EGT right after takeoff, and lean in the climb to keep that same absolute value on the digital EGT. As you climb, that EGT will drop a little. Lean until it comes back up. Drops a little, lean and bring it back up.

For cruise, first determine your range needs, and set a power setting to maintain the AIRSPEED that will do the job you want. Set WOT, then the RPM and mixture you need to maintain that. LOP is much better, if it will do the job, but use ROP if needed. Cruise outside the red box, or at worst, on the fringes of it.

For descent, use mixture control and RPM to get the desired descent, switching to slightly rich of peak EGT, if you wish to keep the CHTs up. (Remember, 50 ROP on the EGT may be the same absolute value of EGT as 50 LOP, but the CHT will be much hotter when 50 ROP!)

None of this takes a lot of effort, once learned. Most of it will fry your CFI's mind, but it may do him some good in the long run, and force him to examine the issues. You may teach him something.

Finally, and very sadly, unless you have a truly enlightened check pilot or examiner, do it the old way when taking a check ride. It'll feel like you're abusing your engine, but once won't hurt a thing. You will never profit from doing "something different" with the FAA or a check pilot on board.

Be careful up there!"

Well, that is my summary, I hope I did him justice.

Hans
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Old 08-08-2011, 05:10 AM
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Default This thread needs a Sticky!

........just like the ones Paul Dye started on EFIS's.

Great work Hans
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Old 08-08-2011, 08:34 AM
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Allow a moment of devil's advocacy......

First, the scary "red box": The good shops run your brand new engine at full rated HP on a dyno, near sea level, well within the box (80-150 ROP). Why don't they detonate into oblivion?

LOP advocates claim longer top end life. What is the nature of the actual failures?
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Old 08-08-2011, 10:09 AM
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Good point Dan. To expand on that, it has been argued by some that it is not even possible to cause a naturally aspirated Lycoming to detonate. While Im sure that there are some extreme examples (a high compression engine on mogas, perhaps), the fact that our fleet is not falling out of the air on a regular basis has a lot more to do with engine design than our collective skill at engine management. Lets face it, were pilots if there is a way to screw something up, well find it! I dont know for sure, but perhaps detonation in most of our engines is like the boogey man mostly a figment of our imagination and the only thing we really need to worry about is CHT management?

For the record, does anyone have firsthand knowledge of a N/A Lycoming failing due to detonation?
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Old 08-08-2011, 12:09 PM
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Quote:
For the record, does anyone have firsthand knowledge of a N/A Lycoming failing due to detonation?
Several years ago, a fellow at my airport had a Glasair III equipped with an extremely high compression ratio IO-540 angle valve (K-series, I think) engine. He used to race it. I helped him change his cylinders out when he "downgraded" to lower compression, and most of the pistons had busted ring lands with chunks of aluminum falling out, holes melted all the way thru the sides of the piston in the grooves behind the compression rings, the compression rings came out in a bazillion fragments, the valves had been badly overheated but none of the piston tops had shown any signs of being melted. The engine was still running before it was top-overhauled, but I'd imagine a compression check would've been horrible.

The pistons reminded me of my second hot-rod car I had as a teenager... a 1970 Ford Torino Cobra with 429CJ engine (11.3:1 CR). After buying the car, I drove it for most of one summer, and it blew about a quart and a half of oil out the tailpipes per hour of driving. It would still turn mid-13 second quarter mile times in that condition. I pulled that engine apart and found 5 of the 8 pistons were busted up all along the compression ring lands, with a couple holes melted thru the top compression ring grooves in two of the pistons. All the compression rings were cracked into countless little segments, and looked like little oblong roller bearing pieces where they has been turning in what was left of the grooves. Obvious extensive damage due to detonation and/or pre-ignition. I rebuilt that engine, boring it .030 oversize and putting 11:1 pistons back in it. It ran much better, very scary strong, after the rebuild, and I still have that engine today... 3 decades later. Too bad it's far too extremely heavy to even think about some sort of aircraft use for it.
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Old 08-08-2011, 12:45 PM
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and to make a statement "a little roughness won't hurt your engine a bit" really?? 2000 hrs of roughness isn't going to hurt anything else? A lot more to an engine than just the cyl and pistons - bearings do not like vibrations
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Old 08-08-2011, 03:48 PM
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Quote:
Originally Posted by Neal@F14 View Post
...Several years ago, a fellow at my airport had a Glasair III equipped with an extremely high compression ratio IO-540 angle valve (K-series, I think) engine. He used to race it...
A big, high compression 6 in an airplane that has to be going just below the speed of sound to cool properly requires a high degree of attention. I can expect that something like this would see higher instances of trouble.

...But how about our "run of the mill" 8.5 CR engines? If you can keep the CHT in check, is it even possible to hurt it with the mixture? It's possible we are spending way too much time worrying about an event that is so rare in practical terms to be statistically insignificant.

I guess my point is, if it was possible to hurt an engine by mismanaging the power settings, 172s and their student pilots would be falling out of the skies left and right. The fact that they arent leads me to believe the risk is far less than most of us are willing to accept.
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