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Did any attend Mike Busch's "Leaning the Right Way" forum?

Dean Pichon

Well Known Member
I attended the Tuesday session and like all his talks it was very interesting and informative. Later that day, it occurred to me that throughout his talk, during which he described how most any aircraft can run LOP, he never once mentioned that it was necessary to have a "GAMI spread" of some value (or less).

I had attended another talk at a previous Airventure, at which I recall Mike describing the importance of establishing a flow balance between each cylinder. It seemed odd that no mention was made of this during this session on how to lean.

Can anyone shed any light on whether this may have been an oversight, or perhaps it is only nice to do, but not required?
 
I would assume not mentioning GAMI spread was just the way the discussion went - not some new insight.

On the RV-10 (stock Van?s IO-540-D4A5) the GAMI spread started out well over a gallon per hour on the first LOP test. The engine was simply not happy as reflected in it running very rough. It took just three runs to identify which injector nozzles needed to be replaced and by what value. Four nozzles swapped out ($26 each from Air Flow Performance) and GAMI spread is consistently 0.1 gph. LOP operations are now smooth, even further into LOP than is practical.

I consider tuning injector nozzles to get GAMI spread below 0.3 gph a hard requirement for any RV. Why abuse your engine when it is so easy to do this?

Carl
 
I was lucky. Did GAMI spread testing to see if I could improve over factory nozzles, but spread was right around 0.5 gph. Close enough for me.
 
I attended that forum & he did mention the GAMI spread.
There was a pilot with a IO-550 waiting to get his GAMI injectors and was concerned on the timing of his cylinders peaking. Mike said the whole idea with the spread is to manage the fuel flow & close the timing gap of the cylinder achieving their peak.
If I understood it correctly.
 
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I attended that forum & he did mention the GAMI spread.

Must be I missed that part! Two years ago, I attended another of his forums that was specifically on flow balancing. I went home and installed an engine analyzer and found my cylinders were reasonably well balanced (~0.2-0.3 gph depending upon altitude).

This morning I went for a flight and ran LOP for the first time. It seemed to work well enough and significantly reduced my fuel flow. I did, however, find that the number one cylinder was running much cooler than the hottest, so I may need to do some further balancing.

My issue now is that my cylinders are running too cool. This morning, they only reached ~280F when running LOP. Typically, they are about 320F. I will experiment with running a bit less lean.
 
My issue now is that my cylinders are running too cool. This morning, they only reached ~280F when running LOP. Typically, they are about 320F. I will experiment with running a bit less lean.

And what damage or problems do you expect might be caused by running at 280 vs 320 CHTs?

Larry
 
And what damage or problems do you expect might be caused by running at 280 vs 320 CHTs?

Larry

Good question. One of the Mike's leaning videos posted to YouTube indicated that at temperatures below 300F, the valves are more likely to accumulate deposits. I don't have much to go on, though.
 
49clipper

My cylinders run much cooler than 300f a lot. According to Lycoming, I am lucky and they said my cylinders should last til TBO. He said, call me when they get below 175f and we'll talk. Cool cylinders are a plus not a problem.
Jim
 
In his book, and on my Savvy reports, Mike says a .5 spread is the acceptable GAMI spread limit for smooth LOP operations.
 
My cylinders run much cooler than 300f a lot. According to Lycoming, I am lucky and they said my cylinders should last til TBO. He said, call me when they get below 175f and we'll talk. Cool cylinders are a plus not a problem.
Jim

This was my understanding as well. You want the heads as cool as possible. Water cooled engines are running comparable CHTs under 200* 180-200 is the sweet spot. Much less and you give up a bit of combustion efficiency. You do need enough combustion heat to keep lead deposits from forming. However, unti someone publishes correlated charts that show CHTs related to combustion temps and cooling airflow, I don't know how we use CHTs to ensure we are above the critical temps, beyond trial and error. My speculation is that at any meaningful RPM, no matter how far LOP, we are well above that temp. In our world, it is the low temps at the 1000 RPM area that do not create enough heat to convert the lead to lead bromide (this conversion eliminates the deposits).

Larry
 
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Strange Approach

The really strange thing about Mike's approach to leaning is that he does not advocate using EGT as a reference. That's counter to practically every other source I've read. And, most engine monitors that provide a Peak Lean function, do so based on EGT. Mike has you use CHT alone and in such a way that you never enter the peak cylinder internal pressure zone. His approach seems reasonable but seems somewhat less accurate to me than the EGT base method.
 
My cylinders run much cooler than 300f a lot. According to Lycoming, I am lucky and they said my cylinders should last til TBO. He said, call me when they get below 175f and we'll talk. Cool cylinders are a plus not a problem.
Jim

This is what my Lycoming contact told me also. He said cool is good for engines.

Chris
 
The really strange thing about Mike's approach to leaning is that he does not advocate using EGT as a reference. That's counter to practically every other source I've read. And, most engine monitors that provide a Peak Lean function, do so based on EGT. Mike has you use CHT alone and in such a way that you never enter the peak cylinder internal pressure zone. His approach seems reasonable but seems somewhat less accurate to me than the EGT base method.

Yeah, sorta. Except that Mike would agree that running LOP is a good thing, and you cant confirm whether you are lean of peak without EGT data.

erich
 
The really strange thing about Mike's approach to leaning is that he does not advocate using EGT as a reference. That's counter to practically every other source I've read. And, most engine monitors that provide a Peak Lean function, do so based on EGT. Mike has you use CHT alone and in such a way that you never enter the peak cylinder internal pressure zone. His approach seems reasonable but seems somewhat less accurate to me than the EGT base method.

EGT isn't really a reliable measure of much because the probes don't really see the exhaust gas most of the time. Plus, they are being alternately exposed to hot and cold gas many times per second. The time constant of a thermocouple just isn't capable of responding that fast. At best you are seeing an average of some set of conditions over many seconds (hundreds of cycles of the exhaust valve). I wouldn't really trust that an EGT sensor is doing much more than telling you "the combustion conditions in this particular cylinder may be running hotter (or cooler) than they were a few minutes ago." I definitely wouldn't try to compare EGTs even between cylinders on the same engine. The conditions in which they run and the geometry of mounting them differs too much.

I can't see how an EGT probe can be said to be "accurate" in anything other than a relative sense. Running Lean of Peak is just that...relative to a peak, but the peak is just an average and may change from time to time. It will change based on engine and ambient conditions.
 
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Yeah, sorta. Except that Mike would agree that running LOP is a good thing, and you cant confirm whether you are lean of peak without EGT data.

erich

Well, you clearly haven't read Mike's book. He doesn't look at EGT at all when leaning his engine. He relies solely on CHT, coming at it from either an economy a performance perspective. When performance is the goal, he leans to his CHT target (380 Continential, 400 Lycoming) or, if efficiency, he leans to his fuel flow goal, desired fuel reserve being the objective. Either way, he carefully avoids entering the peak CHT / EGT area that this necessary to establish a peak EGT for leaning with regard to that set point.
 
EGT isn't really a reliable measure of much because the probes don't really see the exhaust gas most of the time. Plus, they are being alternately exposed to hot and cold gas many times per second. The time constant of a thermocouple just isn't capable of responding that fast. At best you are seeing an average of some set of conditions over many seconds (hundreds of cycles of the exhaust valve). I wouldn't really trust that an EGT sensor is doing much more than telling you "the combustion conditions in this particular cylinder may be running hotter (or cooler) than they were a few minutes ago." I definitely wouldn't try to compare EGTs even between cylinders on the same engine. The conditions in which they run and the geometry of mounting them differs too much.

I can't see how an EGT probe can be said to be "accurate" in anything other than a relative sense. Running Lean of Peak is just that...relative to a peak, but the peak is just an average and may change from time to time. It will change based on engine and ambient conditions.

I don't understand your statement at all. An EGT probe obviously doesn't display the instantaneous EGT but it should display a very accurate average which is derived over a fairly short period of time. A "few minutes" certainly isn't my experience with the reaction time of my EGT probes. They respond within a few seconds of my changing engine parameters.

I do agree that running LOP is relative to a peak. I just think it is relative to a very accurate peak derived from the very near past, not minutes, but seconds.

My point is that Mike Busch advocates avoiding every seeing LOP CHT or EGT with his method and many others advocate finding peak EGT as a necessary step in leading your engine to whatever performance point your desire.
 
When performance is the goal, he leans to his CHT target (380 Continential, 400 Lycoming) or, if efficiency, he leans to his fuel flow goal, desired fuel reserve being the objective. Either way, he carefully avoids entering the peak CHT / EGT area that this necessary to establish a peak EGT for leaning with regard to that set point.

I would think peak fuel efficiencies would be obtained LOP, and I dont see how LOP can be attained without ever passing through peak EGT, although that could certainly be done relatively rapidly.

erich
 
I would think peak fuel efficiencies would be obtained LOP, and I dont see how LOP can be attained without ever passing through peak EGT, although that could certainly be done relatively rapidly.

erich

You're right about that. Using EGTs and logging fuel flow when each cylinder peaks is necessary for nozzle tuning but not for every day flying. The big pull to an expected fuel flow takes you right through the peak and the dreaded red box and gets you to the LOP condition too quickly for any detonation problems to have developed. Once on the LOP side of things, you can fine tune for whatever cruise power/fuel flow you want with CHTs being primary. Don't care what EGTs read anymore. If the cylinders are not over 380-400 and rising, a dangerous level of detonation is not occurring. Maximum available LOP cruise power will be at your personal maximum continuous CHT on the hottest cylinder. If you want to go faster, set it to max CHT on the rich side of peak. My personal max continuous CHT is 380F.

Ed Holyoke
 
engine efficiency is all about air fuel ratio (leaving CR, valve timing and ignition timing to the side for this discussion) Peak EGT is almost directly related to stochiometric (14.7:1 AFR) as well as being quite fast at identifying this. It is therefore a very good tool for working to a baseline AFR and operating at some point from or at that baseline. This would be quite difficult to with CHT, as it reacts very slowly to combustion temperature and is also impacted by ambient temps and cooling airflow volume. I am not even sure where combustion temps peak, though I suspect it is also at stochiometric. I am sure most have seen how CHTs can be higher or lower, based upon ambients or airspeed, at the same power setting (EGT sees very little variability from ambient or airflow). Therefore a peak must be found individually for every circumstance and this would take a long time, due to the slow reaction.

Larry
 
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My point is that Mike Busch advocates avoiding every seeing LOP CHT or EGT with his method and many others advocate finding peak EGT as a necessary step in leading your engine to whatever performance point your desire.

Obviously he does pass through peak EGT, as you can't get from rich to lean and back any other way. He just doesn't look for it. I suspect he knows exactly where a particular fuel flow puts him in relation to peak.

Consider Busch is flying a twin, so setting mixture by peak EGT would be 12 probes to track. Within caveats, he can do the same with matched FF needles at some familiar power setting

Reference to peak is an accurate indicator. The need for accuracy is high at high power settings. Accuracy is not necessary at low power settings, which is where he operates.
 
You're right about that. Using EGTs and logging fuel flow when each cylinder peaks is necessary for nozzle tuning but not for every day flying. The big pull to an expected fuel flow takes you right through the peak and the dreaded red box and gets you to the LOP condition too quickly for any detonation problems to have developed. Once on the LOP side of things, you can fine tune for whatever cruise power/fuel flow you want with CHTs being primary. Don't care what EGTs read anymore. If the cylinders are not over 380-400 and rising, a dangerous level of detonation is not occurring. Maximum available LOP cruise power will be at your personal maximum continuous CHT on the hottest cylinder. If you want to go faster, set it to max CHT on the rich side of peak. My personal max continuous CHT is 380F.

Ed Holyoke

Ed has perfectly summarized the Mike Busch approach to LOP operations and my personal technique. Big Pull to far below peak EGT/CHT then richen to desired fuel flow limited by maximum desired CHT. No need to ever reach or establish peak EGT.

One big caveat though, If you've read Bill Ross's book on Engine Management, he'll tell you that if you are operating below Maximum Recommended Cruise Power (MRC), it just plain doesn't matter what you do with the red knob. You can't hurt your engine below MRC. Bill defines MRC as , ?the power setting at or below, which full leaning authority is safe, allowable and engines are tested for durability.? This maximum setting can be found in your POH or engine operations manual. He takes care to note that MRC is not Maximum Continuous Power (MCP), which is the power setting the engine can run 24/7 in a full rich condition.

So, if you are operating at or below MRC, feel free to feel for that EGT peak using your engine monitor. You can't harm your engine. If you are operating above that performance level though, Bill Ross and Mike Busch would agree that you should avoid the Red Box, Red Fin, Zone of Maximum Cylinder Internal Pressure, etc. Knowing where MRC is in your experimental engine / experimental installation, is the trick. For me, since I don't know precisely where MRC is in my airplane, I'll just use Mike Busch's technique and avoid the danger zone altogether.
 
Ed has perfectly summarized the Mike Busch approach to LOP operations and my personal technique. Big Pull to far below peak EGT/CHT then richen to desired fuel flow limited by maximum desired CHT. No need to ever reach or establish peak EGT.

One big caveat though, If you've read Bill Ross's book on Engine Management, he'll tell you that if you are operating below Maximum Recommended Cruise Power (MRC), it just plain doesn't matter what you do with the red knob. You can't hurt your engine below MRC. Bill defines MRC as , ?the power setting at or below, which full leaning authority is safe, allowable and engines are tested for durability.? This maximum setting can be found in your POH or engine operations manual. He takes care to note that MRC is not Maximum Continuous Power (MCP), which is the power setting the engine can run 24/7 in a full rich condition.

So, if you are operating at or below MRC, feel free to feel for that EGT peak using your engine monitor. You can't harm your engine. If you are operating above that performance level though, Bill Ross and Mike Busch would agree that you should avoid the Red Box, Red Fin, Zone of Maximum Cylinder Internal Pressure, etc. Knowing where MRC is in your experimental engine / experimental installation, is the trick. For me, since I don't know precisely where MRC is in my airplane, I'll just use Mike Busch's technique and avoid the danger zone altogether.

This might apply with stock compression ratios, fixed mag timing and 100LL. Certainly a blanket statement like this is not accurate if you're running higher CRs, mogas and EI with more advance than a mag. All that original validation goes out the window with these common changes in the Experimental world.
 
This might apply with stock compression ratios, fixed mag timing and 100LL. Certainly a blanket statement like this is not accurate if you're running higher CRs, mogas and EI with more advance than a mag. All that original validation goes out the window with these common changes in the Experimental world.

Well, what would be your advice for those of us running higher compression ratios and electronic mags Ross? I'm all ears, seriously!
 
Well, what would be your advice for those of us running higher compression ratios and electronic mags Ross? I'm all ears, seriously!

Too many variables to give one answer here but blanket statements like this are irresponsible in my view. I'd prefer to give advice on an individual basis.

If these folks are commenting about certified configurations running on 100LL, possibly valid.

It's unlikely that 35-40 degrees of timing (common on some EIs) combined with 9.5 or 10 to 1 pistons and 87 or 91 octane mogas is safe with hot heads and high IATs even at medium cruise rpms and say 25 inches MAP which is less than 75% power.
 
CHT is CHT

This might apply with stock compression ratios, fixed mag timing and 100LL. Certainly a blanket statement like this is not accurate if you're running higher CRs, mogas and EI with more advance than a mag. All that original validation goes out the window with these common changes in the Experimental world.

I'm running 10:1, dual electronic ignitions, and 100LL. Haven't tried mogas - probably won't. What I said before works for my engine. No runaway CHTs - no worries. My red box is probably a bit bigger than with a stock engine and that means that with high manifold pressure, I have to stay rich of peak to keep it cool. With a bit more nozzle tweaking I think I'll be able to get LOP far enough to run it that way down low even with high MAP. Right now, it runs rough when I try. Remember, LOP means less power which means lower CHTs. All that said, I can run about 27", 2300 RPM and about 10.5 GPH at 3K' and make about 175kt true with 90* OAT. Contrary to what they teach, my engine runs cooler oversquare. Lower RPM = lower power = lower heat production and lower prop drag. If I find it running hotter than I like, I reduce MAP and/or go further rich (or further lean if it will run smooth). If I'm willing to accept ~160kt TAS, I can do that at 3K' at about 8.5 GPH LOP at lower MAP.

Ed Holyoke
 
No runaway CHTs - no worries.

Ed Holyoke

You should do some research before being comforted by that statement. detonation has many levels of severity and many don't create "runaway CHT's" but can do a lot of damage over an extended time.

Larry
 
I don't understand your statement at all. An EGT probe obviously doesn't display the instantaneous EGT but it should display a very accurate average which is derived over a fairly short period of time. A "few minutes" certainly isn't my experience with the reaction time of my EGT probes. They respond within a few seconds of my changing engine parameters.

I did edit my reply to correct "minutes" to "seconds" but you may have been writing your reply before I did so.

My point is that while the thermocouple might give a useful piece of data, it is not an "accurate" measure of anything other than its own average temperature. It bears a very loose relationship to anything going on in the cylinder itself.

However, that data can be very useful for adjusting engine operation and for diagnosing problems. Watching a trend of the values can expose problems such as developing valve damage, as Busch shows in his new book.

It wouldn't make much sense to compare EGTs between cylinders, however, because of the major differences in what each thermocouple will see from jug to jug.
 
All that said, I can run about 27", 2300 RPM and about 10.5 GPH at 3K' and make about 175kt true with 90* OAT. Contrary to what they teach, my engine runs cooler oversquare. Lower RPM = lower power = lower heat production and lower prop drag. If I find it running hotter than I like, I reduce MAP and/or go further rich (or further lean if it will run smooth). If I'm willing to accept ~160kt TAS, I can do that at 3K' at about 8.5 GPH LOP at lower MAP.

Ed, I too would suggest caution about running 27" and 2300. Not sayin' you can't, but it's the bleeding edge of what Lycoming allows for stock compression.

Less RPM = less CHT is not contrary to teaching, not if you go back to the old NACA papers. They expressed it as mass flow, but that's the same as RPM.

8.5 GPH for 160 KTAS is not very good. Climb to a more efficient altitude, set WOT and maybe 2400 RPM, then record speeds at three mixture settings, 100 ROP, peak EGT, and perhaps 25 LOP.

25~30 LOP, IO-390, 2014, loaded with camping gear and a few cases of Cow:

LOP%20From%20OSH14%20with%20notes.jpg


Peak EGT will net about 181 KTAS, which illustrates how fast power falls off on the lean side. 100 ROP will 185, but fuel burn will be near 11 GPH, which illustrates how fast fuel burn rises running ROP, for not a lot more speed.

Most recent cruise experiment, again an OSH return. Popped up high to get the tailwind. MPG was 25.7 nautical:

Lean%20Cruise%2023%20deg%20timing%20OSH%2008A%202018%20800w.jpg


Note higher temperatures, which I attribute to reduced air density. See the Lycoming cooling air charts. So much for leaning to CHT.

Also note higher AOA compared to photo at 11.5K. The stubby wing seems to be a bit out of its sweet spot. All around, probably not the best altitude for this RV-8.
Where's that RV-9 wing when I need it? ;)
 
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I attended the "Leaning the right Way" this year but I have also watched the webinar on youtube, https://www.youtube.com/watch?v=X-tKyiUZ3ts before. I also experimented a lot on the way home from Oshkosh. I have RV7, IO 360, Bendix Mags, burning 100LL. Experimenting at 8500' cruise, I lean with a fast pull until my engine looses considerable power to get on the LOP side, then moving slowly rich to find peak EGT's on my Dynon engine monitor. All my cylinders peak fairly close. When cruising at 8500', I have always ran full throttle and about 50 degrees LOP because I had read this gives the best fuel economy. This configuration gives me about 157 Kts per hr TAS at about 8.2 GPM. But, I have discovered, if I lean to about 15-20 degrees LOP, then pull the throttle back to establish a fuel flow of 7.9 GPH, I get 160 Kts per hr and my CHTs are the same running about 320-350. It seems, at any altitude, my engine is happy and getting best economy when I reduce my throttle to 65% power, then lean to 15-20 LOP....I wonder if this is normal? By the way,my engine does not run noticeably rough at 50 LOP.
 
my engine is happy and getting best economy when I reduce my throttle to 65% power, then lean to 15-20 LOP....I wonder if this is normal? By the way,my engine does not run noticeably rough at 50 LOP.

That matches my expectations and cruise SOP. My research tells me that max MPG will occur 10-20* LOP and seems to be validated in the testing on my 6A. While WOT can reduce pumping loss, it does not offset the efficiency of running at 10-20 LOP in my experience. The CS guys can tweak MAP to allow WOT at most any point LOP. Us FP guys need to throttle back to get into the efficient range.

Larry
 
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With a fixed pitch prop, excluding prop efficiency factors, max fuel economy will be primarily a composite of L/D ratio vs. fuel burn which is itself, governed by many engine factors. Speed and altitude come into play with the L/D ratio. Thinner air equals less drag up to a point where L/D starts to degrade with too high alpha.

C/S props give you many more options, allowing more efficient engine operation over a wider range of altitudes and speeds.

When it comes to the engine, it is most efficient at WOT and relatively low rpms. WOT reduces pumping losses, low rpm (below torque peak rpm somewhat) reduces frictional losses. Volumetrically, the engine is most efficient at torque peak.

Theoretically the engine produces the best BSFC well lean of peak as shown below:



I've posted this graph before and it shows best BSFC occurs around 18 to 1 (.055 here) AFR. Peak EGT on avgas is around 15.2 AFR by comparison (.065 here). Best power is around 13 to 1 AFR (.077 on the graph).

So from this graph we'd lose around 8% hp at best economy mixture but drop fuel flow 25%.

We see some incredible MPG figures from Dave Anders flying high (17.5 is typical) at WOT and very low rpms, at peak or LOP.

If someone is flying with a wideband AFR meter, it would be interesting to equate AFR with EGT degrees ROP and LOP and do a flight comparison to see how the theory compares. It's important to keep in mind that EGT is affected by ignition timing as well so it would be necessary to keep that set the same on EIs when doing this testing.
 
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Howdy Guys,

For sure I can do better at higher altitudes. Drag falls off faster than power. It is also difficult to make it detonate at high altitudes. When I speak of runaway CHTs, I'm talking about blowing right through 400* on it's way to oblivion. If I'm running at 360*-380* and stable, I really doubt that it is detonating. Lycoming's power setting charts are based on "best power" - just ROP - right where the red box is the largest. Yup, 27" and 2300 RPM at 30* ROP is not a good place to be. That's why I don't do that.

Detonation is promoted by high pressure and high CHTs and causes CHTs to rise even more by disrupting the thermal boundary layer next to the cylinder, head, and piston. With the boundary layer disrupted, localized heating will happen and that's where damage can occur. This extra heat will be seen in rising CHT, (and possibly oil temp). If it is bad enough, the CHT will rise very rapidly, the detonation will get a lot worse, and stuff can get hot enough to glow causing pre-ignition. Not good. Either running very ROP or even slightly LOP increases detonation margin by slowing the combustion event so that peak pressure occurs well after TDC, about 17*. Advanced timing at high MAP would cause the peak pressure to occur earlier and would promote detonation. Most EIs don't begin to advance until MAP falls off significantly and so should not be a problem for high power ops.

So - even with high compression, I have several ways to prevent detonation at fairly high manifold pressure. I don't have a detonation sensor, none of us Lycloning operators do, so I use CHT trends to track what's going on. At take off and full rich I see about 17.5 GPH and the CHTs climb smoothly to 325* or so. I can lean it to around 12.5 GPH at full throttle remaining ROP and the CHTs will climb to about 360* or so. So far so good. Fairly quickly pulling more mixture, right past peak EGTs will put me at 9 GPH or so and the CHTs will be about the same and not rising. The catch is that my nozzles aren't yet balanced quite well enough to smoothly get far enough LOP at WOT and 3K' where I often do some cross town commuting, so I have to use other tricks - stay ROP, cowl flaps partially open and/or reducing MAP as necessary for CHT control. What we really want to avoid is a combination of high CHTs and high MAP. And I really don't want to see a very rapid increase in CHT as that would be an indication that the combustion event is proceeding more rapidly than normal - probably detonation. Another trick is reducing RPM. Though that will decrease detonation margin by decreasing the amount of time for combustion, it also decreases power produced which tends to lower CHTs. Either way rich or LOP, never right near peak at high MAP.

Ed Holyoke
 
When I speak of runaway CHTs, I'm talking about blowing right through 400* on it's way to oblivion. If I'm running at 360*-380* and stable, I really doubt that it is detonating.

"Stable" being the key.

Lycoming's power setting charts are based on "best power" - just ROP- right where the red box is the largest. Yup, 27" and 2300 RPM at 30* ROP is not a good place to be. That's why I don't do that.

Reference please. I would place best power mixture somewhat further ROP than 30, although yes, in detonation surveys (all parameters quite hot), max power is usually just short of detonation.

Either running very ROP or even slightly LOP increases detonation margin by slowing the combustion event so that peak pressure occurs well after TDC, about 17*.

And there is where the fun starts. Much of the GAMI/APS and Busch teaching is based on a goal of longevity, the key being avoidance of very high peak cylinder pressure. Out here in the field, none of us have the equipment to measure ICP in real time, so we're left to guess, or derive it from CHT, or as a proxy, oil temperature. When I see temperature rise without a corresponding increase in output (usually speed increase, but one might argue for high MPG), I back off to a more conservative mixture and/or timing, as I'm cautious about the mechanical stress.

Nigel's timing survey (published in Kitplanes) is a good example. Two major conclusions were that (1) advanced timing is always accompanied by higher CHT, the result of higher ICP, and (2), the same speed or efficiency increase can be found with less advance by climbing to a little higher altitude, without the mechanical stress.
 
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Just throwing a number at it. Could just as easily said 50* or 70*. I certainly don't set it up that way to measure it. ;-) Yup, stable. If the CHT is rising and doesn't show any signs of stopping by 380, I do something - richen, lean, reduce MAP, nose over for more air, cowl flaps. I agree, extra heat without extra airspeed is more stress for no good reason.

Ed Holyoke

"Stable" being the key.



Reference please. I would place best power mixture somewhat further ROP than 30, although yes, in detonation surveys (all parameters quite hot), max power is usually just short of detonation.



And there is where the fun starts. Much of the GAMI/APS and Busch teaching is based on a goal of longevity, the key being avoidance of very high peak cylinder pressure. Out here in the field, none of us have the equipment to measure ICP in real time, so we're left to guess, or derive it from CHT, or as a proxy, oil temperature. When I see temperature rise without a corresponding increase in output (usually speed increase, but one might argue for high MPG), I back off to a more conservative mixture and/or timing, as I'm cautious about the mechanical stress.

Nigel's timing survey (published in Kitplanes) is a good example. Two major conclusions were that (1) advanced timing is always accompanied by higher CHT, the result of higher ICP, and (2), the same speed or efficiency increase can be found with less advance by climbing to a little higher altitude, without the mechanical stress.
 
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