Detonation - cylinder destroyed

[Fly Boomer]

1 hour ago, A64Pilot said:

uncontrolled and or pre-ignition is detonation.

This only touches on the pre-ignition/detonation issue obliquely, but is interesting.  John Deakin isn't big on being politically correct.  This was written more than 20 years ago as a rebuttal to an article from Lycoming asserting that nobody could possibly know more than they.  Clearly, we had less engine instrumentation in our airplanes 20 years ago, and Lycoming has slowly come around to LOP, but the APS and GAMI guys had arguably the best instrumented test stand in the world.  John Deakin had something like 39,000 hours of 747 time in his logbook.  And yes, I understand that 747s are powered by jets.  Point is, he had been around for a long time, and knew some stuff.  That said, George Braly (also GAMI/APS) is the engineer.

Experts Are Everywhere - The Rebuttal sm.pdf

[Pinecone]

You can have pre-ignition without detonation. You can have detonation without pre-ignition.

But you can have pre-ignition that leads to detonation.

 

[Fly Boomer]

2 hours ago, Pinecone said:

You can have pre-ignition without detonation. You can have detonation without pre-ignition.

But you can have pre-ignition that leads to detonation.

Hmm...  I'm no expert, but I recommend this article from John Deakin's Pelican Perch series.

043 - Detonation Myths sm.pdf

[A64Pilot]

2 hours ago, Pinecone said:

You can have pre-ignition without detonation. You can have detonation without pre-ignition.

But you can have pre-ignition that leads to detonation.

 

Usually unless there is some kind of existing defect, detonation comes first. detonation supplies the heat necessary for pre-ignition, you could almost say pre-ignition is most often advanced detonation. Normal operating engine doesn’t have something that can essentially function as a glow plug, busted plug is possible I guess as is maybe a piece of helicoil hanging down into the combustion chamber, but neither of those are normal, and the classic detonation damage is destroyed spark plugs, so finding broken insulator isn’t the Aha moment it could be.

Classic cause of pre-ignition in racing motors is wrong heat range spark plug, that can function as a glow plug, but that shouldn’t happen in an airplane.

So yes it possible to get pre-ignition without detonation, but it shouldn’t happen that way in a properly maintained aircraft engine. However a pilot can induce detonation in a perfect engine, with perfect fuel etc. All it takes is to be too lean at too high a power, of arguably not lean enough. I say properly maintained as a piece of helicoil hanging down isn’t proper.

Edited May 23, 2022 by A64Pilot

[jaylw314]

2 hours ago, Pinecone said:

You can have pre-ignition without detonation. You can have detonation without pre-ignition.

But you can have pre-ignition that leads to detonation.

You can also have detonation that leads to pre-ignition  

It's probably easiest just to say that detonation and pre-ignition are "interdependent"

[Pinecone]

13 hours ago, jaylw314 said:

You can also have detonation that leads to pre-ignition  

It's probably easiest just to say that detonation and pre-ignition are "interdependent"

MAY be interdependent.

Pre-ignition does create much higher cylinder pressures, that can lead to detonation.  But not always.

Pre-ignition can be caused by a hot spot (the afore mentioned helicoil tab).

Detonation can definitely occur without pre-ignition.

[A64Pilot]

2 hours ago, Pinecone said:

 

Detonation can definitely occur without pre-ignition.

It’s my unsubstantiated belief that detonation is about the only way a properly maintained aircraft can get into pre-ignition, and that likely if you drill down into it, that it’s likely that pre-ignition does most of the damage.

There are still a lot of mechanics that believe fine wire plugs can cause pre-ignition, I think maybe if the fine wire is heated from detonation first.

[PT20J]

Detonation and preignition are just definitions.

Detonation is the spontaneous uncontrolled combustion of the end gas (the unburned part of the air-fuel mixture in the combustion chamber ahead of the flame front) due to excessive temperature and pressure.

Preignition is simply the ignition of the air-fuel mixture before the spark occurs.

Detonation can be caused by fuel contamination (lowers octane rating), high power at lean mixture (increases combustion chamber temperature), overly advanced ignition timing (increases peak pressure and temperature).

Preignition requires some ignition source other than spark. The spark plug is the hottest cylinder component which is why thermocouple CHT probes placed here often read high. The spark plug cools through its insulator. A damaged insulator can cause the spark plug to overheat and become an ignition source. That’s why you are not supposed to reuse spark plugs if you drop them — there is a possibility that the insulator has cracked and the plug will run hot. Improperly installed spark plug helicoil inserts (or ones that have been damaged perhaps with a thread chaser or by overtorque) can protrude into the combustion chamber and become an ignition source.

Preignition has a similar effect as advanced ignition timing and can cause destructive detonation. If detonation damages a spark plug, it could lead to preignition, but by that point, the damage has already been done.

Skip

[Shadrach]

4 hours ago, A64Pilot said:

It’s my unsubstantiated belief that detonation is about the only way a properly maintained aircraft can get into pre-ignition, and that likely if you drill down into it, that it’s likely that pre-ignition does most of the damage.

There are still a lot of mechanics that believe fine wire plugs can cause pre-ignition, I think maybe if the fine wire is heated from detonation first.

Perhaps with a turbocharged engine that’s the case. The detonation margins built into FAA certified aircraft engines make it almost impossible for a pilot to induce detonation in a normally aspirated engine, no matter how ham-fisted. It’s certainly possible to run them hot by modern standards, but inducing detonation in something like a normally aspirated Lycoming four cylinder would require deliberately selecting the most abusive power and flight scenarios for such a period as to allow a significant  amount of heat to build up before detonation would become a factor. Something like leaned to max power at low altitude while in slow flight (behind the power curve).

The certification margins depicted in the attachment are the minimum for FAA certification. We have no idea how far over the conservative side of the line our engines are. It’s why I say that stressing about “big mixture pulls” and the like is wasted worry… it also clearly debunks the notion that it’s more dangerous to operate on the lean side of peak vs rich. 

 

[A64Pilot]

Since your a Gami guy, that’s not what they say, according to them one engine operates detonating very often and detonation is relatively easy to get into, and almost never causes any real damage. But then I don’t believe aluminum or aluminum pistons melt at less than 1000F either. 

I disagree of course, it’s my opinion that detonation is or can easily become very destructive, heck don’t we have two threads running right now about detonated to death cylinders?

I also agree that’s it’s difficult to get into detonation, but all it takes really is someone to go-around and climb while still leaned out, the slow speed, possibly closed cowl flaps and lean mix can do it.

But at normal cruising altitudes and airspeeds, I agree an average N/A engine isn’t likely to detonate.

 

[carusoam]

Jungle’s graphs 

Do show an interesting loss of control of CHT….

But,

apparently I ran out of memory and don’t have the option to post the pic…

Bummer,

-a-

[PT20J]

On 5/24/2022 at 9:01 AM, Shadrach said:

Perhaps with a turbocharged engine that’s the case. The detonation margins built into FAA certified aircraft engines make it almost impossible for a pilot to induce detonation in a normally aspirated engine, no matter how ham-fisted. It’s certainly possible to run them hot by modern standards, but inducing detonation in something like a normally aspirated Lycoming four cylinder would require deliberately selecting the most abusive power and flight scenarios for such a period as to allow a significant  amount of heat to build up before detonation would become a factor. Something like leaned to max power at low altitude while in slow flight (behind the power curve).

The certification margins depicted in the attachment are the minimum for FAA certification. We have no idea how far over the conservative side of the line our engines are. It’s why I say that stressing about “big mixture pulls” and the like is wasted worry… it also clearly debunks the notion that it’s more dangerous to operate on the lean side of peak vs rich. 

 

That graph made me curious, so I did some searching into what the FAA actually requires. The regs (both FAR 33 and the older CAR 13) both just say that a test must be performed to show that the engine is free from detonation over its intended operating conditions.

AC 33.47-1 provides non-regulatory guidance. Basically it recommends doing mixture sweeps at various power settings and with maximum inlet air, oil, and CHT temps and setting the approved mixture 12% richer than that that produces detonation. Since Lycoming generally allows leaning at power settings below 75%, and Continental below 65%, this would imply that there is at least a 12% margin at those power settings. In the real world, some engines are probably much better and some are probably closer to the limit.

Skip

AC_33_47-1.pdf

[A64Pilot]

No AC is regulatory, but try Certifying something without complying with them. Cert wise most everything is up for discussion, some things you can get them to give on, but usually only if you give them something that they want, but can’t make you do. It’s way more of a tit for tat thing than I expected, way more.

AC’s drove our flight test DER bat crap crazy, he would tell them straight to their faces if you want me to do something, put it in the CFR’s then I will, he didn’t win all the arguments but won a surprising amount. 

[PT20J]

There are some interesting threads on BeechTalk including George Braly comments:

https://www.beechtalk.com/forums/viewtopic.php?f=37&t=186351

https://www.beechtalk.com/forums/viewtopic.php?f=37&t=113931&start=0

https://www.beechtalk.com/forums/viewtopic.php?p=1423152

https://www.beechtalk.com/forums/viewtopic.php?f=37&t=43455

https://www.beechtalk.com/forums/viewtopic.php?f=37&t=137121

[T. Peterson]

On 5/23/2022 at 2:04 PM, A64Pilot said:

Usually unless there is some kind of existing defect, detonation comes first. detonation supplies the heat necessary for pre-ignition, you could almost say pre-ignition is most often advanced detonation. Normal operating engine doesn’t have something that can essentially function as a glow plug, busted plug is possible I guess as is maybe a piece of helicoil hanging down into the combustion chamber, but neither of those are normal, and the classic detonation damage is destroyed spark plugs, so finding broken insulator isn’t the Aha moment it could be.

Classic cause of pre-ignition in racing motors is wrong heat range spark plug, that can function as a glow plug, but that shouldn’t happen in an airplane.

So yes it possible to get pre-ignition without detonation, but it shouldn’t happen that way in a properly maintained aircraft engine. However a pilot can induce detonation in a perfect engine, with perfect fuel etc. All it takes is to be too lean at too high a power, of arguably not lean enough. I say properly maintained as a piece of helicoil hanging down isn’t proper.

I very much enjoy your posts and highly respect your credentials. What you say about high power and lean mixture potentially harming the engine makes sense to me. If my very unsophisticated understanding is correct, the situation arises because adding more MP by pushing in the far left knob admitting  more gas/air mixture (determined by the far right knob) can cause detonation if that mixture has too much air and not enough gas. This is because at high power more of the wet stuff (gas) is required as part of mixture to ensure proper cooling. If I have understood this correctly, would my engine monitor not warn me I was getting close to the bad place? Should I not see high cylinder, EGT or TIT temps before I damage the engine?

The reason I ask is because I don’t use the LF feature of my EDM 700 anymore. When I first got the airplane I did do so but after a few times it seemed redundant and predictable. I would end up at 12.5 gallons an hour and then would readjust from there anyway. 

Now I just get to cruise altitude ( 10 to 12 thousand feet), set 2300-2350 rpm, MP 28-30, and roll the mixture back to 11 gallons an hour. I carefully monitor all temps. If TIT exceeds 1580, or my hottest cylinder (#3) exceeds 380, I either decrease MP or enrich just a tad depending if I am in the mood to go faster or save fuel. Sometimes I can even roll back to 10.5 and keep all temps happy. Other times it may take 11.5. I assume these differences are due to varied atmospheric conditions.

Am I okay doing it this way, or did I just demonstrate my colossal ignorance to everyone on MS as well as using a procedure that could damage my engine!?!?

Torrey

P.S. I don’t have GAMI injectors.

[T. Peterson]

4 minutes ago, T. Peterson said:

I very much enjoy your posts and highly respect your credentials. What you say about high power and lean mixture potentially harming the engine makes sense to me. If my very unsophisticated understanding is correct, the situation arises because adding more MP by pushing in the far left knob admitting  more gas/air mixture (determined by the far right knob) can cause detonation if that mixture has too much air and not enough gas. This is because at high power more of the wet stuff (gas) is required as part of mixture to ensure proper cooling. If I have understood this correctly, would my engine monitor not warn me I was getting close to the bad place? Should I not see high cylinder, EGT or TIT temps before I damage the engine?

The reason I ask is because I don’t use the LF feature of my EDM 700 anymore. When I first got the airplane I did do so but after a few times it seemed redundant and predictable. I would end up at 12.5 gallons an hour and then would readjust from there anyway. 

Now I just get to cruise altitude ( 10 to 12 thousand feet), set 2300-2350 rpm, MP 28-30, and roll the mixture back to 11 gallons an hour. I carefully monitor all temps. If TIT exceeds 1580, or my hottest cylinder (#3) exceeds 380, I either decrease MP or enrich just a tad depending if I am in the mood to go faster or save fuel. Sometimes I can even roll back to 10.5 and keep all temps happy. Other times it may take 11.5. I assume these differences are due to varied atmospheric conditions.

Am I okay doing it this way, or did I just demonstrate my colossal ignorance to everyone on MS as well as using a procedure that could damage my engine!?!?

Torrey

P.S. I don’t have GAMI injectors.

I should have mentioned that I have a 1979 231 with a Merlyn wastegate, but no intercooler.

[Shadrach]

6 hours ago, T. Peterson said:

I very much enjoy your posts and highly respect your credentials. What you say about high power and lean mixture potentially harming the engine makes sense to me. If my very unsophisticated understanding is correct, the situation arises because adding more MP by pushing in the far left knob admitting  more gas/air mixture (determined by the far right knob) can cause detonation if that mixture has too much air and not enough gas. This is because at high power more of the wet stuff (gas) is required as part of mixture to ensure proper cooling. If I have understood this correctly, would my engine monitor not warn me I was getting close to the bad place? Should I not see high cylinder, EGT or TIT temps before I damage the engine?

The reason I ask is because I don’t use the LF feature of my EDM 700 anymore. When I first got the airplane I did do so but after a few times it seemed redundant and predictable. I would end up at 12.5 gallons an hour and then would readjust from there anyway. 

Now I just get to cruise altitude ( 10 to 12 thousand feet), set 2300-2350 rpm, MP 28-30, and roll the mixture back to 11 gallons an hour. I carefully monitor all temps. If TIT exceeds 1580, or my hottest cylinder (#3) exceeds 380, I either decrease MP or enrich just a tad depending if I am in the mood to go faster or save fuel. Sometimes I can even roll back to 10.5 and keep all temps happy. Other times it may take 11.5. I assume these differences are due to varied atmospheric conditions.

Am I okay doing it this way, or did I just demonstrate my colossal ignorance to everyone on MS as well as using a procedure that could damage my engine!?!?

Torrey

P.S. I don’t have GAMI injectors.

I’m betting that you’ve been flying a while. I too learned that surplus fuel is “required” to ensure proper “cooling”. I remember a pilot in the 80s describing how the evaporation of surplus fuel cooled the cylinders. Seems intuitive. Mixture rich, EGTs fall, CHTs fall. However, that’s not really what’s happening. Surplus fuel does not really cool anything (it does have a slight cooling affect on exhaust gas temp). Surplus fuel simply slows the speed of the combustion event by forcing the flame front to go around non combustible fuel molecules that are not paired with O2 molecules (at ROP mixtures, there simply are not enough O2 molecules for all of the fuel). We therefor manipulate how fast the flame front propagates with mixture. This allows us to move the point of peak cylinder pressure closer to or further from TDC. Closer to TDC = more pressure= more heat and typically more power (whole ‘nother discussion there). Further from TDC = less pressure = less heat.  
 

A well conforming engine can use a surplus of air instead of fuel to control the combustion event. The up side is that air is free, the down side is that the surplus is limited by ambient pressure…unless we have a turbo! Then we are limited by MP restrictions and TIT. This is why turbo normalized engine operators rarely use ROP settings other than full rich, max power climb. As I posted earlier, these are the book Power settings for the 2013 SR22T. Note cruise power settings up to 85%. That’s 268hp on 18.3gph. No way to do that ROP. Cirrus does not even provide ROP power setting tables…and why would they? It would be a dirty, hot, inefficient and bone headed way to run that engine.

 

 

[A64Pilot]

6 hours ago, T. Peterson said:

I very much enjoy your posts and highly respect your credentials. What you say about high power and lean mixture potentially harming the engine makes sense to me. If my very unsophisticated understanding is correct, the situation arises because adding more MP by pushing in the far left knob admitting  more gas/air mixture (determined by the far right knob) can cause detonation if that mixture has too much air and not enough gas. This is because at high power more of the wet stuff (gas) is required as part of mixture to ensure proper cooling. If I have understood this correctly, would my engine monitor not warn me I was getting close to the bad place? Should I not see high cylinder, EGT or TIT temps before I damage the engine?

The reason I ask is because I don’t use the LF feature of my EDM 700 anymore. When I first got the airplane I did do so but after a few times it seemed redundant and predictable. I would end up at 12.5 gallons an hour and then would readjust from there anyway. 

Now I just get to cruise altitude ( 10 to 12 thousand feet), set 2300-2350 rpm, MP 28-30, and roll the mixture back to 11 gallons an hour. I carefully monitor all temps. If TIT exceeds 1580, or my hottest cylinder (#3) exceeds 380, I either decrease MP or enrich just a tad depending if I am in the mood to go faster or save fuel. Sometimes I can even roll back to 10.5 and keep all temps happy. Other times it may take 11.5. I assume these differences are due to varied atmospheric conditions.

Am I okay doing it this way, or did I just demonstrate my colossal ignorance to everyone on MS as well as using a procedure that could damage my engine!?!?

Torrey

P.S. I don’t have GAMI injectors.

I think your fine as it sounds as if your LOP

First let me clarify what I mean by too lean at too high a power, by too high a power I mean above the power where the manufacturer says you can lean, and by too lean I mean being close to best power, we can run best power mixture of course, but not at high power, our engines don’t have enough cooling to get away with that. If your LOP very often you can’t make enough power to hurt things, exception being I guess boosted motors, the deeper LOP you get the more power you lose and the safer you are.

Lastly used to be everyone understood fuel had two Octane ratings, for example 100 / 130, what that means is that at a lean mixture the fuel tests at 100 Octane, but at a rich mixture it tests as 130 Octane, meaning of course a whole lot more detonation margin at rich mixtures. Our 100LL is actually 100/130 fuel, I have no idea why it’s not stated as two different Octanes as all other fuels were. But by saying it’s 100, people forgot all about that second number. But 30 Octane points is big, it’s 30% higher.

[T. Peterson]

2 hours ago, Shadrach said:

I’m betting that you’ve been flying a while. I too learned that surplus fuel is “required” to ensure proper “cooling”. I remember a pilot in the 80s describing how the evaporation of surplus fuel cooled the cylinders. Seems intuitive. Mixture rich, EGTs fall, CHTs fall. However, that’s not really what’s happening. Surplus fuel does not really cool anything (it does have a slight cooling affect on exhaust gas temp). Surplus fuel simply slows the speed of the combustion event by forcing the flame front to go around non combustible fuel molecules that are not paired with O2 molecules (at ROP mixtures, there simply are not enough O2 molecules for all of the fuel). We therefor manipulate how fast the flame front propagates with mixture. This allows us to move the point of peak cylinder pressure closer to or further from TDC. Closer to TDC = more pressure= more heat and typically more power (whole ‘nother discussion there). Further from TDC = less pressure = less heat.  
 

A well conforming engine can use a surplus of air instead of fuel to control the combustion event. The up side is that air is free, the down side is that the surplus is limited by ambient pressure…unless we have a turbo! Then we are limited by MP restrictions and TIT. This is why turbo normalized engine operators rarely use ROP settings other than full rich, max power climb. As I posted earlier, these are the book Power settings for the 2013 SR22T. Note cruise power settings up to 85%. That’s 268hp on 18.3gph. No way to do that ROP. Cirrus does not even provide ROP power setting tables…and why would they? It would be a dirty, hot, inefficient and bone headed way to run that engine.

 

 

I have flown quite a bit, but I am an absolute rookie when it comes to piston engines. I am very grateful for your detailed explanation in paragraph one. I am a little fuzzy about paragraph two, but I will have to ponder it a bit to intelligently pose my question. I need to respond to A64 as he also graciously responded to me and then I have to get outside and get some work done before my wife confiscates my iPad!

gratefully,

Torrey

[T. Peterson]

23 minutes ago, A64Pilot said:

I think your fine as it sounds as if your LOP

First let me clarify what I mean by too lean at too high a power, by too high a power I mean above the power where the manufacturer says you can lean, and by too lean I mean being close to best power, we can run best power mixture of course, but not at high power, our engines don’t have enough cooling to get away with that. If your LOP very often you can’t make enough power to hurt things, exception being I guess boosted motors, the deeper LOP you get the more power you lose and the safer you are.

Lastly used to be everyone understood fuel had two Octane ratings, for example 100 / 130, what that means is that at a lean mixture the fuel tests at 100 Octane, but at a rich mixture it tests as 130 Octane, meaning of course a whole lot more detonation margin at rich mixtures. Our 100LL is actually 100/130 fuel, I have no idea why it’s not stated as two different Octanes as all other fuels were. But by saying it’s 100, people forgot all about that second number. But 30 Octane points is big, it’s 30% higher.

Thank you so much. You and Shadrach have both given me information on which to ruminate!

gratefully,

Torrey

[PT20J]

1 hour ago, A64Pilot said:

Lastly used to be everyone understood fuel had two Octane ratings, for example 100 / 130, what that means is that at a lean mixture the fuel tests at 100 Octane, but at a rich mixture it tests as 130 Octane, meaning of course a whole lot more detonation margin at rich mixtures. Our 100LL is actually 100/130 fuel, I have no idea why it’s not stated as two different Octanes as all other fuels were. But by saying it’s 100, people forgot all about that second number. But 30 Octane points is big, it’s 30% higher.

I believe that the aviation rich rating was created for supercharged engines at high power back in the day when the radials ruled the skies. They were all supercharged and generally had higher takeoff power ratings with time limits and lower METO (Maximum except takeoff) power ratings that could be used continuously. By the time 100LL came out, those engines were not common and it was probably simpler just to call it 100 and 100LL. The attached ASTM specification describes how the two ratings are tested.

AvGas_100LL_specs_--_ASTMD910.pdf

 

[Shadrach]

13 minutes ago, PT20J said:

I believe that the aviation rich rating was created for supercharged engines at high power back in the day when the radials ruled the skies. They were all supercharged and generally had higher takeoff power ratings with time limits and lower METO (Maximum except takeoff) power ratings that could be used continuously. By the time 100LL came out, those engines were not common and it was probably simpler just to call it 100 and 100LL. The attached ASTM specification describes how the two ratings are tested.

AvGas_100LL_specs_--_ASTMD910.pdfUnavailable

 

I skimmed it but it appears the 100 and 100LL are functionally Identical in performance but Low Lead (LL) has half as much TEL.

[A64Pilot]

I know it was for high powered boosted motors, because of course those are the guys that had to have it.

Interesting to me though was we went through a lot of WWII on I believe 87 octane fuel and maybe only the allies had 100 / 130 Octane.

https://legionmagazine.com/en/2020/07/gassed-up-the-juice-that-fuelled-victory-in-the-battle-of-britain/

I believe the US was the first source of 100 Octane but it was made to a British spec, it was 100 / 130. It gave the Spit 30 mph increase in speed and that coupled with them being allowed to build the Hamilton Standard constant speed prop under license really woke up the Spit, until just before the Battle of Britain the Spit ran a two pitch prop prop and 87 Octane fuel and compared to what it became, it was a dog.

‘The Me-109 couldn’t benefit from the high octane fuel as it couldn’t pull the boost I don’t think, The Germans used methanol / water injection to increase power down lower and Nitrous Oxide at altitude.

http://spitfiresite.com/2010/06/battle-of-britain-1940-constant-speed-propellers.html

115 / 145 didn’t become available until 1944 I don’t think and the German’s literally didn’t have a fighter that could get to B-29 cruising altitude so they developed a special variation of the FW-190.

Took me a minute to find as it was known as the TA-152

https://en.wikipedia.org/wiki/Focke-Wulf_Ta_152

By the end of the war we were so technologically superior, where four years earlier we didn’t have much, yes I know about the 262 but the engine was so underveloped it wouldn’t, couldn’t have been a serious contender

[A64Pilot]

15 minutes ago, Shadrach said:

I skimmed it but it appears the 100 and 100LL are functionally Identical in performance but Low Lead (LL) has half as much TEL.

LL is close, it actually is higher at lean settings but lower in rich, probably because of the lead bonus

When LL came about there was some increase in Detonation until people learned not to boost so much.

https://generalaviationnews.com/2015/03/24/testing-the-octane-rating-of-aviation-fuel/

[A64Pilot]

11 minutes ago, jaylw314 said:

Maybe a slightly more intuitive way to think about things:

• Enriching from peak more effectively converts heat to mechanical power up to best power.  At best power, it produces lower EGT because you're "sucking" more heat out each cylinder stroke. 
• Leaning to peak less effectively converts heat to mechanical power, so EGT increases since you're "sucking" less energy per cylinder stroke.  However, you're reducing fuel flow even more, so fuel efficiency still goes up

I know, I know, that terrible, but it's not any worse then the idea of "fuel coolling"  

Pretty close I think, I know I was surprised to find that an IO-540’s peak EGT was significantly lower than the lower compression O-540.

‘I can’t prove it, but I suspicion the higher compression engine is more efficient and the lower EGT is from more heat being extracted, these things are heat engines, develop power from expanding air from heat, they don’t get power from “explosions”