Just ran across a possible explanation for my Cad 500's high EGT's in an
article by Roger Zerkle in Ultra-Flight magazine. He's specifically
addressing 2-stroke engine, but the principle seems applicable:

-------------------------------------------------------------------------------------------
The most common, and misunderstood, condition affecting EGT is engine
​
loading. If the engine is not loaded sufficiently, it will run high EGTs.
If the
​
mixture is richened, the EGTs will go up, rather than down. A lightly loaded
​
engine can be enriched to the point that it will barely run, and still have
high
​
EGTs. The explanation is found in one of Newton's laws. Simply stated, it
says
​
that energy cannot be created, nor destroyed.

When we burn a fuel-air mixture inside our engine, we are converting
chemical
​
energy into heat energy. When applying the pressure created by the
expansion of
​
the burning gasses inside the combustion chamber, to a movable piston, we
are
​
converting a portion of the heat energy to mechanical energy. The heat
energy
​
not used in turning the crankshaft is given off through the exhaust and the
​
engine cooling system. If we add all this up, it would equal the amount of
energy
​
present in the fuel before it was introduced into the engine.

So it would be correct to assume that, for the same amount of fuel-air
mixture
​
being burned in the combustion chamber, if less heat energy is being
utilized to
​
turn the propeller, then more heat energy will be going out the exhaust. In
other
​
words, a lightly loaded engine, will throw considerably more heat out of the
​
exhaust than the same engine, with the same throttle setting, carrying a
heavier
​
load. So, not only will a heavier loaded two-stroke engine have lower EGTs,
but
​
the engine will produce more power for the same amount of fuel burned. If a
​
lightly loaded engine, with a high EGT, has the mixture enriched, the extra
fuel
will go out the exhaust, increasing the EGT even more.
-------------------------------------------------------------------------------------------

​I have noticed that I don't get higher EGT's when climbing hills, though I
don't recall seeing lower readings. ​I've always thought that I could
re-install a 3.07 final drive with the Cad 500; now I think I SHOULD -- at
least for a trial. This also plays well with a comment Doc Frohmader made
to me about engine loading and wasted energy out the exhaust.

Ken H.
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Just ran across a possible explanation for my Cad 500's high EGT's in an article by Roger Zerkle in Ultra-Flight magazine. He's specifically addressing 2-stroke engine, but the principle seems applicable: (It Is - Matt)

-------------------------------------------------------------------------------------------
The most common, and misunderstood, condition affecting EGT is engine loading. If the engine is not loaded sufficiently, it will run high EGTs.
If the mixture is richened, the EGTs will go up, rather than down. A lightly loaded engine can be enriched to the point that it will barely run, and still have high EGTs. The explanation is found in one of Newton's laws. Simply stated, it says that energy cannot be created, nor destroyed.

When we burn a fuel-air mixture inside our engine, we are converting chemical energy into heat energy. When applying the pressure created by the expansion of the burning gasses inside the combustion chamber, to a movable piston, we are converting a portion of the heat energy to mechanical energy. The heat energy not used in turning the crankshaft is given off through the exhaust and the engine cooling system. If we add all this up, it would equal the amount of energy present in the fuel before it was introduced into the engine.

So it would be correct to assume that, for the same amount of fuel-air mixture being burned in the combustion chamber, if less heat energy is being utilized to turn the propeller, then more heat energy will be going out the exhaust. In other words, a lightly loaded engine, will throw considerably more heat out of the exhaust than the same engine, with the same throttle setting, carrying a heavier load. So, not only will a heavier loaded two-stroke engine have lower EGTs, but the engine will produce more power for the same amount of fuel burned. If a lightly loaded engine, with a high EGT, has the mixture enriched, the extra fuel will go out the exhaust, increasing the EGT even more.
-------------------------------------------------------------------------------------------

​I have noticed that I don't get higher EGT's when climbing hills, though I don't recall seeing lower readings. ​I've always thought that I could re-install a 3.07 final drive with the Cad 500; now I think I SHOULD -- at least for a trial. This also plays well with a comment Doc Frohmader made to me about engine loading and wasted energy out the exhaust.

Ken H.

<snip>
I have noticed one peculiarity about the bank-to-bank EGT differential: On acceleration, the RIGHT bank becomes the hotter one! As soon as the acceleration is over with, the Left again registers hotter. If the manifold didn't distribute mixture from each injector to both banks, I'd think one injector was richer than the other -- despite the test results from the Wizard that they're nearly identical. Maybe I'll swap their positions before we start home, just to be sure there's nothing strange
going on.

Ken H.

Ken,

I think I said this before but I monitor EGT on all cylinders independently on my airplane. I can be at cruise at say 8000 feet at WOT with the mixture set for the leanest cylinder -50 degrees on the rich side. It will stay at the same EGT on each cylinder for hours. BUT.... If I put the airplane in a minor to moderate climb or minor to moderate decent WITHOUT changing anything else (throttle or prop pitch), the leanest cylinder changes to a different one. It is also very predictable. #4 is always the leanest one in a climb while #3 is the leanest one at level cruise. If I change altitudes to say 6,000 or 10,000 the engine performance, aircraft speed, and fuel economy will change, but the leanest cylinder will remain the same. ie. #3 at level cruise. Also the front 2 cylinders are always about 175 degrees colder EGT.

I do not understand why this happens. I am just reporting what it does. Economy lean at cruise for me is around 1500 to 1525. I usually richen the mixture to between 1450 to 1475.

Ken B.

Ken,

That's interesting. Must have something to do with the manifold, wouldn't
you guess?

Since I don't follow general aviation (nor any other very much any more),
I'm not up on it, but I thought throttle setting philosophy changed a few
years back. Do you not believe in the "lean of max EGT" throttle setting?