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The absolute value of the EGT reading is less important than when a cylinder
peaks while leaning compared to other cylinders. Ideally they should all
peak within 0.3 gallons per hour of fuel flow according to GAMI information.
But the absolute value of the number is of little use because of the
difficulties of getting truly accurate temperature readings at highly
elevated temperatures with pulsing flow.
Consider: An EGT probe is getting pelted with exhaust flow pouring out of
the cylinder for maybe 180 degrees of rotation, and that temperature is
falling throughout the pulse as the gas expands out of the cylinder. After
the pulse is finished, you get "nothing" for the next ~540 degrees of
rotation.
The thermocouple in the tip of the probe gets heated by a blast of variable
temperature exhaust gas. The probe conducts heat down the stem to the
cooler outside world which lowers the thermocouple temperature. Thermal
radiation from the hot probe tip to the cooler exhaust pipe wall further
lowers temperature. Differences in air flow over the exhaust pipe and over
the probe mounting point also lead to differences in conduction and
radiation losses which affect probe temperature. Adding to the confusion
are pressure waves oscillating back and forth as exhaust pulses travel out
the exhaust manifold and other nearby cylinders release blasts of exhaust
flow.
After a moment of thought two things become evident:
1) There is no single number that accurately reflects the "true exhaust gas
temperature."
2) There are big sources of error due to differences in convection (heating
and cooling from movement of gas), thermal radiation, and conduction heating
and cooling that affect each individual probe.
So the absolute value of the EGT number means little.
The RELATIVE values of the temperatures shown over time or versus fuel flow
ARE useful for leaning, or in comparing behavior of a cylinder over time.
Example: if one cylinder was formerly cooler than its colleagues, and starts
showing much higher (or lower) EGT under the same flight, power, and
mixture conditions, then it is likely the fuel flow in that injector may be
changed, and a bit of investigation may be warranted.
It is most useful to keep records of EGT versus fuel flow (for "evenness" of
leaning) and over time (to check for plugged injectors). But whether an EGT
reads 1250F or 1350F by itself means little.
Fred Moreno
PS of your CHT's are within 20-30F, you are doing very well and need not
worry about uniformity of cylinder cooling. Differences of 50F are not
uncommon.
I have been chasing a problem with my EGT's and have hit a wall. My #3 EGT
is about 80 degrees above the rest of the cylinders. It also has the
highest CHT, but only by 10 degrees over #4. Yet #4 has one of the coolest
EGT's... #2 is the first to peak while leaning.
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