The EGT Raw value is not important. We only care about the relationship to peak EGT."
I concur. Relationships of the temperature indications count, not the absolute, accurate, calibrated temperature number which is NOT displayed on the instrument. Truly accurate measurement of gas temperature is not available with our instrumentation, so concern about calibration and "accurate" absolute values is misplaced.
Colin asked: "Fred, About that TIT number. We care about this I guess mainly because it relates to turbine RPM which is what we really want to limit. Do you have any suggestions how to determine how close to abusing the turbine you are coming?"
To which Walter replied:
"The TIT raw number is important. The turbine is rated to withstand max RPM at the TIT temp limit indefinitely. If the temp is above the limit at [that] RPM, blade creep will allow the blades to scrape against the housing. Fortunately we rarely operate at max altitude requiring max turbine RPM, so there is a tiny bit of wiggle room. How much wiggle room? We don't have any way to know from the cockpit."
There you have it, from someone who knows.
<<Start rich, Lean down in steps waiting, say, one minute per step while you record data. Then reverse, going from lean back richer. Then plot the data. The curve going leaner will not lie on top of the curve going richer. Why? Because when you took the reading, it was still changing, slowly, but still changing. It takes a lot of time and patience to get good [steady state] data. >>
Walter kindly added: "While that's a logical explanation, that's not why the peak temps are different when coming from ROP or LOP. The reason is the change in volumetric efficiency of the cylinders due to the difference in CHT, therefore a minor change in F:A ratio. If one leans slow enough, the temps and volumetric efficiencies stabilize and the numbers will be the same."
I think we are saying the same thing, Walter. It takes a long time to come to equilibrium after a mixture, power, or speed change. CHTs are changing very slowly after a mixture adjustment for the reasons cited, and so the EGT is drifting slowly to equilibrium for several minutes after your hand leaves the mixture knob. If you want "equilibrium values" (unchanging), wait a long time.
But the data produced from a quicker scan is valuable because it is good enough to show which cylinder leans first, which second, etc at particular fuel flows so that "spread" can be evaluated. Again, the absolute value of the EGT numbers, what Walter termed "raw value" displayed in the cockpit, is not important, but the relationship between the numbers is.
Remember: "better" is the enemy of "good enough."
Fred