On a July afternoon, my EGT will not lower below 400 regardless of how long I idle. That is because the IAT rises when parked, and that keeps EGT high. Yet if I switch off my AC, I can lower egt 50 degrees. Does that mean that my turbo is 50 degrees cooler, by turning off the AC?
I am not arguing, just posing questions. Does my pre-turbo gas temp lower when the turbo is cool enough? No, it lowers when the cylinder cools off, among other things. When reading EGT, you are reading the temp of a gas that has not even been to the turbo yet, so there is no correlation at all.
I know it has been done this way forever, I hear you Duke. I'm saying it doesn't make sense, though if you had a stopwatch and no egt gauge, you would have very similar results.
I too just make it a point to idle in, it was an old pilot trick also, because those turbos need to cool off slow. They were not water cooled however.
Anyway, not picking on any particular technique, just making for a thoughtful and provoking discussion.