OK this is going to require some (much?) more experimental design. I repeated the test as follows: unpowered CPU, no fan --> unpowered CPU + Rotron fan --> boot to CPUBurn with fan--> turn off fan.
Then I got the bright idea that maybe the temperature drop was only due to the increased airflow to the heatsink directly. Made a "roof-shaped" cardboard deflector to put over the 80mm fan. Repeated testing, and now CPU temperatures rise when the Rotron is turned on. This seems to be because the hs/fan's intake air temps also rise when the Rotron is on.
Looks like the important thing here is whether the supply of fresh air to hs/fan is improved or inhibited and not the airflow to the mobo area.
This is not an easy thing to just "test" Maybe a bunch of 40mm fans arranged over all the hot spots would be more useful?
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