Honolulu, I wish I could provide a scientific answer, and a sample of one (me) may not be sufficient for a general conclusion. Wouldn't it be interesting to know why BMW put it there anyway, other than to annoy us at oil change time. The turbulence theory seems as good as any.Inquiring minds crave to know: dBower, how can it happen that the engine runs cooler with the undertray than not?
With the undertray in place the engine compartment is more sealed than without. What happens to air heated by passing through the radiator when the undertray is in place, unless it provides some venting at the rear with enhanced aero/venting capabilities?