Definitely was not in neutral and the ICE did not give any indication of firing. Everyone says it was spinning and my understanding of the PSD is that the engine had to turn, but there was no brief lighting of the ICE in this instance and that caught my attention. I'm accustomed to feeling that transition as I pass 41, but on that downhill glide it didn't give the familiar engine start even at 45. I've noticed a few times before that I could sometimes "sneak" into 42 and back out quickly without feeling the engine start, but this was well past that speed. Perhaps it is just silky smooth and I can't always feel it even when I'm tuned in looking for it, but the butt dyno is fairly reliable at detecting actual transition in acceleration/momentum. I'll play with this downhill stretch some more as I get the chance. There are some aspects of the Prius control system that vary just enough that repeatability is an issue. In this instance, once I regenned and turned for a 1.1 mile flat stretch the ICE wanted to run for the first half of it--most likely because the coolant had dipped below temp during the downhill glide (it should have just reached temp before the downhill.) I could not feather it out and accelerate even though SOC was good. I usually run that leg 100% in glide/electric only.
