Originally Posted by
Weads
I had read, and don’t quote me but I believe it was from a Prouty or Coyle book that said the 407 was most effective and flew the best at a slower Nr but the engineers at Bell wanted to increase the Nr for a reason not known to me.
I don't remember the particulars but Bell used the lower Nr capability to market their Quiet Cruise Option which allows ops at around 92%. Look for the RFM supplement which may give more info. However, the whole intent of the mod was not for efficiency but rather noise reduction for certain flight profiles.