The engine-driven cabin environment compressors mounted to the inboard engines of the P-3 are what gives her distinctive whistling sound on the ground when compared to the C-130, which runs essentially the same engine. I've spent many hours on both the T-56A-14 and -15 and they are a superb engine - very strong and very powerful. They incorporated a very clever relationship between the typical hydromechanical fuel systems of the day and the beginnings of an 'electronic' exhaust temperature controlling/limiting system. There was much to learn about what happened and when for any movement of the power levers by the crew.
The industrial turboshaft version of the T-56/501 would churn out 8000+ horsepower for months on end if needed.
With regard to the loiter mode, an outboard engine shutdown wasn't always available to the crew depending on where the task took the aircraft and what the possible consequences were of an unsuccessful restart prior to recovery.