The 120NC is slightly more powerful than the F-120.
The F-120 is the original production of the 120 series dating
back almost 23 years. The F-120 was last produced almost
14 years ago and preceded by the 120 AC. The 120 NC
followed. There are big differences in the carbs and
internally such as piston design,crankshaft design and
intake valve and exhaust valve diameters just to name
a few. The F-120(or 120SF) and 120NC Were excellent
performers on 10 lb airframes and could fly airframes
weighing as much as 16lbs. The newer engines,120SC
120FZ crank out as much as 6/10ths of a horse more.
Personally I think the 120SC was the most powerful
of all. It took advantage of a high compression ratio
to put the super charger at full affect.