The 91FSR is a fine engine for a scale model, not so great for sport/pattern or current 3D type flying- too big physically and too heavy.

Where the current 91FX is relatively light and compact based on the 61FX case, the 91FSR was heavy duty and based on a larger, heavier case than the 61FSR of the same timeframe.

The 91FSR was later increased in displacement to become the 1.08 which basically made the 91 obsolete. The 91 & 1.08 FSR's were intended for the exploding giant scale scene in the late 80's...torquey, reliable engines that would turn a big prop and fly some fairly good size models. In those days there were not all that many options for an engine this size. Saito, OS & Enya all had 1.20 four strokes out but these early models were none too powerful. Gas engines from Quadra and Zenoah were around but for bigger models altogether.