From pilot Sam Graves- "The stock Stearmans that were used during the war were 220 hp Continental or Lycoming PT-13s or PT-17s. After the war, crop dusting really came of age and the perfect combination was a converted Stearman with the 450 horse Pratt &Whitney engine. These became know as the Super Stearman. A few were also converted to 600 hp (or 650 hp as is the case with 121R cause of the pressure carburators) for use as dusters and airshow work. These I suppose are Super Streamans on steroids. 121R was originally a 1943 Boeing Stearman PT-13. It went from 220 to 650 hp...
quite a jump."