I ran NTernational Nitro's O.P.S Power (Torque version) last summer. As I understand things, these two engines are the same, both being manufactured by O.P.S. The 621 has a different carb and Ive heard internal work may be a little different, but very little difference. I absolutely loved this engine. Definately the best Ive run. Easy to tune, held a tune, and ran great all day long.