The Speed 70 is a rare and desirable engine. It is mechanically stronger and better balanced than the speed 80, which is just a bored 61LS. Prop it the same as a 60 (13000 rpm) and enjoy better power and fuel draw. An ABC-ring setup lets the engine handle a lot of heat while being able to eat dirt with minimal damage. It can use the same muffler/header as OS61fsr and Enya 60X.