If the engine on an A36 you are considering is due for replacement it can be very cost effective to upgrade from the original IO-520 rated at 285 horsepower to the 300 hp IO-550. The engines are nearly identical externally so the 550 fits right into the 520 space with the most minor of modifications. Continental offers trade-in credit on a 520 for a factory rebuilt or factory new 550 so there is little cost difference when replacing the engine. Not all propellers approved for use with the 520 can be used on the 550, so be sure to check that out. However, the new Hartzell propellers are considerably better in performance and durability so a change to the Top Prop is valuable in any event. The 550, particularly with a new Hartzell three-blade, will give you a few more knots speed for a little more fuel, plus better takeoff and initial climb. And, subjectively, I think the 550 runs just a tad smoother.