The difference between that .91's 1.6 horsepower and the .46's 1.6 horsepower is that the horsepower of the .91 four-stroke was measured at an rpm that model airplane engines actully turn in sport aerobatic airplanes. The .46 makes it's horsepower at something like 17500 rpm. To get that engine to rev up that high, you would probably have to put a 9 inch prop on it like pylon racing planes use. Unfortunately, these small props are a total mismatch for most trainers and sport planes so the engines power mostly gets used to accelerate air backwards instead of pulling the plane forwards. The engine makes 1.6 horsepower but the prop wastes most of it. That's why .46 two stroke usually uses a 11 to 12 inch prop. Its the best tradeoff between letting the engine rev enough to make horsepower and prop efficiency. In real world use, that .46 is probably making no more than 1 to 1.2 horsepower.