ORIGINAL: XJet

Lots of the current generation of 90-sized engines are run with larger props to keep noise down and because they have quite a flat torque curve (due to their narrow transfer ports and over-square configurations.

My ST90s give maximum usable power with a 13x6 prop. The power drops noticeably even if you go up to a 14x6 so I've never tried a 15x6.