Complex sentence: The single-speed single-stage supercharger fitted to the Allison V-1710 engine had been designed to produce its maximum power at a low altitude; above 15,000 feet, the supercharger's critical altitude rating, the power dropped off rapidly.
Split into simpler sentences: The single-speed, single-stage supercharger fitted to the Allison V-1710 engine, had been designed to produce its maximum power at a low altitude. Above 15,000 feet, the supercharger's critical altitude rating, the power dropped off rapidly.