 The Merlin 65 series engine was utilized in all the prototypes as it was identical to the Merlin 66 powering the Spitfire Mk IX, allowing for a closer comparison.Due to the speed of the conversions, engines were often swapped from aircraft to aircraft as well as being replaced by newer units.The high-altitude performance was a major advance over the Mustang I, with the Mustang X serial "AM208" reaching 433 mph (697 km/h) at 22,000 ft (6,506 m) and "AL975" tested at an absolute ceiling of 40,600 ft (12,375 m).