 The Mustang Mk.I was 30 mph (48 km/h) faster than the Spitfire Mk VC at 5,000 ft (1,524 m) and 35 mph (56 km/h) faster at 15,000 ft (4,572 m), despite the latter having a significantly more powerful engine than the Mustang's Allison.Rolls-Royce engineers rapidly concluded that the Mustang powered by a two-stage Merlin 61 would result in a significant improvement in performance and started converting five Mustangs to Merlin power as the "Mustang Mk.X" (i.e., Mk.10) With a minimum of modification to the engine bay, the Merlin engine neatly fitted into the adapted engine formers.A smooth engine cowling with an additional "chin" radiator was tried out in various configurations as the two-stage Merlin required a greater cooling capacity than could be obtained with the standard Mustang radiator alone.