Rolls-Royce Merlin exhaust flow and waste-gates, meant that this option was rejected in favour of the two-stage supercharger. Fitted with the two-stage two-speed supercharger, the Merlin 60 series gained 300 horsepower (224 kW) at over the Merlin 45 series, at which altitude a Spitfire IX was nearly faster than a Spitfire V. The two-stage Merlin family was extended in 1943 with the Merlin 66 which had its supercharger geared for increased power ratings at low altitudes, and the Merlin 70 series that were designed to deliver increased power at high altitudes. While the design of the two-stage supercharger forged ahead, Rolls-Royce also
Rolls-Royce Crecy fighter. Studies on the de Havilland Mosquito also showed it to raise complex problems with Crecy installation. In 1942 Rolls-Royce Hucknall received a North American P-51 Mustang for engine installation trials. This prompted a series of studies for a Crecy version and the Mustang turned out a more suitable mount than the Spitfire. However these studies were not taken further. As the possibility of a flight-worthy engine approached, on 28 March 1943 Hawker Henley "L3385" was delivered to Hucknall for fitting with a Crecy. However the engine never became available and the aircraft remained at Hucknall until when it was