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 RB401 Rolls-Royce RB401 The Rolls-Royce RB.401 was a two-spool business jet engine which Rolls-Royce started to develop in the mid-1970s as a replacement for the Viper. RB.401-06 prototype engines were already being manufactured when a decision to develop the higher thrust RB.401-07 was taken. Although ground testing of both the -06 and -07 continued into the early 1980s, a lack of funds caused the project to be cancelled. Although the basic configuration of both engines was almost identical, the -07 variant had a larger fan diameter. The -06 version HP compressor was based on the eight-stage version of the RC34B research