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 25/30 Rolls-Royce 25/30 The Rolls-Royce 25/30 built between 1936 and 1938 is an updated version of the 20/25 with larger engine to provide more power, as often, over-large bodies had been fitted to the earlier model leading to complaints about its performance. The in-line 6-cylinder, overhead-valve 25/30 hp engine is similar to that used in the 20/25 but increased in capacity to by increasing the bore from to , with the stroke remaining at . The compression ratio is 6:1. A single proprietary Stromberg downdraught carburettor replaced the Rolls-Royce one, and magneto ignition was no longer fitted, but a standby coil