 The tropical filter and fuel tanks were to be used on later variants of the Hurricane.The improved Merlin XX (Mk.20) engine appeared in 1940 featuring a new two-speed supercharger that could have its impeller speed changed by the pilot depending on the outside air pressure (altitude).At about (effective), it would be switched to a higher speed gearing ("FS ratio" – Full Supercharge) for added compression, while below that, at its lower speed gearing, ("MS ratio" – Moderate Supercharge), it "robbed" less power from the engine.