Hooker's developments allowed the aircraft they powered to maintain a crucial advantage over the German aircraft they opposed throughout World War II despite the German engines being significantly larger in displacement.[17][18] Two-stage superchargers were also always two-speed. After the air was compressed in the low-pressure stage, the air flowed through an intercooler radiator where it was cooled before being compressed again by the high-pressure stage and then possibly also aftercooled in another heat exchanger. Two-stage compressors provided much improved high altitude performance, as typified by the Rolls-Royce Merlin 61 powered Supermarine Spitfire Mk IX and the North American Mustang.