The majority of high-altitude aircraft engines used during World War II used mechanically driven superchargers, because these had three significant manufacturing advantages over turbochargers. Turbochargers - used by large American aircraft engines such as the Allison V-1710 (used in the P-38) and the Pratt & Whitney R-2800, required additional ducting expensive high-temperature metal alloys in the gas turbine and preturbine section of the exhaust system, but they were very useful in high-altitude bombers and some fighter planes. The size of the ducting alone was a serious problem.