The Packard V-1650 was a version of the Rolls-Royce Merlin V-12 aircraft engine, produced under licence in the United States by the Packard Motor Car Company. The engine was licensed in order to provide a 1,500 hp class design at a time when U.S. engines of this rating were not considered ready for use even after years of development. The '1650' designation relates to the engine capacity of 1650 cubic inches (27 litres).
The first V-1650s, with a simple one-stage supercharger, were used in the P-40F Kittyhawk fighter. Later versions included a much more advanced two-stage supercharger for greatly improved performance at high altitudes. It found its most famous application in the North American P-51 Mustang fighter, where it vastly improved that aircraft's performance at altitude, transforming the Mustang into an outstanding fighter with the range and performance to escort heavy bombers over the European continent. By 1944, P-51B, P-51C and P-51D "Merlin" Mustangs were able to escort Allied heavy bombers in daylight all the way to Berlin and yet were still capable of combating German fighters attempting to intercept the bombers.
The use of a foreign-designed engine in US military aircraft wrankles to this day, giving rise to nonsensical claims about differences and relative merits of the Packard and the Rolls-Royce-built versions of this engine.
Packard built 55,523 of the total 168,176 Merlins produced. Rolls-Royce produced 82,117 in three factories, Ford (Manchester) 30,428, with small numbers built by others, including Continental in the USA.