Grace's Guide is the leading source of historical information on industry and manufacturing in Britain. This web publication contains 146,105 pages of information and 231,598 images on early companies, their products and the people who designed and built them.
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 at high altitude. 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 to the effect that Packard 'redesigned' the engine, or built them in larger numbers than Rolls-Royce were able to.
Of the total 168,176 Merlins produced, Packard built 55,523. Rolls-Royce produced 82,117 in three factories, Ford (Manchester) 30,428, with small numbers built by others, including Continental in the USA.
Later Packard Merlins had a number of significant differences to the Rolls-Royce-produced engines, principally in the fuel supply arrangements and in the use of epicylic gears instead of Farman drives for the two-speed superchargers.
A detailed account of how the Merlin came to be adopted and adapted for the P-51 Mustang was was written by David Birch and published in 1997 .
See here for links to some Packard Merlin manuals.