It was the supercharger technology that made the Merlin superior to the Allison orginally fitted to the P51. It took the Yanks to refine the Merlin for mass production.
As much as Rolls Royce has everyone believe that they produced the finest engines, they were effectively hand made. Each pistoin individually fitted to each bore and each cranckshaft individually fitted to each journal. You couldn't take a piston out of one cylinder and fit it to another. The Roll Royce produced ones are/were a nightmare to own.
What is the basis for this assertion?
Wikipedia (fwiw) gives the following:
" By the end of its production run in 1950, almost 150,000 Merlin engines had been built; over 112,000 in Britain and more than 37,000 under license in the U.S"
Wikipedia breaks down the numbers:
" Factory production numbers:
- Rolls-Royce: Derby = 32,377
- Rolls-Royce: Crewe = 26,065
- Rolls-Royce: Glasgow =23,675
- Ford Manchester= 30,428
- Packard Motor Corp = 55,523 (37,143 Merlins, 18,380 V-1650s)
- Overall: 168,068"
Ref Gunson 1995