[RayChuang88]

From what I've read about the Mustang's development, everyone knew the P-51 was a great fighter, but the early P-51A was hobbled by the Allison V-1170 engine, which was not useful at higher altitudes. It was when the Mustang switched to the Packard Merlin engine--which was already being built in the USA for certain later-production P-40 models--that the Mustang became the best fighter of World War II.

[C19fan]

The article claims it was a Brit pilot who noticed the Merlin and Allison both were the same sized and he came up with the idea of swapping engines.

[Son-Joshua]

The Rolls-Royce Merlin engine transformed the P51 from an average fighter to the best fighter in the European theater. A good friend of mine flew them in WW11 and he shared many amazing stories.