Re-engining a fighter plane

Roger A Freeman; Mustang at War, Doubleday 1974

The Mustang became a complete fighter plane when the larger and more powerful Roll Royce engine replaced the original Allison. The top photo shows the prototype installation of the test Rolls Royce Merlin 65. The bottom photo shows the later production Packard Merlin installation.
 The new engine increased top speed by 70 mph and more importantly, accomplished that speed at 25,000 feet from 13,000.