By Stephen Sherman, Apr. 2002. Updated Sept. 26, 2012.
North American Aviation originally designed the Mustang in response to a British specification. They agreed to produce the first prototype only 4 months after signing the contract in April 1940. By the end of 1941 North American had delivered the first Mustang to England for test flights. These first Mustangs were powered by the Allison V-1710 engine, a good engine, but one which didn't operate well at high altitudes.
A Better Engine
In April, 1942, a British test pilot, Ronald Harker, flew the Mustang and was very impressed by it. He suggested that the new plane would be a natural fit with the Rolls Royce Merlin 60-series engine, well-suited to high altitudes. At the prodding of Major Thomas Hitchcock, the Americans began working along the same lines (using the Packard license-built version of the Merlin), and the first Merlin-equipped Mustang, the P-51B, flew in November, 1942. The results were impressive, to say the least. At 30,000 feet, the improved Mustang reached 440 MPH, almost 100 MPH faster than the Allison-equipped Mustang at that altitude. Both Robert Goebel and Bud Anderson flew Mustangs. Their comments follow.
Bob Goebel on the P-51:
Robert Goebel flew Mustangs with the 31st Fighter Group, based at San Severo, Italy, in the MTO (Mediterranean Theater of Operations). Like Bud Anderson, he had flown P-39s earlier on. At San Severo in Spring 1944, he got his first crack at the P-51:
We soon found out that the P-51 Mustang was indeed a different breed of airplane. It was fast, for one thing. ... The P-51 was red-lined at 505 and, though it was no Spitfire, its turning ability wasn't bad at all - especially if you sneaked down 10 degrees of flaps. It was pretty good in the climbing department too, and accelerated very fast in a dive. But the thing that really set the Mustang apart from any other fighter, friend or foe, was its range. With a 75-gallon tank slung under each wing, it could perform the unheard-of: It could fly six-hour missions.
Physically, it was pleasing to the eye and looked fast, even sitting on the ground. Power was provided by a V-1650 Rolls-Royce Merlin engine built under license in the States by Packard, the luxury automobile company. The V-1650 was a fine engine and could be taken up to 61 inches of manifold pressure at 3,000 RPM for take-off or, if needed in combat, 67 inches for up to five minutes in Emergency Power. Normally aspirated engines tended to run out of power as altitude increased, usually between 15,000 and 20,000 feet.
The P-51 had a two-stage blower in the induction system that was controlled automatically with a barometric switch. Around 17,000 feet, when the throttle had been advanced almost all the way forward just to maintain normal cruise, the blower would kick into high, the manifold pressure would jump up, and the climb could be continued to 30,000 feet. The P-51 could be taken a lot higher than that, but above 30,000 feet the power was way down and the controls had to be handled gingerly.
quoted from "Mustang Ace", available at Amazon.com