V-1710 said:
My mistake, it was the P-51J that had the 2 stage Allison. I think only 2 were built, and they had about 2,300 h.p.. The idea was that because Packard was going to have to start paying royalties to Rolls-Royce to continue Merlin production after the war's end, thought was given to going back to the Allison on the P-51.
The P-51J used an Allison V-1710-119, rated at 1720 max output, not 2,300 hp.
From Joe Baughers excellent website;
"The last prototype in the lightweight NA-105 series was the XP-51J, which was similar to the F and G models except that it reintroduced the Allison V-1710 engine to bring the Mustang full circle. The Allison engine was, however, the V-1710-119 version with a two stage, gear-driven supercharger, rated at 1500 hp for takeoff and 1720 hp with water injection at 20,700 feet. Unlike earlier Allisons, this engine had an updraft carburetor. The nose geometry was substantially modified, and all air inlets in the nose were completely eliminated. Instead, the carburetor air was taken in through a ram inlet at the front of the radiator duct and piped to the engine. A dorsal fin was fitted.
Two XP-51J prototypes were ordered, with serial numbers being 44-76027 and 44-76028. 44-76027 made its first flight on April 23, 1945, piloted by Joe Barton. The XP-51J weighed 6030 pounds empty and 7550 pounds normal loaded. It was anticipated that a maximum speed of 491 mph could be achieved at an altitude of 27,400 feet, but this was never achieved during tests because the new Allison had not yet been cleared for full power operations. XP-51J Ser No 44-76027 was, in fact, loaned to Allison so that they could use it to iron out the bugs in their engine. The other XP-51J prototype, Ser No 44-76028, was never actually flown, but was scavenged for spare parts to keep the other example flying. The end of the war in the Pacific brought all further work on the XP-51J to an end. "