Editing Allison V-1710 (section)

===Turbo-supercharger===
The USAAC had earlier decided to concentrate on [[turbo-supercharger]]s for high altitude boost, believing that further development of turbo-superchargers would allow their engines to outperform European rivals using crankshaft driven superchargers. Turbo-superchargers are powered by the engine exhaust and so do not draw much power from the engine crankshaft, whereas displacement superchargers are coupled directly by shafts and gears to the engine crankshaft. Turbo-superchargers do increase the exhaust back-pressure and thus do cause a very small decrease in engine power, but the power increase due to increased induction pressures more than makes up for that decrease. Crankshaft-driven superchargers require an increase in directly driven percentage of engine power as altitude increases (the two-stage supercharger of the Merlin 60 series engines consumed some {{convert|330|-|380|hp|kW|abbr=on}} at {{convert|25,000|ft|m|abbr=on}}. General Electric was the sole source for research and production of American turbo-superchargers during this period, from its four decades worth of [[steam turbine]] engineering experience.

Turbo-superchargers were highly successful in U.S. bombers, which were exclusively powered by radial engines. The P-47 fighter had the same combination of radial engine ([[R-2800]]) and turbo-supercharger and was also successful, apart from its large bulk, which was caused by the need for the ductwork for the aft-mounted turbo-supercharger.

However, mating the turbocharger with the Allison V-1710 proved to be problematic. As a result, designers of the fighter planes that used the V-1710 were invariably forced to choose between the poor high-altitude performance of the V-1710 versus the increased problems brought on by addition of the turbo-supercharger. The fates of all of the V-1710 powered fighters of World War II would thus hinge on that choice.

The original XP-39 was built with a V-1710 augmented by a General Electric Type B-5 turbo-supercharger as specified by Fighter Projects Officer Lieutenant [[Benjamin S. Kelsey]] and his colleague [[Gordon P. Saville]].<ref name=Bodie19>Bodie 1991, p. 19.</ref> Numerous changes were made to the design during a period of time when Kelsey's attention was focused elsewhere, and Bell engineers, [[National Advisory Committee for Aeronautics|NACA]] aero-dynamic specialists and the substitute fighter project officer determined that dropping the turbocharger would be among the drag reduction measures indicated by borderline wind tunnel test results; an unnecessary step, according to aviation engineer and historian Warren M. Bodie.<ref name=Bodie20>Bodie 1991, p. 20.</ref> The production P-39 was thus stuck with poor high-altitude performance and proved unsuitable for the air war in Western Europe which was largely conducted at high altitudes. The P-39 was rejected by the British, but used by the U.S. in the Mediterranean and the early Pacific air war, as well as shipped to the Soviet Union in large numbers under the [[Lend-Lease|Lend Lease program]]. The Soviets were able to make good use of P-39s because of its excellent maneuverability and because the air war on the Eastern Front in Europe was primarily short ranged, tactical, and conducted at lower altitudes. In the P-39, Soviet pilots scored the highest number of individual kills made on ''any'' American, or British fighter type.  

The P-40, which also had only the single-stage, single-speed-supercharged V-1710, had similar problems with high-altitude performance.

The P-38 was the only fighter to make it into combat during World War II with turbo-supercharged V-1710s. The operating conditions of the Western European air war – flying for long hours in intensely cold weather at {{convert|30000|ft|m}} – revealed several problems with these engines. They had a poor manifold fuel-air distribution and poor temperature regulation of the turbo-supercharger air, which resulted in frequent engine failures ([[Engine knocking|detonation]] occurred as the result of persistent uneven fuel-air mixture across the cylinders caused by the poor manifold design). Specially formulated fuels were a necessity for the P-38 as were specific spark plugs needed for specific cylinders. The turbo-supercharger had additional problems with getting stuck in the freezing air in either high or low boost mode; the high boost mode could cause detonation in the engine, while the low boost mode would be manifested as power loss in one engine, resulting in sudden fishtailing in flight. These problems were aggravated by sub-optimal engine management techniques taught to many pilots during the first part of WWII, including a cruise setting that ran the engine at high RPM and low manifold pressure with a rich mixture. These settings can contribute to over-cooling of the engine, fuel condensation problems, accelerated mechanical wear, and the likelihood of components binding or "freezing up."<ref>Whitney 1998, p. 127</ref> Details of the failure patterns were described in a report by General Doolittle to General Spatz in January 1944.<ref>Ludwig 2004, pp. 188–189</ref> In March 1944, the first Allison engines appearing over Berlin belonged to a group of P-38Hs of [[RAF Nuthampstead#55th Fighter Group|55th Fighter Group]], engine troubles contributing to a reduction of the force to half strength over the target.<ref>Bodie 2001, p. 223.</ref> It was too late to correct these problems in the production lines of Allison or GE, and as the numbers of Merlin-engined P-51 Mustangs based in England mounted up through the end of 1943 and into 1944, the P-38s were steadily withdrawn from Europe until October 1944 when they were no longer used for bomber escort duty with the [[United States Strategic Air Forces in Europe|Eighth Air Force]]. A few P-38s would remain in the European theater as the F-5 for photo reconnaissance.

The P-38 had fewer engine failures in the Pacific Theater, where operating techniques were better developed (such as those recommended by [[Charles Lindbergh]] during his [[Lockheed P-38 Lightning#Charles Lindbergh|development work]] in the theater),<ref>Kirkland 2003, pp. 29–35</ref>) and the Japanese did not operate at such high altitudes.<ref>Bodie 2001, p. 220</ref> Using the same P-38Gs which were proving difficult to maintain in England, Pacific-based pilots were able to use the aircraft to good advantage including, in April 1943, [[Operation Vengeance]], the interception and downing of the Japanese bomber carrying Admiral [[Isoroku Yamamoto]]. New P-38 models with ever-increasing power from more advanced Allisons were eagerly accepted by Pacific air groups.

When Packard started building Merlin V-1650 engines in America in 1942, certain American fighter designs using the Allison V-1710 were changed to use the Merlin. The P-40F, a Lend Lease export to Britain, was one of the first American fighters to be converted to a Packard-Merlin engine. However, the installed engine was the V-1650-1 (a Packard-produced [[Rolls-Royce Merlin#Early engines|Merlin XX]]) with a slightly improved single-stage, two-speed supercharger, yielding only modest gains over the Allison V-1710.

The last Allison powered P-51, the Mustang I(II)/P-51A, used the single-stage, single-speed Allison V-1710-81, with a 9.6:1 blower ratio. This allowed the P-51A to reach a maximum speed of {{convert|415|mph|km/h kn|abbr=on}} at {{convert|10,400|ft|abbr=on}} and maintain {{convert|400|mph|km/h kn|abbr=on}} at {{convert|23,000|ft|abbr=on}}.<ref>Memorandum Report on Pursuit Single Engine P-51A-1-NA, 2 April 1943</ref> This was more than {{convert|70|mph|km/h kn|abbr=on}} faster than the Merlin 45-powered Spitfire V at {{convert|10,000|ft|abbr=on}}, and more than {{convert|30|mph|km/h kn|abbr=on}} faster at {{convert|25,000|ft|abbr=on}}.<ref>Aeroplane and Armament Experimental Establishment, AA873 Report, 8 March 1942</ref> Its speed impressed the British, and the RAF quickly realized the airplane would possess excellent high altitude performance if the Allison V-1710 engine were replaced by the [[Rolls-Royce Merlin#Supercharger|60-Series Merlin]]. A similar proposal to cure the P-38's problems by replacing its Allisons with Merlins was quashed by the USAAF, after protests from Allison.<ref>White 1995, p. 92</ref>  
[[File:Allison V-1710 Engine 1.jpg|thumb|Cutaway of Allison V-1710]]

Starting with the V-1710-45 around 1943 (after the P-51 had been fitted with a Merlin 61 by Rolls-Royce), Allison attached an auxiliary supercharger to some of its engines in an effort to improve high-altitude performance. The two-stage supercharged Allison was essentially developed as an "add on" to the single-stage engine, and required minimal changes to the base engine. While it lacked the refinement, compactness and after-cooler of the two-stage Merlin, the Allison used a pressure-altitude governed variable-speed first stage. Various configurations of this auxiliary supercharger were used in production versions of the V-1710 that powered aircraft such as the [[Bell P-63 Kingcobra|Bell P-63]] and [[North American F-82 Twin Mustang|North American P-82]]E/F/G series. In addition, it was tried or studied as the powerplant for many experimental and test aircraft such as the [[Curtiss-Wright XP-55 Ascender|Curtiss XP-55 Ascender]], [[North American P-51 Mustang|North American XP-51J "lightweight Mustang]]", [[Boeing XB-38 Flying Fortress]], and [[Republic P-47 Thunderbolt|Republic XP-47A]] (AP-10), both of the latter with turbo-superchargers.