Packard as the V-1650 and used in the P-51B Mustang and subsequent variants. The USAAC had specified that the V-1710 was to be a single-stage supercharged engine and, if a higher altitude capability was desired, the aircraft could use their newly developed turbo-supercharger as was featured in the XP-37(YP-37), P-38, and XP-39. The benefits of a two-stage supercharger eventually became so clear that Allison did make some efforts in this direction. Allison attached an auxiliary supercharger in various configurations to the existing engine-mounted supercharger and carburetor. Early versions of these two-stage supercharger engines were used on the P-63. No intercooler, aftercooler, or backfire screen (flame trap) were incorporated into these two-stage V-1710 engines (except for the V-1710-119 used on the experimental P-51J, which had an aftercooler). The two-stage Merlin engines had all of these features, which were designed to prevent detonation from charge heating and backfire into the supercharger. The G-series V-1710s installed on the F-82 E/F/G models had only anti-detonation injection (ADI) to deal with these problems, and not surprisingly had severe reliability and maintenance problems. In one record, it was stated that the F-82 required 33 hours of maintenance for each hour of flight. Although the early V-1710 powered P-39, P-40 and P-51A were limited to combat operations at a maximum of about they were available in comparatively large numbers and were the mainstay of some Allied Air Forces in all but the European theater of war. The engines proved to be robust and little affected by machine-gun fire. In total, over 60 percent of the post-June 1941 USAAF's pursuit aircraft operated during WWII were powered by the V-1710. Allison slowly but continuously improved the engine during the war. The initial rating of was incrementally increased; the final V-1710-143/145(G6R/L) was rated for . By 1944, the War Emergency Power rating on the P-38L was . The most powerful factory variant was the V-1710-127, designed to produce at low altitude and at . This engine was static tested at and was planned for installation in an XP-63H aircraft. The end of the war ended this development, so this promising experiment never flew. The extra power of this version was derived from using exhaust turbines, not to drive a turbo-supercharger, but to return that energy to turning the crankshaft, called a turbo-compound engine. Improvements in manufacturing brought the cost to produce each engine from $25,000 down to $8,500 and allowed