Allison V-1710

The Allison V-1710 was the only American liquid-cooled V-12 aero engine to reach mass production during the Second World War. Designed at the Allison Engine Company in Indianapolis and first run in 1931, the V-1710 entered service in 1937 and powered four of the principal U.S. Army Air Forces fighters of the war: the Lockheed P-38 Lightning, the Bell P-39 Airacobra, the Curtiss P-40 Warhawk, and the early P-51A Mustang. Allison built roughly 70,000 V-1710s between 1937 and 1948 at its Speedway plant in Indianapolis, making it the second-most-numerous Allied V-12 of the war after the Rolls-Royce Merlin.

The V-1710 was a 60-degree V-12 of 1,710 cubic inches (28 litres) displacement, with a bore of 5.50 inches and a stroke of 6.00 inches. Cylinders were cast as separate aluminium blocks bolted to a forged crankcase, then liquid-cooled with a 70/30 ethylene-glycol mixture. The engine was modular: the same core could be configured as a tractor (propeller-forward) or pusher, with either a left-hand or right-hand crankshaft rotation. This let Allison ship handed pairs to Lockheed for the counter-rotating propellers on the P-38, eliminating the asymmetric torque problem that plagued other twin-engine designs.

Power output rose across the production run from 1,000 hp on the C-series (V-1710-33, used on the P-40B) to 1,150 hp on the F-series (V-1710-39, P-40E and P-51A), 1,325 hp on late F-series (V-1710-89/91, late P-38) and 1,475 hp on G-series builds (V-1710-111/113). Experimental late-war V-1710s with two-stage mechanical superchargers reached 2,300 hp on the bench but never entered service. The standard production engine used a single-stage single-speed mechanical supercharger that limited rated altitude to roughly 12,000 ft — adequate for ground-attack and low-level air combat, but a hard ceiling for high-altitude bomber escort.

The supercharger limitation drove two of the war's most consequential engineering decisions. On the P-38 Lightning, Lockheed paired each V-1710 with a General Electric B-series turbocharger fed by the booms behind each engine, recovering high-altitude performance and giving the Lightning genuine 40,000 ft combat ability. On the P-51 Mustang, however, the Allison's single-stage supercharger left the early P-51A weak above 15,000 ft. RAF testing in 1942 demonstrated that swapping in the two-stage Rolls-Royce Merlin 61 (and its licence-built Packard V-1650) transformed the airframe into the long-range high-altitude escort that decided the air war over Germany. The V-1710-engined P-51A and F-6A variants soldiered on in low-altitude reconnaissance and ground-attack roles.

The V-1710 also powered the Bell P-63 Kingcobra, supplied in large numbers to the Soviet Union under Lend-Lease, and the experimental Curtiss XP-46 and Lockheed XP-49. The P-39 and P-63 lines together shipped roughly 9,000 airframes to the Soviet VVS, where Soviet pilots removed the 37 mm nose cannon's interrupter gear and used both types as low-altitude air-superiority and tank-busting fighters across the Eastern Front. Aleksandr Pokryshkin scored 59 victories on the V-1710-engined P-39 to become the second-highest-scoring Allied ace of the war.

Post-war the engine found a second life on the Reno Unlimited air-racing circuit, where modified V-1710s on Strega and similar P-51 racers produced 3,500+ hp on nitromethane fuel. The Allison's modular cylinder-block design, its accessible rear-drive accessory section, and its rugged simplicity made it a favourite of racers and restorers long after its wartime obsolescence. Roughly 70,000 V-1710s were built between 1937 and 1948, ranking it second only to the Rolls-Royce Merlin among Allied V-12 production figures of the Second World War.