of equivalent power. This empty weight does not include the radiator and coolant fluid. Generally, air-cooled engines are lighter than their equivalent horsepower water-cooled counterparts. For example, the Bentley BR.2 rotary put out and weighed , Clerget 9B rotary , . The new engine was presented to the French Ministry of War in February 1915, and tested for 15 hours at full power. This was standard procedure for a new engine design to be admitted into military service. However, because of lobbying by French engine manufacturers, the Spanish-made engine was ordered to undergo a bench test that no French-made engine had yet passed: a 50-hour run at full speed. The HS-31 was therefore sent back to Chalais-Meudon on July 21, 1915, and tested for 50 hours, succeeding against all expectations. The design also promised far more development-potential than rotary engines. This was despite being the most common type, then in use, for most aircraft. Also, rotary engines were getting close to the limits of their development at this time. Rotary engines of increased power generally had increased weight, which in turn increased the already serious gyroscopic torque generated by the engine's rotation. A further increase in torque was considered unacceptable, and the power to weight ratio of the new rotary engines under development did not appeal to aircraft designers. French officials ordered production of the 8A to be started as soon as possible and issued a requirement for a new single-seat high-performance fighter aircraft using the new engine. The Louis Béchereau-designed SPAD VII was the result of this requirement and allowed the Allies to regain air superiority over the Germans. Variants Some data from: British Piston Engines and their Aircraft Note: Hispano-Suiza company type numbers were prefixed by HS- or written in full as Hispano-Suiza Type 31, but military designations used the conventional system of Hispano-Suiza(engine manufacturer) 8(no of cylinders) A(engine series) b(variant) r(attribute), thus Hispano-Suiza 8Abr. 8 (HS-31) , initial production and test engines, with few applications, including early Nieuport 14s. 8Aa (HS-31) at 2000 rpm, entered production in July 1915. Early HS-8A engines were plagued with various problems which required further work and was the standard powerplant for early-production SPAD VIIs and the Curtiss "Jenny" JN-4H variants. The demand for the Hispano-Suiza engine was such that other manufacturers began producing it under licence, in France, Great Britain (Wolseley Adder), Italy (Nagliati in Florence and Itala/SCAT (automobile) in Turin)