and Russia. Total production of the HS-8Aa amounted to some 6,000 engines. 8Ab (HS-34) at 2,100 rpm, increasing the compression ratio from 4.7 to 5.3, Birkigt was able to increase the power output . The 8Ab began replacing the 8Aa on SPAD VIIs in early 1917. 8Ac 8Ad (1929) bore x stroke, for take-off. 8B (HS-35) , compression ratio 5.3:1, geared at 0.75:1. The HS-36 was the 8B with a Lewis gun firing through the propeller boss. 8B twin (HS-39)Coupled 8B engines 8Ba at 2,300 rpm, low compression ratio of 4.7:1, spur geared at 0.585:1. 8Bb , compression ratio of 4.8:1, reduction gear 0.75:1. However the reduction gear system was fragile, and often broke down, sometimes with spectacular results ending up with the entire propeller, driveshaft and driven gear parting company from the airframe. Progressive refinement of the engine brought the available power to by the end of 1917. The 8B, 8Ba and 8Bb were used (a) to power the earliest versions of the S.E.5a, (b) along with the 8Bd, the SPAD S.XIII, (c) front-line active versions of the Sopwith Dolphin, and (d) several other Allied aircraft types, with its gear reduction easily identifiable in vintage World War I photos, from its use of a clockwise (viewed from in front, otherwise known as a left hand tractor) rotation propeller. 8Bc , compression ratio of 5.3:1, reduction gear 0.75:1. 8Bd , compression ratio of 5.3:1, reduction gear 0.75:1. 8Bda 8Be , compression ratio of 5.3:1, reduction gear 0.75:1. 8BeC (HS-38) The 8Be fitted with the SAMC Model 37 cannon, or a similar weapon, firing through the propeller boss. A reduction gear equipped power-plant with a resultant clockwise rotation propeller like the 8B, produced at 2,100 rpm. Two known weapons fitted were the SAMC with a rifled barrel and a smooth-bore cannon firing canister ammunition. The moteur-canon could fire a single shot at a time through the hollow drive shaft without propeller interference. This cannon mount required an "elevated" intake manifold design, bringing the intake "runners" straight off the inner surfaces of the cylinder banks to the updraft carburetor's plenum chamber. The engine was used on the SPAD S.XII. 8Ca/220 Cannon-equipped at 2,100 rpm with 5.3:1 compression. Given the company designation HS Type 38 8Cb/180 Cannon-equipped at 2,000 rpm with 4.7:1 compression. Given the company designation HS Type 44 8Cc/220 Cannon-equipped at 2,100 rpm with 5.3:1 compression. Given the company designation HS