Curtiss D-12 The 375-hp Curtiss D-12 engine, introduced in 1921, became one of the most successful aircraft engines of the 1920s. Developed from Curtiss' K-12 engine used in World War I, the D-12 replaced the gears connecting the crankshaft to the propeller with a more reliable direct-drive connection, hence the "D" for direct-drive. Also, Curtiss attached a new propeller designed by acoustics engineer Dr. S.A. Reed. It could rotate at a higher speed than conventional propellers, allowing the engine to use its full power. This combination made the D-12 the most advanced power plant in the world. After powering the famous series of Curtiss racers to numerous victories in the early 1920s, the D-12 influenced inline military and racing engines through the start of World War II. The Army installed them in such fighter aircraft as the Curtiss PW-8 and Boeing PW-9. TECHNICAL NOTES: Type: Liquid-cooled, V-12 Displacement: 1,145 cu.in. Maximum hp: 375 (435 in later models) Maximum rpm: 2,000 (2,300 in later models) Click here to return to the Early Years Gallery.