Curtiss D-12 engine

Curtiss D-12 engine at the National Museum of the United States Air Force in Dayton, OH -- (U.S. Air Force photo)

The 375-hp Curtiss D-12 engine, introduced in 1921, was a liquid-cooled V-12. It was one of the most  successful aircraft engines of the 1920s. 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. However, as demonstrated by the Macchi M.33 in the 1925 race that was powered by a 1923 Curtiss D-12 engine rated at 507 horsepower, total horsepower was often the deciding factor in the Schneider races..