Harley-Davidson models. In the late summer of 1929, Harley-Davidson introduced its 45 cubic inches (737 cm³) flathead V-Twin to compete with the Indian 101 Scout and the Excelsior Super X. This was the "D" model, produced from 1929 to 1931. Riders of Indian motorcycles derisively referred to this model as the "three cylinder Harley" because the generator was upright and parallel to the front cylinder. The Great Depression began a few months after the introduction of their 45 cubic inch (737 cm³) model. Harley-Davidson's sales fell from 21,000 in 1929 to 3,703 in 1933. Despite this, Harley-Davidson unveiled a new lineup
Harley-Davidson Evolution engine until the introduction of the Twin Cam engine, the last of the line of single cam, overhead valve motors tracing their lineage back to the seminal Knucklehead design penned by founder Bill Harley. It is also the largest aftermarket supported Harley-Davidson design, to date, with clones of the engine being produced by third parties like S&S Cycle and several others. Harley reverted to a single cam design with the Milwaukee-Eight engine, first introduced for touring models in 2016. Harley-Davidson Evolution engine The Evolution engine (popularly known as Evo) is an air-cooled, 45-degree, V-twin engine manufactured from 1984 by Harley-Davidson for