Harley-Davidson seems to work on a completely different clock, with new engine designs arriving every 15 years or so. Between 1936 and 2003, engine designs released by Harley represented a constant tweaking of the same basic V-twin, 45-degree, air-cooled engine design. In 2001, Harley released its first truly new design in a commercial motorcycle, yet it was still a V-twin. If you ignore the first few years of the company's history as a period of experimentation, there really have been only seven major engine revisions during the company's 100 year existence:

Flathead engines - Manufactured between 1909 and 1974. Flatheads did not have overhead valves. Instead, the valves ran alongside the engine and opened upwards into a chamber beside the combustion chamber. The advantage of a flathead was simplicity -- no pushrods or rocker arms, and the head was a simple casting with a hole in it for the sparkplug. A typical Flathead engine had a displacement of 45 cubic inches (742 cc) and produced about 22 horsepower.

Knucklehead engines - Manufactured between 1936 and 1947. Knucklehead engines were the first overhead valve engines produced by Harley-Davidson. The knucklehead came in 60 cubic inch (990 cc) and 74 cubic inch (1,200 cc) variations able to produce 40 and 45 horsepower respectively.

Panhead engines - Manufactured between 1948 and 1965. The panhead also came in 60 cubic inch (990 cc) and 74 cubic inch (1,200 cc) variations and produced 50 and 55 horsepower respectively. Big differences between the knucklehead and the panhead included aluminum heads on the panhead and internal oil lines, as opposed to external lines on the knucklehead.

Shovelhead engines - Manufactured between 1966 and 1985. Shovelheads displaced 74 cubic inches (1,200 cc) and produced 60 horsepower.

Evolution engines - Manufactured between 1984 and 1999. Displacement is 81.8 cubic inches (1,340 cc), and the engine produces 70 horsepower. Although the Evolution 1340cc is no longer in production, the Sportster® model line of motorcycles receives Evolution engines with 883 cc and 1200 cc displacements (manufactured 1986 to present) .

Twin Cam 88 engines - Manufactured starting in 1999. The Twin Cam gets its name from the fact that it has two cams in the crankcase to activate the valves. At 88 cubic inches (1,450 cc) of displacement, it is the largest production Harley motorcycle engine, and it produces 80 horsepower. The engine remains air-cooled, and uses overhead valves activated by pushrods. The 88B version of the engine, which came out in 2000, contains counterbalancing shafts to reduce engine vibration.

Revolution engines - Manufactured starting in 2001. The Revolution engine is currently used on only one Harley production model -- the VSRC. While all of the engines previously mentioned are largely the same and represent incremental improvements, the Revolution engine is different. This engine is water-cooled rather than air-cooled and its V angle is 60 degrees rather than 45. It has four overhead cams rather than two cams in the crankcase and is fuel injected. This engine is smaller -- only 69 cubic inches (1,130 cc). It has a much shorter stroke, allowing it to rev to 9,000 RPM and it produces 115 horsepower.