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 the company's motorcycles. It was made in the 1,340 cc (82 cu in) displacement for Harley-Davidson Big V-twins bikes, replacing the Shovelhead engine until 2000 when the last EVO was placed in a production factory custom FXR4 (FXR2 and FXR3 were the first CVOs). In 1999, it was replaced by the Harley-Davidson Twin Cam 88 in the Touring and Dyna model and in 2000 in the Softail models. Also available in the Sportster model beginning in 1986, it was made in the 1,100 cc (67 cu in) displacement until 1988 and is still made in the 883 cc (53.9 cu in) and 1,200 cc (73 cu in) displacements for the Harley-Davidson Sportster, replacing the ironhead Sportster engine.

Most analysts consider the Evolution to be the engine that saved the reorganized Harley-Davidson company from certain bankruptcy. Harley-Davidson's official name for the engine was likely related to the company's attempt to reform its image following the 1981 management buyout from previous owner American Machine and Foundry (AMF).

Both the heads and cylinders of the Evolution engine are made from aluminum to reduce weight compared to a cast iron design. Air cooling efficiency is improved as aluminum is a superior thermal conductor to cast iron. The blocky rocker boxes (thus becoming nicknamed "block head" which never caught on), aluminum heads and cylinders (also referred to as "jugs") are the only part of the Evolution engine that can be said to be essential; the Big Twin and Sportster incarnations of the Evolution are significantly different.

The unit construction of the Harley-Davidson Sportster, which has essentially been unchanged since its inception as the side-valve 750cc "K" Model in 1952, was retained with the Evolution engine upgrade in 1986, resulting in a unique valve train configuration. Unlike almost any other engine in production today, the Sportster Evolution uses one cam per engine overhead valve, resulting in four individual, single-lobe, gear-driven camshafts. The cam lobes are thus all located one behind another, and pushrods are arrayed in pairs (front and rear) parallel to the cylinder axis as a result. This allows each lifter and pushrod to deflect from the cam lobes perpendicular to the lobe plane. This configuration is friendly to radical, high-output cams, making the Sportster Evolution a natural choice for the once Harley-Davidson owned line of Buell Motorcycle Company sportbikes from 1986 up to late 2009 (2010 model year).

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Wikipedia