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 CVO's).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.

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.

Above, a color-coded approximate diagram of the Sportster Evolution valve train superimposed over an image of a Sportster Evolution.

Crank output is purple; cams are red; pushrod/lifters are yellow; rockers are blue; valves are dark green, with seats shown in light green.

The unit construction of the Harley-Davidson Sportster, which has been part and parcel with the highly successful model line since its inception as the sidevalve 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.

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).

Though a major design advance for Harley-Davidson in many ways, the Evolution is most distinct from earlier Harley-Davidson engine designs by virtue of its reliability, oil tightness, and ability to be run hard under all kinds of circumstances for tens of thousands of miles farther than any of its predecessors.

Both the heads and cylinders of the Evolution engine are made from aluminum to reduce weight compared to a cast iron design.