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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). In 2001, it was replaced by the Harley-Davidson Twin Cam 88. 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).
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. Air cooling efficiency is improved as aluminum is a superior thermal conductor to cast iron. The use of aluminum for both heads and block reduces wear on the head gasket seals, compared to a combined cast iron and aluminum design which could induce stress on the head gasket seals as a result of different rates of expansion and contraction between dissimilar metals. The blocky rocker boxes (thus becoming nicknamed "block head"), 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 quasi-unit construction of the Harley-Davidson Sportster, which has been part and parcel with the highly successful model line since its inception, 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 parallel fashion 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 2003.
The Sportster Evolution engine has remained largely unchanged from 1986 to 2006, though changes to the transmission, final drive and motor mounts have necessitated changes to the Sportster Evolution case. Construction is almost entirely the same between 883 cc and 1,200 cc versions; the chief difference between the two is a smaller bore on the 883 cc, along with slightly different heads. Conversions from 883 cc to 1,200 cc are relatively inexpensive and commonplace, and cheaper than the price premium to go from an 883 to 1,200 engine on a new bike. Carburetors were standard on Sportster engines until 2007, when they were replaced by the Delphi Electronic Sequential-Port Fuel Injection (ESPFI) system.
The Evolution Big Twin saw a fifteen-year run in Harley-Davidson's "big bikes," in the Dyna, Softail, FXR, and Touring frames, and is basically a newer style top end and stroker flywheel assembly upon the slightly modified shovelhead case.
The Big Twin utilizes a single, four-lobe, gear-driven camshaft located just above the crankshaft axis. While this simplifies camshaft replacement, it complicates the Big Twin valve train with tappet/lifters and pushrods that each deflect from the camshaft at wildly different angles. The Big Twin pushrods have a distinct helical appearance because the vertical plane formed by each cylinder's rockers (front-to-back) is exactly perpendicular to the vertical plane formed by the cam lobes (left-to-right). The need for one lifter and pushrod set to reach all the way out to the most outboard cam lobe gives rise to the Big Twin's tell-tale offset lifter assemblies, where the forward lifter assembly is located slightly farther out and rotated to enable the valve gear to make the reach.
The Big Twin has been accompanied by a number of different primary drives and transmissions, both on production Harley-Davidson motorcycles and in custom applications. The aftermarket selection of accessories for these closely related systems is wide, as it is for the engine itself.
The Evolution Big Twin motor is, for now, the last of the line of single cam, overhead valve motors tracing their lineage back to the ground breaking Knucklehead design penned by founder Bill Harley. In its 1994 to 2000 (FXR4) final configuration, with rockerbox and base gasket leaks fixed, it proved to be a robust, durable, and versatile power plant for all of the Big Twin platforms. It is also the largest aftermarket supported Harley-Davidson design, to date.