The Evolution engine (popularly known as Evo or Blockhead) is an air-cooled, 45 degree, V-twin motorcycle engine manufactured since 1984 by the Harley Davidson Motor Company. It was made in the 1340cc displacement for Harley-Davidson bikes, replacing the Shovelhead engine, until 1999 when it was replaced by the Twin Cam 88s. It was made in the 1100cc displacement and is still made in the 883cc and 1200cc displacements since 1986 for the Harley-Davidson Sportster, where it also replaced 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 reformed image following the 1981 employee-led buyback from AMF. The nick-name "Blockhead" is derived from the tradition of nick-naming Harley-Davidson engines based on their rocker box shape, which on the Evolution are distinctly rectangular.
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, which is both lighter than iron (reducing overall vehicle weight) and a superior thermal conductor to iron (improving air cooling efficiency). A problem is avoided thereby which occurs when the heads and cylinders are of different materials. They expand and contract at different rates which induces a relative motion; this motion ruins the gasket seal and necessitates replacement. In water-cooled vehicles the oil and water will often mix and contribute to lubrication failure and engine destruction. The blocky rocker boxes, 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.
Evolution on the Sportster
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 valvetrain 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 Harley-Davidson-owned line of Buell 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 1200 cc versions; the chief difference between the two is a much smaller bore on the 883 cc, along with slightly different heads. Conversions from 883 cc to 1200 cc are relatively inexpensive and commonplace. Until 2007, all Sportster Evolution engines (and in fact, all Sportster engines of any kind) were only built with carburetors; today, however, all new Sportster models come standard with fuel-injected Evolution engines.
Evolution on the big bikes
The Evolution Big Twin (herein referred to simply as "Big Twin") saw a fifteen-year run in Harley-Davidson's "big bikes," in the Dyna, Softail, FXR, and Touring frames.
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 valvetrain with lifters & 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 & 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 1998 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.