Kick Starting a Harley Evo ? ? ?
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 (80 cubic inch) displacement for Harley-Davidson Big Twin bikes, replacing the obsolete 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.
Some long time Harley owners considered the Evolution to be something of a sell-out because it was more reliable, burned less oil, and required less routine maintenance and less frequent overhauls than previous engines, serving the rider with little or no mechanical knowledge or aptitude and making it too easy to buy into the Harley mystique. 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). 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.
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 due to the fact that 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 1994-1998 final configuration, with rockerbox and base gasket leaks fixed, it proved to be an incredibly robust, durable, and versatile power plant for all of the Big Twin platforms.
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