Evolution On The Big Bikes
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 & 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 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.
Read more about this topic: Harley-Davidson Evolution Engine
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