The Evolution engine 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 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 1999, it was replaced by the Harley-Davidson Twin Cam
88 in the Touring and Dyna model and in 2000 in the Softail models. Also
available in the Sportster model beginning in 1986, it was made in the
1,100 cc displacement until 1988 and is still made in the 883 cc and 1,200 cc
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.
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, aluminum heads and cylinders 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 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 parallel to the cylinder axis 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 late 2009.
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 system. 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. Sixteen year run if you include the 2000 FXR4 Limited production
CVO. 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 is exactly
perpendicular to the vertical plane formed by the cam lobes. 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 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. See also
Harley-Davidson engine timeline References
External links Detailed Evo Engine Animations