Twin Cam engine (Harley-Davidson)

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The Harley-Davidson Twin Cam 88 engine was released for the 1999 model year in September of 1998. The engine was a successor to the Evolution engine ("Evo"). The engine design differed considerably from it's predecessor the "Evo" although shared some design elements with the Sportster line. The 88 represents the displacement of the standard engine in cubic inches. 88 cubic inches equals approximately 1450 cc. The motor company released a "95 inch" upgrade kit around the same time to increase the displacement to 1550 cc.

[edit] Differences to predecessor (Evolution engine)

Larger displacement

The stock Evo has a displacement of 1340 cc's. The stock Twim Cam displaced 1450 cc's.

Internal oil pump

The Twin Cam oil pump is connected directly to the drive shaft. It also uses a more efficient and higher volume design.

Twin Cams

The Evo motor uses a single cam with four lobes to control all four push rods. The Twin Cam used one cam per cylinder. The allows the push rods to be better aligned with the rocker arms.

Chain driven cams

The Evo used gears to drive the cam. The Twin Cam used a "silent chain". Many users installed after market gear driven replacements to increase timing accuracy.

Transmission attachment

The Twin Cam motor attached directly to the transmission casing.

Internal oil tank

The Twin Cam holds it's oil reserve under the transmission housing. Essentially the oil tank, transmission, and engine cases and all bolted together as a single unit.

Bathtub combustion chamber

The combustion chamber shape was changed from the Evo's "D" shape to a more efficient "bathtub" shape.

No wasted spark

The ignition system uses an electronic ignition with no wasted spark. The plug sized went from 14mm in the Evo to 12mm.

Better cooling

To increase cooling more fin areas was used and a oil jet was added to spray the bottom of the pistons.

The Twin Cam generated considerably more horse power and torque than the Evo. All components were also built to tougher standards. Upgrading to 1550 cc's became the norm among most enthusiasts.

When the Twin Cam engine was released, Harley-Davidson put much emphasis on the supposedly stronger case and bottom end. They claimed the older Big Twin design, with the cam bearing positioned directly above the main bearing on the right side of the engine, precluded increasing main bearing size to optimal and created structural weakness in the right side case because of the small space between the two bearings. However, after market companies have successfully used the old design in engines displacing 144 cubic inches and more, producing well over 120 foot pounds of torque and 120 horsepower without case failure, casting doubt on Harley-Davidson's claims.

When the Evolution Big Twin engine was released, Harley-Davidson did not protect its design elements as thoroughly as they have with the Twin Cam. In fact, H-D relied on the after market to add value to their products and broaden their appeal. Since the Evo's release in 1984, though, H-D has moved to catalogue thousands of accessories, including engine hop-up parts and services. The Motor Co. moved to the Twin Cam not because the Evo had reached its power limits as a design, but because H-D could not prevent other manufacturers from making virtual clones of the design. With the Twin Cam, H-D was able to pre empt cloning via the U.S. Patent Office, thereby making it a lot more difficult and expensive for the after market to compete with the Motor Co. in the development and sale of hop up products and services, or complete motors.

[edit] Wear of cam tensioner shoes

The Twin Cam motor has cam chain tensioners with nylon composite shoes, and these have proven to be a wear item even though they are not on the regular maintenance schedule. Some riders have seen these shoes wear down until there was metal to metal contact against the cam chain, while others have experienced no wear. Harley-Davidson changed from spring loaded to hydraulically loaded tension on the shoes with the 2007 model year, but shoe wear and the variability of cam timing introduced by inconsistent tension on the cam chain continue to be an issue best resolved by after market gear drive cam sets.

[edit] See also

Harley-Davidson engines
Engine	1900s	1910s	1920s	1930s	1940s	1950s	1960s	1970s	1980s	1990s	2000s
One-cylinder engines
unnamed	1903–1910
unnamed			1926–?
Big twin engines
F-head (IOE)		1914–1929
Flathead			1929–1936
Knucklehead				1936–1947
Panhead					1948–1965
Shovelhead							1966–1985
Evolution									1984–1999
Twin Cam										1999–
Sport engines
Sport		1919–?
XA					1942
WR					1946-?
K						1952–1956
Ironhead						1957–1985
Evolution									1985–
Water-cooled engines
Revolution											2002–