Harley-Davidson Twin Cam engine
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The Harley-Davidson Twin Cam 88 engine was released for the 1999 model year in September 1998. The Twin Cam 96 was released for the 2007 model year. Although these engines differed significantly from the Evolution engine, which in turn was derived from the series of single cam, overhead valve motors that were first released in 1936, they share a number of characteristics with nearly all previous Harley-Davidson engines. Both engines have two cylinders in a V-twin configuration at 45 degrees, are air-cooled, and activate valves with push-rods. The crankshafts have a single pin with a knife and fork arrangement for the connection rods. These are sandwiched between a pair of flywheels.
88B and 96B Engines
The Twin Cam wasn't initially used in the Softail model family before the year 2000. This was due to the chassis design and vibration transfer to the Softail frame as a result of the direct (hard) mounting of the engine. Dyna models are "rubber mounted", damping the majority of vibration transfer to the frame and rider. Harley solved the issue for the Softail family by designing an engine variant known as Twin Cam 88B. It is basically the same engine as the original (now called "Twin Cam 88A"), but with a modified engine block design that incorporates twin chain-driven counter-balancers. The Twin Cam 96B engine was released at the same time as the Twin Cam 96A model, for the 2007 model year, and is currently used on all Softail models.
Differences from predecessor (Evolution engine)
The engine design differed considerably from its predecessor the "Evo" although shared some design elements with the Sportster line. The 88 represents the displacement in cubic inches of the standard engine in cubic inches. The bore is 95.3 mm (3.75 in) and the stroke is 101.6 mm (4.00 in), meaning the displacement is 88 cu in (1,449 cc). The Twin Cam 96 displaces 96.7 cu in (1,584 cc). The company released 103 cu in (1,690 cc) for 2012 Softail models and 110 cu in (1,802 cc) for Screamin' Eagle/CVO Models.
|Displacement||82 cu in (1,340 cc)||88 cu in (1,449 cc) 96 cu in (1,584 cc) 103 cu in (1,690 cc) 110 cu. in. (1,803 cc)|
|Oil pump||external. Connected through a series of gears.||internal twin-gerotor. Connected directly to the right side pinion shaft. This pump is more efficient, maintains a higher pressure and larger volume.|
|Cams||Single, with 4 lobes||One per cylinder, each with 2 lobes. This allows the push rods to be better aligned with the rocker arms.|
|Cam drive||gears||silent chain. This change was reported as necessary to meet EPA noise requirements. Many users installed after market gear driven replacements to increase timing accuracy.|
|Transmission attachment||Displaced from motor||Transmission casing is attached directly to the engine|
|Oil tank||Usually surrounds battery box under the seat||Below and behind the transmission assembly (except Softails, which are same as noted for the Evo)|
|combustion chamber||"D" shape||"bathtub" shape. Allowed for more efficient combustion and has higher compression.|
|spark firing||wasted spark (both plugs fire at the same time). The ignition system uses single coil. plugs are 14 mm (0.55 in)||no wasted spark. The ignition system employs dual coils. plugs are 12 mm (0.47 in).|
|cooling||air-cooled||also air-cooled. Increased cooling fin areas and an oil jet that sprays the bottom of the pistons provide additional cooling capability.|
Design Issues and Recalls
Design and development problems
In the book The Harley Davidson Century (edited by Darwin Holstrom, Barnes and Noble Press, 2005) Steven Anderson, detailing the Twin Cam story in the chapter "The Twin Cam Era," explains the huge development problems Harley-Davidson had while trying to bring this engine to market. The early prototype Twin Cam engines had considerable trouble with the oiling system. These problems delayed release of the engine as scheduled for the 1997 model year. When the engines were run, oil came out any gasketed joint as well as the breather. Harley sought the help of Paul Troxler, a young engineer from Southwest Research Institute and eventually the problem was traced to a design which drained the cam case into the crankcase, and used a single scavenge pump. Due to airflow through ports in the crankcase wall, the cam case wasn't draining properly. After much testing, the solution was to seal the cam case from the crankcase, and use a dual scavenge system. However, oil was still not scavenging properly from the crankcase, and this was traced to an acoustic phenomenon due to the caliber of the scavenge inlet. Restricting the diameter of the inlet, a counter-intuitive solution, solved that problem.
A more serious problem was that of overheating. Piston temperatures in particular were troubling. Nicolae Glaja, a Romanian-born engineer, initially tackled the problem by installing oil jets in the crankcase, aimed so that they sprayed the piston crown from underneath. While this cooled the pistons, the oil temperature then became intolerably high. Skip Metz, engineering project leader for the engine, and his team then came up with installation of an oil cooler, with good results. However, the styling and marketing departments felt that this fix looked too much like a band-aid solution. It seemed to be an admission that Harley-Davidson had designed an engine that ran too hot. Management agreed, and the 1998 release date was also passed. Returning to the drawing board, the engineers examined the entire oil system. The excessive oil temperature was not just the result of heat coming from the piston crowns, but from the cylinder head and rocker boxes. Engineer Ben Vandenhoeven then initiated a series of test runs restricting oil flow to specific areas of the engine, with surprising results. In the initial design the concept was that flowing large amounts of oil through the rocker boxes (much more than was actually needed for lubrication) would help cool the heads. In actuality, not much cooling was provided for the heads. Rather, the heads were heating the oil. The oil flow to the rocker boxes was cut down to about one-sixth of the initial design flow, still adequate for lubrication, and this brought the operating oil temperatures to around 220 °F (104 °C). The engines were released for the 1999 model year.
Recalls and other mechanical problems
Wear of cam tensioner shoes
The Twin Cam motor includes 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 in the Dyna line for the 2006 model year and for all Twin Cams with the introduction of the 96-cubic-inch Twin Cam for the 2007 model year. Some believe shoe wear and the variability of cam timing introduced by inconsistent tension on the cam chain continue to be an issue best resolved by third-party gear drive cam sets, such as those made by S&S Cycle and other aftermarket firms.
Cam retention bolt recall
In 1999 a recall was made to replace the bolts that hold the cams in place. The bolts were replaced with Grade 5 bolts to prevent a catastrophic shearing.
Notes and references
- Petersen, Donny (2011-01-19). "VII". Donny's Unauthorized Technical Guide to Harley-davidson, 1936 to Present: The Twin Cam. iUniverse. pp. 307–. ISBN 9781450267724. Retrieved 6 March 2013.
- "2006+ Twin Cam 96" Motor to EVO Frame Adapter Plate :: Twin Cam Engine Parts :: Motorway Engineering Co.". Vulcan Engineering. Retrieved 6 March 2013.