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 were the successors to the Evolution engine ("Evo"), 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.
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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", dampening 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. This caused some controversy among Harley veterans, claiming that the "Harley vibes" were a part of their motorcycles. 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.
The engine design differed considerably from its predecessor the "Evo" although shared some design elements with the Sportster line. The 88 represents the displacement of the standard engine in cubic inches. The bore is 3.75 in (95 mm) and the stroke is 4.00 in (102 mm), meaning the actual displacement is 88.357 cubic inches (1,447.91 cc),[1] commonly rounded up to 1450 cc. The motor company released a 95 cubic inch upgrade kit a year later to increase the displacement to 1,550 cc. Likewise, the Twin Cam 96 is approximately 1,584 cc. The company has released 103-cubic-inch (1,688 cc) and 110-cubic-inch (1,803 cc) kits for the TC96.
| Change | Evo | Twin Cam |
|---|---|---|
| Displacement | 82 cu in (1,340 cc) | 88 cu in (1,442 cc) and 96 cu in (1,584 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 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 degrees. The engines were released for the 1999 model year. High operating temperatures have continued to plague the Twin Cam engines, although the engineering refinements have resulted in a reliable and smooth-running engine.
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 (2,360 cc) and more, producing well over 120 ft·lb (160 N·m) of torque and 120 hp (89 kW) 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 third-party firms to add value to their products and broaden their appeal. Since the Evo's release in 1984, though, the company has moved to catalogue thousands of accessories, including engine upgrades. The company 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.[2] With the Twin Cam, H-D was able to preempt 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 upgrades or complete motors.