Completing the Oval Piston Engine

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The NR500 (2X) machine that helped Kengo Kiyama to win the 1981 Suzuka 500-Kilometer Race, giving Honda its first victory with the oval piston engine



<< 1. A 4-stroke Engine: The Natural Choice for a Comeback
<< 2. We Want to Create the Best Engine
<< 3. Will the Engine Really Rev?
<< 4. A Miserable Debut
<< 5. Completing the Oval Piston Engine
<< 6. In a Drawer, a Piece of a Dream
 


Key problems in the team's engine design included the gear train and valve system. In the former, a reduction gear was initially used to turn the cam at half the rate of crank revolution. However, since it had been a source of frequent failures, a normal cam-reduction gear train system was adopted. Still, the problem refused to disappear. Numerous options were tried, after which the team came up with the idea of a rubber damper that would mesh with the cam gear. Fortunately, the design was a success, allowing the valve system to turn properly. Moreover, it resulted in higher power output.

Additional areas of concern were the over-effectiveness of engine braking and a sudden burst of power when the throttle was opened (the so-called "bang"). The problem of engine braking was quickly resolved through the use of a device called a "back-torque limiter." However, the team couldn't find an absolute solution for the "bang."

The NR500 improved slowly but steadily, thanks to the team's dedicated effort. In 1982, their 2X modified engine achieved 135 horsepower, and in 1983, a 3X unit demonstrated output of 130 horsepower. The oval piston engines were at last on par with their rivals, at least in terms of output.

Despite their enhanced output, the performance of Honda's engines was not so impressive on the track. Even though the Honda team earned a victory in the 1981 Suzuka 500-Kilometer Race using an oval piston engine, that was to remain their only triumph. The World Grand Prix series was an ongoing struggle.

Weight was a major handicap. Since the four-stroke engine required a larger cylinder head, its weight was greater by around 20 kg. The added mass surrounding the head also affected the machine's center of gravity and overall balance. Measures were taken in order to reduce weight, including the replacement of iron with titanium and aluminum-already a light material-with magnesium. However, the precious improvement gained was quickly lost, as Honda's rivals began using the same approach.

Other measures were taken to save weight. These included a reduction in the thickness of the outer crankshaft, which was subject to a relatively minor dynamic load. Often, before a race, the staff would labor overnight, grinding the parts with a pencil grinder. Still they were unable to overcome the weight disadvantage. It was a problem that would not be completely solved until a successor to the NR500 could be built.

Three years had passed since Honda's highly anticipated return to the World Grand Prix series, but the NR500s had yet to win a race on the international circuit. Still, no losing streak could last forever, and Honda knew it. There was increasing pressure, both in Japan and elsewhere, for Honda to take the checkered flag.

As a compromise in the effort to get on the winners" podium, in 1982 GP series Honda introduced NS500 machines powered by two-stroke engines. The NS machines gradually replaced the NRs and, with that, came to play a dominant role on the world stage of motorcycle racing.
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