Soichiro Honda was a man of conviction. When the FIM announced in 1968 that the 500 cc engine class was limited to four cylinders, that gave a significant advantage to teams utilizing two-stroke engines. Honda made it clear that he was committed to the cleaner burning four-stroke engines as was covered in this article on the 1966 Honda RC 166. This forced Honda's hand. They withdrew from motorcycle racing to focus more attention on the automotive side of the business.
Two-strokes continued to dominate Grand Prix motorcycle racing into the late 70s, but Honda was convinced they could develop a high tech four-stroke race bike solution. In April 1978, after a nearly 12 year hiatus, Honda announced publicly it would return to Grand Prix motorcycle racing, specifically utilizing a four-stroke.
The two-stroke engines of the day were producing around 120 horsepower. While there is more to winning than horsepower, the development team knew their goal was to exceed the competition's output. Since the four-combustion chamber limitation prevented the same innovative approach of adding pistons used on the RC generation, the team decided to go another route. For a four-stroke engine to be as powerful as a two-stroke unit with the same piston count, it basically has to double its normal rpm. To achieve that, the team had to dramatically improve the intake efficiency and design a valve system with higher resistance to friction and heat buildup at high revolutions.
Honda's idea was a 32 valve V8 with each pair of cylinders having linked combustion chambers. A creative interpretation of the rule. Eventually that design idea evolved into a four cylinder with oval-shaped pistons. That allowed each cylinder to house the eight valves and two spark plugs they determined were needed. In a sense, it was a high tech DOHC V8 head design, but on a V4 cylinder block. According to their calculations, the eight-valve oval-piston engine would offer an estimated output of 23,000 rpm and 130 horsepower.
Toshimitsu Yoshimura, an engineer charged with development of the NR500's oval piston engine recalled "The reason was simply that we were all so young. We had nothing to fear. You could even say we had no preconceived notion that a piston had to have a circular cross-section. We were determined that the oval design was the key to outperforming two-stroke engines."
They quickly ran into issues. Each piston had two connecting rods, and at engine speeds over 10,000 rpm, they would distort, causing wrist pin failure. Even something as simple as the piston rings became not only a multiple iteration design exercise, but one of manufacturing capability. To attain their target, they needed tolerances higher than the equipment of the time could produce.
One issue at a time, they persevered. In 6 months from kick-off, they had a viable test buck of a single oval piston engine. By April 1979, they had version 0X, their first 100 degree V4 engine ready for testing. They were anxious to get out of the lab. They targeted the 11th race of the season, the British Grand Prix race at Silverstone. The engine was already producing 100 horsepower at 16,000 rpm, but they had a long way to go.
"The emphasis was to create a difference-not just any difference but the difference that would work to our definite advantage. That's why we decided that Honda should go with four-stroke engines. We wanted to achieve our target through innovative technology, and in so doing have the edge over our competition... When I look back at it, I'm not sure if we were experimenting with cutting-edge technologies or obsessed with foolish ideas. At least we were doing something that was beyond the realm of conventional thinking. I'm not just talking about us, who were designing the engine, but also those who were creating the body," shared Yoshimura.
In addition to the highly exotic engine, the NR 500 also utilized a monocoque aluminum body, and inverted forks - a standard practice on most sport bikes today. Honda also used 16 inch wheels versus the more common 18 inch in order to reduce aero drag.
The NR 500 made the anticipated appearance at the British Grand Prix. Both riders Mick Grant and Takazumi Katayama barely qualified. In the race, Grant fell in oil from his own bike at the very first corner, and retired. Katayama also retired after several laps because of engine trouble. The season didn't end much better. The next and final race, the French Grand Prix, neither even qualified.
They needed to shed weight. It wasn't only an impediment to the spinning engine parts, but the complex four-stroke head added nearly 45 pounds to their bike versus the competition. That negatively impacted the bike's center of gravity and balance. The team got aggressive with materials. Iron was replaced with titanium. Aluminum was replaced with magnesium. They started to see forward movement, but it was short-lived. It wasn't long before other teams started using the same materials in their engines as well, which only increased their advantage.
For three years, Honda refined and improved the NR500. It won the 1981 Suzuka 500 km race, but had yet to win a race on the international Grand Prix circuit. The pressure on Honda was growing. In 1982, Honda introduced NS500, their brutally powerful two-stroke bike. It was much lighter and better handling than the NR500. They ran both versions of the bike for 1982 and 1983.
The 3X version of the NR oval-piston engine for 1983 was the last of the oval piston competition engines. It produced a stout 130 hp from 19,500 rpm from a 500 cc four stroke. They came very close to beating the Kenny Roberts Yamaha, lreading him for a while at their final race in Laguna Seca, but Roberts was too much. Plus they were getting beat by their own NS500 two-stroke V3 engine-powered bike. Freddie Spencer went on to win the 1983 500 cc Grand Prix Motorcycle World Championship aboard the NS500. In the twenty seasons from 1982 through 2002 that Honda ran two-stroke engines in the NS and NSR platforms, they won 11 of them. The NR5
While they hadn't won a race in the Grand Prix series, they had hit their performance goals. "Although it couldn't win a race," said Yoshimura, "the 3X was very close to the complete form of an oval-piston engine, achieving more than 95-percent maturity... The engine was designed for racing, so we wanted it to be a winning design. If we had won at Laguna Seca, we could have been content with that and put a more peaceful end to the engine's racing history."
For years, the oval piston lived on in the Honda NR750 street bike, introduced in 1992. Yoshimura, reflecting on their great experiment, said, "To create anything, you must put your heart and soul to it."