In the 1970s, Honda developed an innovative engine with oval pistons to compete in the Campeonato Mundial of Motovelocidade, currently MotoGP. The NR500 project sought to overcome the dominant two-stroke engines with a four-stroke solution. The initiative came from founder Soichiro Honda, who rejected two-stroke engines as he considered them noisy and inefficient. The 500cc V4 engine adopted oval pistons to allow eight valves per cylinder, totaling 32 valves and eight spark plugs.
The debut took place at the Grã-Britain GP in 1979. The two motorcycles entered did not complete the race and were well behind their two-stroke rivals. The limitations of the rules at the time restricted it to four cylinders and 500 cc, forcing the Honda to seek extreme revs. The engine initially achieved around 100 hp at 16,000 rpm, but the goal was 130 hp at 23,000 rpm to match the power of competitors.
Technical challenges of the NR500 project
Oval pistons required special rings for perfect oblong-shaped sealing. Machining required minimum tolerances, pushing the technologies of the time to the limit. Cada piston had two connecting rods to balance the movement, increasing the weight and reciprocal mass. The long pins suffered from flexion under high rotations, creating durability problems.
Honda continued development in the following seasons. In 1981, the Freddie Spencer driver achieved the best result, finishing fifth in the Grã-British GP before retiring due to mechanical failure. The bike won victories in events such as the 500 km Suzuka race in 1981 with Kengo Kiyama. Apesar of the advances, the extra weight of the complex engine prevented consistent competitiveness.
Engine evolution and transition to two-strokes
Modified versions of the engine, such as the 2X in 1982 and the 3X in 1983, achieved 135 hp and 130 hp, respectively. Engineers incorporated lightweight metals like magnesium and titanium to reduce weight. The slipper clutch emerged as an innovation to control reverse torque. The Honda abandoned the NR500 project in early 1982, focusing on the three-cylinder, two-stroke NS500, which won the world title in 1983.
Innovations that lasted in motorcycling
The project leveraged precision machining technologies and exotic materials. The slipper clutch and inverted forks have become common on modern motorcycles. Lessons from the NR500 influenced the development of Honda V4 engines in later competitions.
The NR750 street version
In 1992, Honda launched the NR750 as a limited edition for public roads. Produzida in around 300 units, the bike used elliptical pistons with curved sides and reached 125 hp at 15,000 rpm. The high price, equivalent to around US$60,000 at the time, meant it was destined for collectors. Hoje, rare specimens reach high prices at auctions.
The NR750 paid homage to the racing project and demonstrated the viability of the technology off the track. The design incorporated electronic injection and high performance elements.
Legacy of oval piston technology
Honda’s boldness in defying conventions marked the history of motorcycling. Apesar despite not having won championships, the NR500 proved that radical innovations can generate lasting advances. The experience influenced generations of engineers and contributed to the brand’s later dominance in four-stroke categories.
