Only Honda could have done the oval-piston NR, says Hirano and he's right of course. Even after two decades, the NR represents unsurpassed engineering brilliance...
Honda produced just 300 units of the NR and that was back in 1992 – more than two decades ago – and yet, for us, that bike remains one of the most intriguing, deeply fascinating motorcycles ever built anywhere in the world. Some of that fascination is down to the NR’s oval-piston engine, of course – the NR’s massively complicated four-cylinder 750cc engine had 8 valves per cylinder and two conrods per piston, which allowed the engine to function as a V8. No, honestly, we don’t really understand how it worked, but the fact that it did, and that Honda actually built a streetbike – one that cost US$50,000 back in 1992 – around this engine was some kind of a miracle. And it really doesn’t matter that with 125bhp at 14,000rpm, the NR engine’s output doesn’t really look anything special today, when compared to modern-day 750cc sportsbikes.
“When I look back at it, I’m not sure if we were experimenting with cutting-edge technology or obsessed with foolish ideas,” says Toshimitsu Yoshimura, speaking to Inspire magazine. “We didn’t think much about whether the engine would actually turn over or even whether it would be practical at all. We weren’t worried about those things since we just wanted to make it work. To create anything, you must put your heart and soul to it. The development of oval piston engines impressed that upon me, as well as on the other young engineers,” adds Toshimitsu, who was one of the engineers who actually worked on the development of the Honda NR’s oval-piston engine.
Toshimitsu’s story is taken up by Makoto Hirano, an R&D engineer who used to be in charge of engineering and production at Honda. “The oval piston’s development started from the racing scene. At that time, during the 1970s, across the world people were starting to be aware of the problems related to pollution. Therefore Honda wanted to try to reduce engine emissions by building a four-stroke racing bike. In those days, the other manufacturers were only building two-stroke engine bikes,” says Hirano.
“The idea of the oval piston came from the need to reduce piston friction in the four-cylinder engine… we had to make more cylinders in the four-stroke-cycle engine and an extremely short piston stroke. So we built a ‘one-cylinder’ which is equal to two cylinders. Which means, to build an eight-cylinder in order to fit the four-cylinder rules limit on the race,” says Hirano. “There were no suppliers who could collaborate with us to make oval piston rings. They said, ‘Oval piston rings won’t go mainstream. Furthermore, we have no special equipment.’ But we strongly requested them many times,” he adds.
“Incidentally, the VTEC (variable valve timing and lift electronic control) was invented during that period. During development of the oval piston, we tried to have the eight valves to four valves. We studied about losing the engine’s ‘startability’ to improve the combustion and to increase the horsepower in the middle-to-low rpm area. A test engineer found this by coincidence and this innovation led to the idea of the engine of the CBR400. If Honda had given up the oval piston development, there wouldn’t have been any Honda VTEC,” says Hirano.
“Honda’s oval piston had extremely high performance alongside that of the round piston engine. Therefore the race regulations at that time prohibited using the oval piston, first in Formula 1 and then in MotoGP. So Honda could not use this technology and could not feedback to the production. But what I can say for sure is that only Honda could develop this technology. The oval piston proved Honda’s highly advanced technology to the world. It was an epoch making technology,” concludes Hirano.
Source: LCR Inspire