Toyota is famous for its 2JZ engine. Mazda has the Wankel rotary. Subaru gets the “boxer.” These engines have earned our respect and admiration, even with their shortcomings. Honda has its own compromised engine which was an abject failure, but ended up being kind of a triumph anyway: the oval piston engine.
That engine was fitted to one of Honda’s most unsuccessful race bikes ever, the NR500, a bike that represented Honda’s ambition to displace two-stroke bikes as the darlings of Motorcycle GP with a four-stroke bike.
At the time, Honda’s motorcycle racing rivals were using two-stroke bikes that made a lot of horsepower, around 120 HP, but Soichiro Honda famously disliked them, comparing them to bamboo contraptions. So, Honda engineers came up with a four-stroke bike with an engine that had oval pistons, as the company describes:
Honda returned to the 500 cc class of the World Motorcycle Grand Prix series in 1979, following a twelve-year hiatus. The machine they had developed for their comeback - an entry in the World Grand Prix’s most prominent class - was the NR500, powered by a four-stroke, DOHC V-four engine. With its oval piston engine incorporating eight valves and two connecting rods per cylinder, plus an aluminum semi-monococque frame complete with an inverted front fork, the machine surprised everyone with its daringly innovative technologies.
Honda has detailed articles about the development of this engine in its archives, which are worth a read. One of my favorite bits in there is from Toshimitsu Yoshimura, who designed the engine. The engineer said, “When I look back at it, I’m not sure if we were experimenting with cutting-edge technologies or obsessed with foolish ideas.”
I think it’s fair to say that Yoshimura was right on both counts. The engine was both cutting-edge and foolish, but it’s still fascinating and the engineers had wildly ambitious hopes for the oval piston engine’s output. Maybe a little too ambitious, if the figures from Honda below are any indication:
With eight valves lined up atop the pistons, each supported by two connecting rods, the team’s new 4-cylinder engine looked like an 8-cylinder. According to Fukui’s calculation, the engine could potentially reach a maximum speed of 23,000 rpm and output of 130 horsepower. Therefore, the target output was set accordingly, at 130 horsepower.
In practice, the NR500 was so bad that it failed to qualify in some races and even needed a push-start at the beginning of the British GP race in 1979. Honda engineers learned that past a certain RPM threshold, it’s very hard to keep the engine from destroying itself:
The phenomenon of sudden disintegration was a significant obstacle, typically arising when the engine speed exceeded 10,000 rpm. The cause for this was the twisting of connecting rods. Unlike a regular piston, an oval piston has two rods. The rods would distort as the engine speed increased, pulling the piston pin out of its proper orientation and causing the parts to break.
After a lot of bad showings, the oval piston bikes would hand Honda just two victories. One was at the 1981 Suzuka 500-Kilometer race. The other was at a qualifying round for an international race, a five-lap heat race at Laguna Seca.
The engineers kept working and were finally able to exceed (or meet) the power figures they had hoped for when they came up with the design, with one oval piston engine making 135 HP in 1982, and another making 130 HP in 1983.
The last engine produced made 130 horses and revved all the way to 19,500 RPM, but the engine and bike did not compete. Honda had to admit the project failed.
I still think Honda failed upwards with its four-stroke project, because after about a decade, the oval piston engine went into one of the coolest bikes the company has ever released, the 1992 NR750. That bike would have never been possible without the NR500 preceding it.
The 1992 NR even wore the “Oval Piston” decals on its fairing, because, of course, the world should know about its weird engine. That bike had beautiful design overall. Just look at (and listen to) this glorious Honda:
The red fairing was rounder than our current bikes, while retaining some aero. The tail section was all there, which contrasts with today’s bikes, which in my opinion look incomplete now that everyone wants factory “tail deletes.”
The under-seat exhaust even reportedly inspired some Ducati designs. Also, look at its beautiful single-sided swing arm. And the sound! The oval piston design might have failed to beat its two-stroke rivals decades ago, but its one of the most memorable chapters in the company’s history, and it produced one of Honda’s best bikes.
