Welcome to Thursday Trivia where we offer up a historical automotive trivia question and you try and solve it before seeing the answer after the jump. It’s like a history test, with cars! This week’s question: What was the most unique feature of Honda’s NR500 Grand Prix Motorcycle engine? If you think you know the answer, make the jump and see if you’re right. Honda has always ben the master of that tricky balance between pushing forward innovation and maintaining a steady stream what just plan works. Their consumer offerings – both bikes and cars – over the years have shown this balance to work. The original Civic was a very straight-forward design, inexpensive to build and to buy, but could be had with a marvelous and advanced CVCC engine that met the then emerging emissions standards while still offering reasonable power and exceptional fuel economy. Of course in competition you need to innovate in order to keep an edge, and here Honda has almost always pushed ingenuity over the status quo. At one time they didn’t, running in the Grand Prix Motorcycle series with four-strokes at a time when the two-stroke was dominant. Honda had run four-strokes with success for so long that the company felt it was an emblematic part of their heritage. There was even a fear that the switch to two-strokes would be an insult to company founder, Soichiro Honda, who once derided two strokes as nothing more than “bamboo tubes.” The choice then wasn’t to switch to two strokes, but how to innovate the company’s four strokes to beat them. And, in 1978, after a 12-year absence, Honda announced that it would return to the 500-cc World Grand Prix series It wasn’t just the competition that stood in Honda’s way, the rules too worked against them. The company’s engineers felt that power could be had through improved breathing, and that meant more valve area. More valve area required bigger bores – affecting rotational inertia – or alternatively, more pistons. The World GP rules however, were limited to engines with a maximum of four cylinders. Honda found an innovative way around that. From Honda Worldwide History:
…for a four-stroke engine to be as powerful as a two-stroke unit, it has to achieve twice its normal rpm. To achieve that, the team had to enhance the intake efficiency and design a valve system with higher resistance to friction and heat buildup at high revolutions. Given these conditions, the idea was born to double the number of valves to eight. As they examined the potential valve positions in the context of their four-stroke engine layout, the team came up with an idea of changing the piston’s shape from a circle to an oval. “The reason was simply that we were all so young,” Yoshimura said. “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.” According to their calculations, the eight-valve oval-piston engine would offer an estimated output of 23,000 rpm and 130 horsepower. With such promising figures, the team set out on its quest for new technologies. They believed they had a winning idea, and now they needed the winning formula.
Unfortunately, the oval piston NR500 wasn’t a winner. Upon its debut the bikes almost didn’t qualify, and both DNF’d, the first after a fall and the second with engine trouble. Further losses piled up as it was realized that the engine was not ready for prime time, and was failing to produce the RPM range and horsepower that calculations had shown it should. Steady improvement advanced the design, and by 1983 the oval-piston engine could finally be considered competitive. It had not yet however won a race. To finally reach the winner’s circle, Honda introduced the NS500 two-stroke in 1982. Over time these pushed the oval-piston four strokes off the grid and gained dominance in the GP series. The NR500 and it’s oval-piston engine may not have been a success on the track, but a lot of learning was gained through its development, and technologies like its back-torque limiter eventually made their way into Honda’s mass-production machines. A swan song, the NR750 commercial bike, was the last of the company’s odd-piston designs, at least to date. Images: Honda Worldwide History