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Which shape is the best shape for sealing something? And it doesn't matter what you're sealing; water, air, combustion, coolant, oil.
The answer is: a circle. Why? Because it's the simplest and most uniform shape of all. Wherever sealing is important chances are very high that you will find a circular shape. So here's another question for you: if I know that a circle is best for sealing than Honda's engineers certainly know it too. So why did Honda invest an absolutely incredible amount of time, money and human resources into trying to reinvent the wheel by developing an oval piston engine? To properly answer that question we need to process a cocktail of history, mechanics and human persistence.

The year is 1960 and Honda has started a very ambitious campaign to dominate the world of motorcycle racing. They soon started winning races left and right with their motorcycles equipped with four stroke engines. But by the mid-sixties most manufacturers and racing teams jumped on the band-wagon of switching to two stroke engines for their racing efforts.

But despite the two stroke trend Honda remained faithful to four stroke engines. Founder Soichiro Honda is famous for disliking two strokes engines, a sentiment he spread throughout the company. But Honda did more than remained faithful to four strokes, they kept trailblazing and pushing forward in the field and by 1966 they did what no other manufacturer managed to do before, and that is to score podiums in every of the five different classes. The RC166 perhaps best exemplifies how far Honda managed to push the four stroke. Inline six, four cams, 24 valves all miniaturized to 250cc in an age before CAD and CNC.

So in 1966 Honda proved their point. Their four stroke engines featured clockwork precision, they were amazing and perhaps most importantly, they could outperform two stroke engines. So with nothing left to prove in 1967 Honda decided to retire from motorcycle grand prix racing and instead focus on the development of mass-produced cars. Instead of chasing power and speed Honda started chasing economy and environmental responsibility. In 1973 Honda introduced the CVCC-equipped Civic model, becoming the first carmaker to offer a model in full compliance with the U.S. Clean Air Act passed by the U.S. Congress.

But soon Honda would see that without racing there is no drive to create new and competitive technology to beat the competition. Without racing there is no suitable environment where you can test and prove these new technologies. By 1977 Honda had realized they would end up lagging behind other manufacturers in terms of technology and innovation, so they announced their return to the World Motorcycle Grand Prix.

But during Honda's absence, things had changed . Two stroke engines were now the norm in grand prix racing. Four strokes simply couldn't keep up anymore. Despite obvious advantages of the two stroke Honda decided to once again remain faithful to their four strokes and try to beat the competition on an uneven playing field right after coming out of a decade long hibernation.
So they quickly started putting together a large team of staff which was to work on the NR project. NR being „new racing“.
The team figured out that beating a two stroke with a four stroke of equal displacement meant that the four stroke needed to rev twice as high. Now to rev to ridiculous rpm you need to make it possible for the engine to take in and push out massive amounts of air very very quickly. But fortunately for Honda the world motorcycle grand prix rule book only said 500cc and 4 cylinders without explicitly defining what they meant by „cylinder“.

But Honda was crazy enough to pursue a non-cylindrical cylinder. So they came up with this: if the cylinder was oval instead of round than you could fit more valves into it and sustain the airflow needed to generate 23.000 rpm.
Of course an oval piston with an oval cylinder was something no one ever attempted before and thus Honda found itself in completely uncharted territory. Chasing the dream of a V4 with 4 oval pistons and 32 valves all packed into only 500cc.

"When I look back at it, I'm not sure if we were experimenting with cutting-edge technologies or obsessed with foolish ideas," are the words Toshimitsu Yoshimura, an engineer involved in the development of the NR500 oval piston engine.

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#d4a #ovalpiston #nr750

00:00 Why reinvent the wheel?
02:53 Pushing the four stroke further
08:55 Making the impossible possible
13:11 The limit of persistence
15:53 Failure is not the opposite of success