In the world of internal combustion engines, we’re used to seeing a handful of different configurations: we’ve got V engines, inline engines, flat engines, and even more unique things like the Wankel rotary engine. But, what if I told you that in the late 1970s Honda developed an engine that didn’t use round pistons, instead, it used oval pistons?
I know it sounds crazy, but it’s true. So, in this article we’re going to take a deep dive into Honda’s oval piston engine, we’ll look at why they developed it, what makes it good, what makes it bad, and ultimately why it failed. Roll that intro.
Honda Joining Grand Prix
To get this story started, we need to rewind the clock to the 1970s, when Honda had been absent from Grand Prix Motorcycle racing since the late 1960s, more specifically they hadn’t competed in 12 years. So, in 1979 they returned to the series. But, at that time, Grand Prix racing was mostly dominated by two-stroke motorcycles.
While two-stroke bikes are cool, and at that time made considerably more power than the average four-stroke engine, Honda didn’t like them, straight-up calling them a “bamboo tube with holes drilled” in it at one point.
I think it’s also worth noting that it’s not that Honda just didn’t like two-stroke engines, but also that they’d previously had success with four-stroke racing engines when everyone else said it couldn’t be done. Dating back to some of Honda’s earliest years with motorcycle racing, they went against the convention of two-stroke engines, and the idea of a four-stroke engine that could actually win races became synonymous with the Honda name.
Well, maybe not that extreme, but the point is that Honda didn’t like two-stroke engines and in their early racing years had already shown the world that four-stroke engines were capable of winning, so when it came time to re-enter Grand Prix racing, a four-stroke engine had to be had. Honda refused to compete with anything less.
There was only one problem though: four-strokes plain and simply were at a disadvantage to compete with two-strokes in terms of power and weight at that time. At the end of the day, two-stroke engines are great at making power with minimal weight. So if Honda wanted to dominate and not just compete, some serious innovation was needed, which is where the oval engine comes into play.
Developing a Four-Stroke Engine That Could Compete
This is also where Toshimitsu Yoshimura comes into play, as he was one of the key engineers involved with the development of the NR500 oval piston engine. Interestingly enough though, he was only involved with Honda for six short years and had literally no experience with Honda’s previous racing efforts.
“Instead of being excited about developing racing machines, our feeling was more akin to raw determination. It was the determination to create something that would represent the very best in technology. We were determined to create an engine to surprise the whole world. We believed that if we could create the best engine, it would bring us a victory.” – he said.
That brings us to the World GP racing regulations of the time, which limited cylinder count to four, and in order for a four-stroke engine to be as powerful as a two-stroke engine, it has to be able to spin twice as fast in order to compensate for having half the combustion cycles. With that in mind, heat resistance, low friction, and intake efficiency were paramount to the development of this new four-stroke engine.
And because this was such a young and relatively inexperienced development team, they weren’t necessarily afraid to try crazy stuff, because the notion that something “couldn’t be done” didn’t really exist to them. This is where the idea of having eight valves per cylinder came into play. But, how are you to fit eight valves next to each other with a circular piston? You don’t. Instead, you elongate the piston to an oval in order for the valves.
“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.” – Yoshimura.
I’m not exactly sure how they calculated this, but according to Honda, their calculations showed the eight-valve oval-piston engine would output 130hp at 23000RPM, which would give them a nice leg-up on their competition, as most competitors were outputting 120hp at that time, and 10hp is a huge jump on a motorcycle.
Of course, calculations are just that, and getting something to work in the real world isn’t always as easy as it is to get it to work on paper or in a simulation. With an oval piston, there are a lot of challenges around sealing, friction, piston stability, and much more. They started with a single-cylinder two-valve oval piston engine and eventually got that work, and as development continued they eventually increased up to the planned eight valves per cylinder.
Struggles of Oval Piston Development
Some of the hurdles they had to get over during the development process include quote sudden disintegration unquote. This would happen because the connecting rods twist at high speeds, but unlike a normal piston where there is only one connecting rod, this one has two, which meant the twisting could cause major binding if they didn’t twist exactly the same.
That also brings up the piston ring, where on a normal circular piston it’s a pretty easy thing to machine and have precisely on the piston and inside the cylinder, but when you elongate said circular piston into an oval, now you also have to figure out to machine piston rings that properly seal the piston.
According to Honda, this was one of the more difficult parts of development, and then they tested dozens of different designs and methods for producing reliable piston rings. Unfortunately, the machines required to make these parts weren’t as accurate back then as they are today, which made the development process even more difficult because simply making the part as you designed it was a challenge in itself.
But lo and behold, after much time spent on development, the engine was finally functioning and ready to hit the races, which is where the 0X engine comes into play. Ironically, development still wasn’t completed and they were dealing with some failing parts, so for some reason, they thought the better development bed would be installing it in a racing machine before it was exactly ready.
Going to the Races
Unsurprisingly, the first race was a complete failure. Honda barely qualified for the race and with two bikes entered, one of them crashed on the very first corner and the other one retired after a few laps because of engine trouble. I couldn’t have seen that coming. A little bit later on at the final race of the season, Honda failed to even qualify for the main race.
So, you could say that right off the bat their crazy engine didn’t perform as they wanted. Not only were dealing with reliability issues, but also they weren’t quite able to get the engine to produce the power they had initially projected and planned for, which made it even harder to compete, even when the engine was functioning correctly.
That takes us forward to 1981 when Honda moved from the 0X engine to the 2X engine and actually saw their first victory at the 1981 Suzuka 500-Kilometer Race. But even though they had finally had their first win and they were actually finally able to get the engine to produce the power they had been aiming for the whole time, they were still struggling to compete in races.
More specifically, they were struggling with weight, which is one of the big benefits of two-stroke engines, which all of their competitors were using at the time. They tried to solve some of these weight issues by using aluminum and titanium parts for the engine rather than iron or magnesium, but even then, they were at a disadvantage.
NS500 Takes Over
At this point, it had been three years into Honda’s return to the Grand Prix racing series and they had yet to have a singular win on the global stage, and as such Honda was growing ever tired of losing. So, in 1982 they introduced the NS500 motorcycles which would race in place of the NR500 four-stroke oval-engine motorcycles.
And funny enough, the new NS500 motorcycles went against everything they had been working towards, by using two-stroke engines. With those new bikes though, Honda began to dominate in racing for years to come, leaving the oval four-stroke engines behind in the graveyard.
That being said, Honda didn’t eventually have a 3X engine bike, which should’ve been a winning oval-engine bike, but with how successful their NS500 bikes quickly became, the 3X powered NR500 never got to see competition, even though it was their best oval engine to date.
The Road-Going Machines
And that takes us off the racetrack, away from the NR500 and NR750 race bikes, to the NR750 road-going bikes, where all the development Honda had been working on was finally put into a mass-production machine. What made the road-going bike different from the race bikes was a decreased V angle, and most notably, an elliptical piston rather than an oval piston.
Understandably, many of these road-going machines struggle with the same reliability issues that plagued the original race bikes, and because of its unique design, getting parts for one of these isn’t exactly easy, which has put many of the NR750 road-going bikes off the road and into collector garages.
Besides just that, they barely produced any of the road-going bikes in the first place, so they were kind of a collector’s item from the get-go.
So that’s the story of Honda’s crazy oval piston engine. We don’t really see engine experimentation like this anymore, as advances in technology make it much easier to develop efficient and powerful engines on a computer and then build them out in real life, but regardless, it’s fun to admire strange engine configurations like this, as manufacturers tried to push the limits and dominate racing series with innovation.