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Honda NR 750

 

 

 

 

Make Model

Honda NR 750 NR40 RC40

Year

1992

Engine

Four stroke, 90°V-four oval cylinder, DOHC, 8 valve per cylinder.
Engine Weight 80.5kg / 177.5 lbs

Capacity

747.7
Bore x Stroke 101.2 x 50.6 x 42 mm
Cooling System Liquid cooled
Compression Ratio 11.7:1
Valve Train Intake valve opens (@ 1mm (0.04 in.) lift) 20-deg. BTDC
Intake valve closes (@ 1mm (0.04 in.) lift) 50-deg. ABDC
Exhaust valve opens (@ 1mm (0.04 in.) lift) 46-deg. BBDC
Exhaust valve closes (@ (0.04 in.) 1mm lift) 16-deg. ATDC
Lubrication Forced pressure and wet sump with oil cooler

Induction

Electronic fuel injection 8x 30mm chokes, 2 injectors per cylinder

Ignition 

Computer-controlled digital  
Charging Triple phase output alternator
Starting Electric

Max Power

125 hp / 92 kW @ 14000 rpm

Max Power Rear Tyre

115.8 hp @14500 rpm)

Max Torque

66 Nm / 6.6 kgf-m @ 11000 rpm
Clutch Multi-plate, wet

Transmission 

6 Speed 
Final Drive Chain 525/ Link 108
Primary Reduction 1.937 (62/32)
Gear Ratio 1st 2.666 (40/15)  /  2nd 2.125 (34/16)  /  3rd 1.777 (32/18)  /  4th 1.545 (34/22)  /  5th 1.381 (29/21)  /  6th 1.381 (29/21)
Frame Triple-square section twin-tube

Front Suspension

45mm Showa inverted telescopic forks, preload,compression and rebound damping adjustment.
Front Wheel Travel 120 mm /; 4.7 in

Rear Suspension

Vertically-asymmetric Pro-Arm swing arm with Pro-Link, Rear damper Nitrogen gas-filled damper with reserve tank
Rear Wheel Travel 120 mm /; 4.7 in

Front Brakes

2x 310mm disc 4 piston calipers

Rear Brakes

Single 220mm disc 2 piston caliper
Front Wheel 16x3.5 in. magnesium alloy
Rear Wheel 17x5.5 in. magnesium alloy

Front Tyre

130/70-ZR16 (Michelin TX11 radial)
Front Tyre Pressure 270 kPa (2.70 kg/cm²; 39 psi.)

Rear Tyre

180/55-ZR17 (Michelin TX23 radial)
Rear Tyre Pressure 290 kPa (2.90 kg/cm²; 42 psi.)
Rake 24°
Trail 88mm / 3.5 in
Dimensions

Length 2,150mm / 84.6 in.

Width  700 mm / 27.6 in

Height1,090mm / 42.9in

Wheelbase 1435 mm / 56.5 in
Seat Height 785 mm / 30.9 in
Footpeg Height 360 mm / 44.2 in
Ground Clearance 130 mm / 5.1 in

Dry Weight

223 kg 491 lbs
Wet Weight 244 kg / 537.9 lbs
Maximum Weight Capacity 115.0k g / 253.5 lbs

Fuel Capacity

17 Litres / 3.8 gal

Consumption  average

12.6 km/lit

Braking 60 - 0 / 100 - 0

12.9 m / 35.9 m

Standing ¼ Mile  

11.4 sec / 197.3 km/h

Top Speed

257.3 km/h
Manual Vsource.org / Service Info

 

The Honda NR750 is possibly the ultimate superbike. It may not be the fastest, and it may not be the best, but it is arguably the best looking,  undoubtedly the most technologically advanced, and unquestionably the most expensive.  What makes the NR750 so special is the amazing high-technology it boasts. It is Honda's way of proving how clever they are, corporate muscle-flexing by a company who wanted to prove that they build quality as well as quantity. Space-age engineering is packed into every cranny of the NR, making it a unique and fascinating machine. But at £36,500 each (and Honda only built 700 of them) you'd expect it to be rare. When it costs five times more than the same company's flag-ship sportsbike, the NR750 would have to be something a bit out of the ordinary. 

What's most special about the NR750 is its engine. The NR is clever in many respects, but the it is the engine that sets it apart from any other motorcycle. This liquid-cooled V4 motor uses oval pistons, no less than eight valves per cylinder and features a highly sophisticated fuel-injection system. A 32-valve fuel-injected V4 750 is an astonishing feat of engineering, and one which allows Honda to produce a lot of power from a relatively small package.  Each of the two cylinder banks has double overhead cams. Each piston (they are actually oblong shaped, but with rounded-off corners, rather than purely oval) has two con-rods, two spark-plugs and eight valves. Why all this high-technology? Engines using lots of valves, all opening and closing quickly, can rev higher than ones with fewer valves. With a rev limit of 15,000rpm and an advanced fuel-injection system, the NR750 produces an impressive 125bhp (although pre-production prototypes were capable of 140bhp and the racing bike on which the NR is based was making more than 160bhp).   But there's more to the NR750 than oval-piston technology. The aluminium-alloy chassis is a work of art, combining strength and rigidity with lightness, and the NR's suspension is the best money can buy. At the front the NR features massive inverted Showa i forks and at the back it uses Honda's fiendishly [clever single-sided swingarm developed for fast wheel changes in endurance racing.  But most breath-taking of all is the body-work, i an impressive mixture of carbon-fibre and fibreglass that costs more than the total price of any other mass-production motorcycle. Even the NR's windscreen is titanium-coated and costs more than most people earn in a month.

The styling of the bodywork is sleek and seductive, the lines beautiful and the finish higher quality than anything seen before from a 'mass-produced' motorcycle.  As far as performance goes, the NR is good rather than exceptional. There are plenty of other motorbikes that will accelerate faster and reach a higher top speed, but the NR750 is one of the best handling bikes on the road, capable of going from 0-60mph in under four seconds and reaching a top speed of 160mph.  The NR750 is so expensive and exclusive that most people will never see one, let alone come across one on a public road. Which is a shame because it is undoubtedly the best looking and most exciting production motorcycle ever made.

Source of review: Super Bikes  by Mac McDiarmid

 

The Oval Piston: Heart of a New and Different Breed

To design a 4-stroke, 4-cylinder engine in the conventional manner would not produce a machine that could out perform its 2-stroke rivals. No, for a 4-stroke engine to generate the same level of output as a 2-stroke engine it had to have twice as many cylinders as its competitor. Moreover, 20,000 r.p.m. was the absolute minimum a 4-stroke engine required to produce superior horsepower.

Increased power meant that the 4-stroke engine would have to consume and exhaust more of the critical fuel-air mixture. In other words, the aperture of the valve would have to increase for enhanced intake and exhaust efficiencies. To do that, the number of valves would increase, also. However, conventional circular pistons would accommodate only four or five valves. Furthermore, such a structure would provide no technical advancements, as compared to the time when Honda was competing in the World GP with its 4-valve DOHC engines. Therefore, the NR project's new challenge was to achieve "innovative technology," being more than a mere refinement of the previous technology.

Winning is the essence of racing. Thus, in winning the race, the team could prove that its technology was superior. Hence, there was no significance in creating a machine that was not capable of winning. That would lead neither to technical progress nor to the fostering of outstanding new talent. However, the Honda NR development team knew that to make a comeback in the World GP meant the establishment of a training ground for young talents; people who would strive to improve their skills through the creation of truly great motorcycles. If there were no chance of winning the race or fostering talent, Honda would be better off not trying at all.

Honda's answer was the adoption of an oval-piston design. 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.

However, the oval-piston design necessitated an extremely difficult manufacturing process, in which machining accuracy would be more critical than ever. Of course, no such design had been adapted for use in a high-performance racing machine. Nevertheless, the spirit of Honda was never to be pessimistic, and the team decided it was worth trying as long as there was a real possibility. In making such a decision the team was well aware of the difficult path ahead. In order to beat 2-stroke engines, they had to transcend common-sense thinking and bring in daring new technologies.

The concept for Honda's new engine was finalized in April 1979, and the NR Block set as its primary goal the realization of an oval-piston engine. The team faced difficulties right from the start. In fact, they had to prove that oval pistons, cylinders, and piston rings could actually be made. They also had to find a manufacturer that could help them do it.

Finally, the team commissioned the production to an associate company, Honda Metal Technology, located in the city of Kawagoe, Saitama Prefecture. Development went forward there, but the process was not entirely free of problems. Since in an oval piston a semicircular curve has to merge into a straight line, continuous curvature could not be maintained and edges were created along the piston. This made machining very difficult. Similar problems were found in the honing of the cylinder's inner diameter, and many hours were spent in production.

The team members in charge of engine design had their own private concerns, but spirits were high in the NR Block, where team personnel worked eagerly to prove that their engine was more than a fantasy.

In July 1978, three months after the start of production, the NR team finally completed a dual-valve head, 125-cc single-cylinder engine prototype, which they named "K00." Contrary to the team's concerns, the engine turned properly on the bench tester. With renewed confidence, the NR Block continued its efforts and the following October completed an eight-valve head, water-cooled, single-cylinder engine, the K0. Each day was spent making a prototype, testing it, modifying the specifications, and testing it again. Repeated testing revealed that the oval-piston design would cause problems at speeds in excess of 10,000 rpm, due to poor machining accuracy and the loss of durability. Regardless, the team members knew they could not continue testing single-cylinder engines indefinitely.

The NR Block was in fact developing a 4-cylinder engine commensurate with the testing of single-cylinder units. In April 1979, they completed a 4-cylinder V-engine called the "0X," which was to become the heart of the NR500. During the bench test, the engine produced 90 horsepower, making it clear that they had not yet reached their target of 130 horsepower.

The NR Block had also formed a new materials group in order to study two key problems-machining accuracy and durability. The group leader was to be Yoshitoshi Hagiwara, then chief research engineer in HGA's Third Research Block, who had been collaborating with the development team since the early stages of development. One of the group's key responsibilities was to collect the engine parts broken during testing and investigate their causes of failure. It was a tedious task in which it was often necessary to go through the pieces of several parts one by one and divide them into rings, valves and so forth. The group even studied new materials such as carbon and complex high-tech materials, along with enhanced manufacturing techniques that might be used to build parts which could withstand high engine revolutions. Ultimately, these efforts were fruitful, producing notable improvements in durability.

It was not just the engine that was unconventional. To increase its competitiveness, the NR500 also employed a frame technology that was simply unheard of in the conventional realm of engineering.

Tadashi Kamiya, a research engineer in HGA's Third Research Block who had joined the NR program in the summer of 1978 as chief of the test group for completed product, proposed several ideas that he was thinking could be adapted for use in production cars. One of them was an aluminum frame called the "shrimp shell."

The shrimp shell, which integrated a monocoque structure with the cowling to form the frame and body, virtually encased the engine, which was then inserted through the rear like a cassette. The engine was fixed with 18 six-millimeter bolts inserted at both sides. Although the panel was paper-thin at just 1 mm, once the engine was in place the frame was able to ensure the required rigidity. Additionally, the frame weighed just 5 kg, which was about half the weight of an ordinary tubular steel frame.

The idea, not surprisingly, encountered outright confusion among the members of the NR Block. In order to convince his colleagues Kamiya created a prototype model and, the members understood the concept of his shrimp-shell frame.

The second idea involved the wheel. Kamiya thought of adopting a 16-inch wheel instead of the mainstream 18-inch wheel.

"By reducing the tire diameter," Kamiya said, "we could reduce the machine's weight by around 4 kg and vehicle height by around 5 cm. The lower air resistance and smaller disc diameter also contributed to a lower moment of inertia, enabling the machine to accelerate faster. Moreover, the reduced frontal projection had the effect of increasing output by several horsepower.

 

"When I compared the 18-inch wheel with the 16-inch wheel, I asked myself which of the two would cross the finish line first in actual races, where average speeds often exceeded 200 km per hour, I was convinced that the 16-inch wheel had greater potential, not simply from the standpoint of partial speeds at corners but by putting all the elements in proper perspective."

Kamiya also adopted a vertical standing screen to be attached to the cowling, instead of the regular semispherical type.

"You could call it an invisible cowling," he said. "With this design we can still achieve sufficient aero-dynamic effect. Since the wind is directed upward after hitting the screen, the rider is subjected to less wind resistance at high speeds. Also, the area of frontal projection becomes smaller."

Another idea involved the swingarm and drive sprocket, which were positioned along the same axis. Because chain length was no longer affected by the upward or downward movement of the swingarm, there was no need to provide extra play. This meant an advantage in reducing shock due to acceleration or deceleration, thus stabilizing the suspension's performance over an entire course.

Kamiya believed that no 4-stroke engine, regardless of its merits, could win with a frame design based on conventional thinking. Thus, the NR's frame was constructed from the standpoint of minimizing volume and weight. The staff sought to create a 125 cc frame capable of carrying a 500 cc engine. These were just examples of the many unconventional, even outrageous, ideas being implemented in the new machine. However, they were all based on strategies that had been calculated to an absolutely meticulous degree.

The NR500 was put to the first test ride in Yatabe, Ibaragki Prefecture, in May 1979. Although the road test brought up problems that the team had failed to identify in bench testing, these were gradually resolved through refinement of the engine in repeated tests at Suzuka Circuit and on the course at Tochigi. The NR500 became more complete with each passing day, and soon it would be ready for its first World GP event. Due to a significant delay in the NR500's overall development schedule, though, its comeback had to be pushed back to the British Grand Prix, which was to be held at Silverstone on August 12.

The NR500 was at last completed in July 1979. Equipped with a 4-cylinder V-engine and 100-degree cylinder banks, the machine had a maximum output of 100 ps at 16,000 r.p.m.

The NR team had finally made it to England, where the machines were receiving their final tuneups in preparation for the GP race. At the same time, Yanase was making various arrangements in order that the team might concentrate on the race free of hindrances. Thus, in addition to setting up the bikes for optimal performance on the track, there were many other things that had to be done for each race, such as arranging transportation and accommodations for the many staff members involved. The World GP, often referred to as the Continental Circus, had most of its races in Europe. Therefore, while it was possible to develop the motorcycles in Japan, it was impossible to manage the races from Japan. The distance was simply too great.

Yanase rented a warehouse in Slough, an English town near Heathrow International Airport, as their team's base of operations in Europe, while in Britain additional preparations were underway through a company called HIRCO (Honda International Racing Corporation). Honda had established HIRCO in December 1978 as a joint venture with Honda UK for the management of all racing activities, including competition in the World GP series.

Honda's riders for the 1979 season would be Mick Grant and Takazumi Katayama, the latter having competed in the 1977 World GP series as a privateer, where he ultimately won the championship title in the 350-cc class.

 

For their part, the NR500s kindled great expectations upon their appearance at Silverstone, portending awesome performance with their original engine design and sleek styling. However, those expectations were mercilessly shattered in the qualifying round. These bikes, which were still in development, barely performed well enough to get through to the final. Even then, Grant fell at the first corner following the start and quickly retired. Katayama also retired after several laps due to ignition problems.

Though they had not expected a win in their very first race, the NR team was deeply disappointed with the outcome of their efforts. The NR500s were brought back to Japan, and improvements were made to further reduce weight and increase output. However, those really were not the fundamental solutions that the project team had long sought.

Harsher realities awaited in the French GP, the twelfth race of the 1979 season where both machines failed to qualify for the final, meaning that no results whatsoever could be garnered from their presence. However, the team could not go back to Japan without data, after having spent so much time and money in preparation for the French event.

Therefore, the staff of the NR Block flew to England, and with help from HIRCO tested the NR500s at Donington Park. The test was a fruitful one, in which new areas of possible improvement were identified. However, there was no improvement in speed, and the machines were still running two seconds behind the lap record. In the world of racing, where one-hundredth of a second determines the winner, two seconds was just too great a handicap.

Having finished the 1979 season with disappointing results, the NR500s had two major problems that were considered unique to 4-stroke engines.

First, was the factor of extreme engine braking. When development began, the team viewed engine braking as an advantage, believing it would assist in overall control of the bike. On the contrary, the engine braking caused the rear wheel to hop. To counteract it, the team developed a back-torque limiter with a built-in, one-way clutch so that the wheel would spin when the force applied to the wheel exceeded a certain level. This methodology was later incorporated in the VF750F, making it the first production bike to be equipped with a back-torque limiter.

Acceleration was the second problem. It simply did not provide the necessary subtlety of control. The ability of the engine to generate ample low-end torque-a characteristic of 4-stroke engines-also made cornering control difficult, resulting in the loss of time. During the 1980 season the team experimented with throttle pulleys based on various shapes, but as yet there was no solution.

Weight was another area in need of attention. Although the first 0X engine was lighter than the average 2-stroke engine, weight gradually increased as durability improved. Before long, the engine had put on as much as 20 kg. The development team conducted an exhaustive review of materials, going to titanium and magnesium in order to achieve weight reduction. However, the new material combination was quickly copied by rival teams, leaving Honda with no advantage in that area.

The shrimp-shell frame, which had so greatly contributed to weight reduction, also had a drawback. Because of its cassette-type mounting structure, the engine had to be removed from the frame in order to perform maintenance. This made it difficult for the engineers to achieve optimal settings within the limited time allowed in the qualifying runs. Accordingly, the team decided to adopt a pipe frame to the machines, beginning in 1980. At the same time, the wheel size was changed to 18 inches.

Ultimately, the problem with the NR500 was that so many new technologies had been introduced that its potential for completion had been compromised. Realizing this, the NR Block prioritized problem areas, placing the top priority on refining the engine. This, they believed, was the most fundamental problem of the bike.

The new and greatly improved NR500s competed in an international race held in Italy. Although it was not a World GP race, Katayama was able to take the podium with a third-place finish. He also put up a good fight in the final round of the British World GP, held that August. The fifteenth bike to pass the finish line, his machine was the first NR500 to finish a GP race. Moreover, Katayama made a strong showing in the West German Grand Prix, finishing in twelfth place.

The 1980 season, however, closed with just two events in which Honda's NR500s made it to the final round. Still, the machines were making steady progress, so it was still possible to build a point total and finish the season in tenth place or higher. However, the reality was that they were still trailing the 2-stroke machines by 10 or more horsepower. Therefore, even if the NR500s could advance in the standings, they did not have what it took to win a GP race.

Yanase recalled a sentence in the plan document he had received in December 1977. It read, "Become the world champion within three years." He knew that 1981 would be the final year in that attempt, so there was not much time left.

The oval piston engine underwent further improvements in order to reduce its weight, enhance output, and improve durability. The improved engine intended for the 1981 season had a smaller body, made possible by dropping the V-bank angle from 100 degrees to 90. Moreover, it had a maximum output of 130 ps at 19,000 rpm. Therefore, beginning with the 1981 season, Honda decided it wouldn't just compete in the World GP, but that it would also enter the All-Japan Championship Series. Honda made the decision to refine its NR500s more rapidly through participation in more races, hoping to build winning machines as quickly as possible.

In the second race of the All-Japan Championship, held at Suzuka Circuit in March 1981, the two NR500s ridden by Katayama and Kengo Kiyama both fell and retired. However, they had demonstrated considerable tenacity during the race, advancing to a point just behind the top group. In the next race held at Suzuka in April, the team saw one of its NR500s finish in fifth place. By this time, the NR500s were performing at levels equal to those of their 2-stroke counterparts.

Yoichi Oguma, the chief research engineer from HGA's Second Research Block who had joined the NR project in 1981 and became the first manager of the Honda Racing Team (HRC) the following year, had one vision: "Tactics and strategies also are important in winning the race. Honda is so preoccupied with the performance of its machines, but we can't win unless the machines, the riders and the team work as one."

This was a conviction that Oguma, a former All-Japan champion in the 125 cc junior class, had acquired through his own experience. Reflecting on that belief, the Suzuka 200-km Race-the sixth All-Japan race held in June 1981-was run under his careful supervision, according to a calculated strategy.

Any 2-stroke engine running a distance of 200 km, or 34 laps of the Suzuka Circuit track, will require at least one pit stop in order to refuel. That means approximately 10 seconds in the pit area. Considering that the bike must decelerate to enter the pits and accelerate again on its return to the course, nearly 20 seconds must be given up. Oguma's strategy was to let his machines run the entire race without a fuel stop by making use of their higher fuel efficiency, which was of course the advantage of 4-stroke design. According to his calculations, the NR500s would save around 0.6 second per lap by eliminating the fuel stop.

The race was carried out according to plan. While the rival machines were making their fuel stops, Kiyama's NR500 gradually advanced, eventually passing the leader on lap 23. It was the first time an NR500 had led a race. What's more, Kiyama maintained his time, leading the race lap after lap. Coming out of the last corner first, his NR500 kept its lead and took the checkered flag. Three years after the start of development, the NR Block and its NR500 had achieved a victory.

The team was again victorious in July, when Freddie Spencer rode his NR500 to a first-place finish in a five-lap heat race held at Laguna Seca in California, which doubled as the qualifying round for an international race. It was with these victories that the NR500s were established as contenders in the ultimate challenge: to win a World GP race.

It was, however, still quite premature to assume that victory in a World GP event was theirs for the taking. That was too high a mountain to climb. After Katayama's thirteenth place finish at the first Austrian Grand Prix in April 1981, the riders continued to retire from subsequent races. The 1981 season ended without any point total for the Honda team.

Hence, the promise to become the world champion within three years was broken. The NR Block had found itself at the crossroads of victory and defeat, survival and abandonment

Source world.honda.com

 

 

Here’s a bunch of pics from the Rayong Province, Thailand. Two friends are out riding their bikes, but not just any old bikes – we’re talking Honda VTR1000 SP2 and the Honda NR750! ‘Given its year of manufacture [1991], the NR still feels fresh and capable, though a tad heavy. It was happy to cruise at around 140km/h with spurts of up to 200km/h. Brakes are sharp and the chassis is more stable than flickable. It isn't an RC30, let alone a modern sportsbike, but it’s an enjoyable bike to ride on a Saturday morning cruise and its able enough that you don't need to make excuses for its performance in the company of the SP2,’ says the guy who owns the NR.

‘The SP2 has had quite a bit of work done to it and felt taught and lively compared to the NR. They are both Hondas and both have RC stamped on their frame but beyond that, they are as different as chalk and cheese. Without wanting to put the NR down in any way, the SP2 is the more involving and sportier bike of the two to ride and by quite a margin. However, it has to be factored in, that the main factor to riding the NR is that you don't really want to be riding it on the limit. There is always a fear of dropping the NR or even a stone flicking up off a truck as you passing it and causing damage to unobtainium paintwork or screen,’ says the NR owner. ‘To a certain extent, with time in the saddle, you get to relax and enjoy the experience. The NR’s retro/modern digital speedo (or is that modern but retro now?) and all the various dials on the dash keep you entertained and the bike is comfy and ergonomic. In addition, once you've arrived, you get the added bonus of having an extra view of the NR to admire when hanging out on the beach. The NR gets attention wherever it is parked. Even non bike enthusiasts take a peek at it,’ he adds.

 


‘You can see just how lardy the NR is. You kind of sit inboard and the bulk is behind and in front of the rider. However, a blindfolded person would think the NR is lithe, from the riding position alone. It is surprisingly narrow with hardly any splaying of legs around the tank. Reach to the bars is natural and sporty while the screen – iridium coated and looks fantastic – is low for most people. Between the NR and SP2, the difference is in facial expressions. The NR is smiley, the SP2 looks almost startled in comparison,’ says the NR guy.

‘The NR’s carbon air-ducts don't actually function though they do hide the screen brace. Seat is made of seude and is definitely not fake. Seems a strange choice for what is essentially a sportsbike but looks and feels good. The side ducts in the seat cowling are functional and the exhaust is otherwise fully enclosed – it gets pretty hot despite those air ducts, but no problems so far. The lower cowling, in carbon, is pure sex. Front forks are massive for the day, at 45mm. They are fully adjustable of course. The twin spotlights mounted in the fairing give the bike a unique frontal presence. Swingarm was the sexiest seen on a production bike until the MV F4 was launched. The frame has some special (some say, titanium) coating and is easy to keep clean and shiny. Swingarm is actually lacquered so you need to be careful with solvents and chips. Still looks good after 18 years,’ says the NR owner.

Look good, it certainly does. An NR may not even be able to keep up with a modern-day CBR600RR, but for its sheer Honda-ness, technology-porn that’s unrivalled to this day and mega exotic-ness, the NR was, and continues to be, right on top. How we envy people who actually own a Honda NR... :-)

Source: Tyga Performance

 

 

 

Honda produced just 300 units of the NR and that was back in 1992 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.

 

It cost US$50,000 back in 1992. 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.

 

Toshimitsu Yoshimura

“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

 

 

 

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