Honda NR 750

Conventional wisdom dictates that a reciprocating internal combustion engine, such as is fitted to most cars and bikes patrolling the roads of the world, should have round pistons - not square, oblong, or oval ones. Since such an engine derives its power from compression (of the fuel/air mixture), sealing the edges of the piston in the bore to achieve this is vital, and can best be achieved with two or three circular piston rings slotted into the circumference of a round piston. QED.  However, conventional wisdom is not something that has often troubled the engineers of Honda's motorcycle racing division HRC.

 They defended their company's beloved four-stroke tradition with success in the 1960s Grand Prix sphere by a succession of ever more complex bikes with smaller and smaller cylinders and more and more revs: the six-cylinder 250, five-cylinder 125 and 50 cc twin represented pinnacles of miniaturization in motorcycle engineering, which carried riders like Mike Hailwood and Luigi Taveri to a succession of GP victories and world titles and, twenty years later, have never been emulated.  The spiralling cost of al\ this achievement caused the FIM (the governing body of motorcycle racing) to introduce new GP race regulations at the start of the 1970s. These banned the use of more than four cylinders, which were only on the 350s and 500s in any case - 250s and 125s were restricted to only two cylinders. This effectively killed off the four-stroke as an effective racing tool, 

Running at astronomical revs of up to 22 000 rpm, the NR500 was an ultra-sophisticated but ultimately unsuccessful motorcycle: to achieve their coveted first-ever victory in the World 500 cc Championship for riders, Honda had to join everyone else in building a two-stroke - the unusual but this time successful V3 NS500. However, Honda always claimed that the lessons learnt in the course of the NR project were invaluable, and that a design of an oval-piston bike engine could still be perfected. By now, too, Takeo Fukui had risen through the ranks to become one of the top men in HRC, and was still eager to prove the NR's validity.

Accordingly, in 1983/84. Honda developed a 250 cc V-twin turbocharged GP engine, based on the NR concept, with eight valves per cylinder and oval pistons - but this had not been seen in public at the time of writing. This in turn led to the NR750, a normally aspirated 750 cc engine, still with twin conrods, oval pistons, and 32 valves, which made its surprise debut in the 1987 Le Mans 24-hour event, where it qualified second fastest in practice thanks to the skills of Australian Malcolm Campbell, who was riding it in rotation with a pair of journalists. In the race, the Honda prototype stunned onlookers with its speed, before retiring with, according to HRC, a faulty big-end assembly incorrectly tightened.  Producing a phenomenal 165 bhp at 15 500 rpm in 'sprint' form, the new generation oval-piston Honda was detuned slightly for the endurance race.

But even so, 155 bhp at 15 000 rpm in a bike scaling only 158 kg with lights and a generator, practically in road-legal trim, guaranteed scintillating performance and seemed to bear out Fukui's insistence that what had begun as a means of circumventing a restrictive set of regulations, in fact turned out almost by accident to offer a significant improvement.  The reason probably lies in an inherent limitation of poppet-valve engineering: a four-stroke engine is restricted in the power it can produce only by the amount of mixture you can pack into the cylinder, and the speed at which you can get rid of the unbumt exhaust gases after combustion; in other words, flow, which in turn is dictated by valve area. How big the overall valve size can be in turn depends on piston area, and therefore the bore.

Having oval pistons enables more valves to be used, in turn offering greater flow, less reciprocating weight,  higher speeds and more power.  The success of the NR project made it almost certain that an oval-piston road bike would be introduced by Honda before very long - most likely a 250 V-twin, maybe even a turbo! Its launch would give the ultimate justification for Fukui-san's perseverance with a project so many discounted so harshly.

Source of review: Dream Bikes Alan Cathcart

NR 750 reviw

Honda had indeed conquered the World GP with its NS500s, but there were mixed feelings among the project staff. They could not forget the NR500s that had made way for the NS500s as Honda's primary machines in 1982, only to vanish from the World GP in 1983.

Oguma sent in a entry with a new machine to compete in the 24 Hours of Le Mans Endurance Race scheduled to be held in July 1987. Honda's machine was to be the NR750, an NR with increased displacement. The NR750 fared well in the qualifying round, but retired in the final. However, it won the Swan Series race held in Australia that December, blazing past its rivals in a decisive show of prowess. That race marked the final appearance by an NR series machine. Nine years after Honda first began their development under the goal of winning back the glory of the World GP series, the NRs had ended with a glorious victory.

Honda decided to fight for the World GP title with the NR500s, because it looked at the races as nurturing grounds for technology. Although the machines labored for years without a single point, they fostered the talents of numerous engineers and left behind precious assets. Among these were technologies that were later incorporated into production models; bikes that employed many of the weight-reduction techniques their NR ancestors had originated.

Honda introduced the new NR production bike in May 1992, powered by a 750 cc 4-cylinder V-engine with oval pistons. The machine employed an array of cutting-edge technologies, including an inverted front fork, aluminum twin tubular frame, and magnesium wheels. Thus, the NR500s completed their mission with the successful transfer of racing technologies to the production world.