998cc DOHC V-4
The V4 Story

Throughout the 1960s, Honda enjoyed outstanding success with its four-stroke engines, both on the road and on the racetrack. During its ten-year absence from Grand Prix racing between 1967 and 1977, Honda applied many of the lessons learned in competition to its road bikes, culminating in the launch of the CB750 — widely regarded as the world's first superbike.

In 1977, journalists gathered in Japan for the launch of Soichiro Irimajiri's six-cylinder CBX. Honda president Kiyoshi Kawashima used the occasion to make another announcement: Honda was returning to Grand Prix racing. The company would compete in the 500cc class in 1979 — and it would do so with a four-stroke.

Honda had previously kept pace with two-strokes on the racetrack by adding cylinders and gears to its four-stroke engines, producing a six-cylinder 17,000rpm 250 in 1964 and a five-cylinder 125 in 1966. But during Honda's absence from racing, the sport's governing body had changed the rules and restricted the 500cc class to four cylinders.

Kawashima had just two years to develop an entirely new engine and placed CBX designer Irimajiri in charge of the New Racing project. Irimajiri would undoubtedly have preferred to add more cylinders, but the new regulations restricted him to four — or, more precisely, four combustion chambers.

In effect, the engine could behave like a V8, but with each pair of adjacent cylinders joined together. Honda filed the patent for its oval-cylinder engine in March 1978, and by 1979 its engineers had produced the NR500, internally designated 0X.

What appeared at Silverstone in August 1979 was radically different from the state-of-the-art motorcycles of the day. Honda's young engineering team, many of whom had no previous racing experience, had attempted to rethink almost every aspect of the racing motorcycle. From suspension and chassis design to features now taken for granted on modern machines, the NR500 was packed with unconventional engineering. At its heart was the extraordinary new oval-piston V4.

Each oval cylinder used four inlet valves, four exhaust valves and twin connecting rods. The opposing cylinder banks were set at an angle of 100°, leaving space for the twin-choke carburettors within the Vee. The NR500's camshafts were gear-driven, and the original 0X engine eventually produced around 115bhp.

Although Honda met its 1979 deadline and the engineering concept proved that an oval-piston four-stroke could work, the NR500 was not a success on the racetrack. Later versions were considerably more competitive. In the 2X model ridden by Freddie Spencer, the angle between the cylinder banks was reduced to 90° and power increased to around 130bhp. A further 5bhp was extracted from the later 3X engine.

The NR500's real success lay not in race victories, but in the knowledge Honda's research and development engineers gained from designing and building it. Oval pistons would not become the future of mainstream motorcycle engines, but the V4 layout had demonstrated enormous potential for both racing and road use.

Turning that concept into a mass-produced road engine presented Honda with a new set of challenges. One disadvantage of the V4 layout is that the rear cylinders sit behind the front pair and deep within the motorcycle, beneath the tank and rider. They therefore receive considerably less cooling air and tend to run hotter, making air cooling impractical for a high-performance engine. Water cooling was the logical solution.

The V4 also brought greater complexity. Its exhaust system was more difficult to package and, compared with an inline four, production required an additional cylinder head, barrels, camshafts, bearings and camshaft-drive components.

There were significant advantages, however. A V4 is narrower than an equivalent inline four, allowing better streamlining and improved ground clearance. Its shorter crankshaft can also reduce the number and size of internal components, while the carburettors fit neatly within the 90° Vee between the cylinder banks. Combined with a 360° crankshaft arrangement, the 90° V4 also offered excellent inherent balance and low vibration.

This basic layout appeared in Honda's first production road-going V4, the VF750S, in 1982. Its four valves per cylinder were operated by chain-driven double overhead camshafts. In a departure from normal Honda practice, valve clearances were adjusted using threaded tappets rather than shims — partly in response to owners and clubs who had complained that shim adjustment made home servicing difficult.

The carburettors breathed through a removable airbox, allowing access to the cylinder heads and tappets while the engine remained in the frame. Lubrication to the top end was supplied through an external oil line feeding the front cylinder's inlet camshaft, with oil then distributed internally through an H-shaped tube.

Honda achieved considerable commercial success in the United States with the 750cc V4, particularly in the VF750S Sabre and V45 Magna. It was also here that the first significant customer problems with Honda's new V4 began to emerge.

One fault manifested itself as uneven idling that could eventually cause the engine to stall. Honda's decision to simplify valve-clearance adjustment was to prove costly, as incorrectly adjusted clearances were identified as one contributing factor. Honda subsequently recommended a special tool to ensure the tappets and their locknuts were adjusted and tightened correctly — an issue that would become part of a much larger series of problems for the early V4 engines.

With the commercial success of the Sabre and Magna established, Honda turned its attention back to competition and the American TT F1 championship in an effort to restore its sporting reputation. The regulations required the major engine castings to be retained from the homologated production motorcycle, so Honda began with VF750 crankcases, barrels and cylinder heads and built an entirely new racing engine within them.

The result was the FWS1000. With a reported $3.5 million invested in the F1 programme, it was perhaps unsurprising that Honda's new V4 racer would overwhelm much of its competition.

One of the most important differences in the FWS engine was its camshaft drive: gears replaced the production engine's chains. Final drive was also by chain rather than shaft. The engine was enlarged to 1024cc and produced more than 150bhp. It sat within a steel cradle chassis beneath a full racing fairing, with Pro-Link rear suspension, a 16-inch front wheel and an 18-inch rear.

A year later, the public would be able to buy a motorcycle born from much of that same engineering: the VF1000R…