Prior to the transition to four stroke engines in 2002, the 500cc 2-stroke class was the top of the food chain in the Grand Prix Motorcycle World Championships. MV Agusta had won 17 consecutive 500cc World Championships, from 1958 to 1974. All but 3 of those championships came from riders John Surtees, Mike Hailwood, and the prolific Giacomo Agostini.
By 1975, the 2-strokes were becoming the technology of choice. The Japanese were advancing the technology rapidly. Agostini fled to Yamaha, and brought them a Championship that year, but Suzuki was advancing quickly. Years earlier they had hired Ernst Degner from the East German motorcycle company MZ, who built quite powerful two-stroke engines. Suzuki broke through and went on to win the 1976 and 1977 World Championships.
Very late in the 1977 season, Yamaha decided to try something different. They equipped their Venezuelan team rider Johnny Cecotto’s YZR500 with an experimental version of their Yamaha Power Valve System (YPVS). The YPVS system was initially developed for single-cylinder Yamaha motocrossers, but Cecotto’s YZR500 OW35K was the first 4-cylinder machine to utilize the technology, according to a Yamaha staffer at the time, Taichi Ito.
The YPVS was actually derived from technology meant for emissions improvements. Compared to a normal 4-stroke, a 2-stroke emits about 10% the amount of NOx (nitrorogen oxide). Conversely, however, a 2-stroke emits much larger amounts of hydrocarbons than a 4-stroke. Reducing these hydrocarbon emissions was an important development theme for 2-strokes as emissions were becoming a topical issue. Additionally, the 2-stroke blow-by trait that caused these increased emissions also caused a torque dip that was a serious issue for racing-grade performance. The YPVS device solved this problem by employing a valve that enabled variable exhaust timing, and was linked to engine speed in order to optimize exhaust timing at any spot in the rpm range.
In certain RPM zones, the exhaust pattern of a 2-stroke engine is such that the pressure in the exhaust pipes is likely to either cancel out or augment the pulse due to the overlapping of the successive exhausts. Yamaha’s engineers realized that negative pressure resulting from the mutual canceling of pulses in the area of the exhaust pipe at the moment of exhaust could also increase exhaust efficiency and be tuned to increase engine power output.
“Improving this unique 2-stroke characteristic, called the Kadenacy effect, was something that had the capability to vastly improve the performance potential. You might call it a critical step in the history to 2-stroke engine development. And, in the long history of Yamaha’s development of the YZR500, no single improvement has led to a bigger improvement in its lap times at the Yamaha Test Course than the adoption of YPVS,” says Masakazu Shiohara, one of Yamaha’s most prolific innovators.
Enter Kenny Roberts. For the 1978 season, Yamaha felt they could not develop a dirt track motorcycle capable of competing with the dominant Harley-Davidson dirt track team. Roberts had been dominating for years aboard Yamaha motorcycless in the AMA Grand National Series. In 1973, he transitioned from the dirt tracks to road race courses. While he did quite well right away, he initially didn’t like how unsettled bikes felt on pavement. Roberts observed Finnish rider Jarno Saarinen win the 1973 Daytona 200 using a riding style where he shifted his body weight towards the inside of a turn. He adapted that style, but took it even further, extending his knee out until it hit the track surface. The ‘knee-dragging’ technique is a staple in motorcycle road racing to this day.
Yamaha USA offered to send Roberts to Europe in 1978 to compete in the World Championship Grand Prix road racing series. The team planned to compete in the 250 cc world championship and the Formula 750 series, but mostly as a way to have more practice time to learn the tracks. Their primary mission was to win the 500 cc class championship.
The season started off with a mechanical failure of the Yamaha at the opener in Venezuela. But Roberts found his groove quicky, finishing second in the next race, followed by three consecutive victories in Austria, France, and Italy. With a late-season controversial win at Silverstone, Roberts became the first American to win the Grand Prix Motorcycle World Championship. Roberts, aboard slightly different evolutions of the YZR500, dominated for three straight years. The Yamaha / Roberts combination ultimately claimed the 1978, 1979 and 1980 world championships.
The Roberts / Yamaha partnership is, to this day, one of the strongest, most recognizable alliances between rider and manufacturer.