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The 500cc class of the World Road Race Championships, which was competed on two-stroke machines until 2001, became a mixed two- and four-stroke competition in 2002 with the start of the new MotoGP regulation. And, as of the 2003 season, the MotoGP class has become in effect a completely four-stroke competition, thus bringing to a close the age of the two-stroke 500cc machines. Many dramas unfolded during Yamaha's three decades of GP500 class racing, a period which saw Yamaha engineers involved in the ongoing challenge of expanding the potential of the two-stroke engine.

The Yamaha YZR500 factory machine won its World GP debut race back in April 1973 at the French GP, breaking the domination of the MV Agusta machines. And, after winning the manufacturers championship the following year, 1974, Yamaha continued to compete in GP racing for thirty years with its evolving YZR500 factory machines. During this period the YZR500 chalked up an enviable record of 115 wins at the hands of 20 different riders while winning a total of 11 rider championship titles and nine manufacturers titles.

The history of the YZR500 is also a history of technological innovation and evolution. In countless technologies from the YPVS born of environmental measures, the rear-exhaust system, the Deltabox frame that contributed to the machine's outstanding handling characteristics and the electronic-control suspension to the release of technologies for the YZR500 machine itself, can be found a history of Yamaha's philosophy of technological development. In this record of the YZR500 and its history, we look at the qualities of the successive models based on the words of the Yamaha engineers who worked on their development.

The date was April 22, 1973, and the place was the opening round of the GP series at the Paul Ricard circuit in France. For Yamaha this race marked its first GP challenge in five years and its first ever in the 500cc class. The riders who would mount the new Yamaha machine that had been developed under the OW20 code name were the previous year's 250cc class champion on a TZ250 Yamaha prototype Jarno Saarinen of Finland and Japan's Hideo Kanaya.

That day, Saarinen would ride the first YZR500 to victory over the 20-lap, 116.2 km course in a time of 45 min. 57 sec. to beat rival Phil Read on the MV Agusta four-stroke machine by a full 16-second margin. Kanaya followed in third place. In the second round of the series at the rainy Saltzburg circuit in Germany, Saarinen and Kanaya would finish one-two, and in doing so herald the start of a new era in GP racing.

It was the start of a new era in that the strength of the two-stroke Yamaha machine ended the domination of the four-stroke MV Agusta in the GP500 class. At the time, some 20 different makes of 500cc machines were competing in the class, including two-stroke machines from Harley-Davidson, Husqvarna, Konig, Ducati, Bultaco, Norton, BMW and Triumph, but none could stop the winning streak of the four-stroke MV Agusta.

With the exception of one win by Honda in 1966, Agusta had won the manufacturers title every year since 1958. The two-stroke models just couldn't keep up and it was an era when there was not even a thought of the possibility of holding a round of the world championship in Japan. Nonetheless, Yamaha was working diligently on the challenge of developing the potential of the two-stroke engine.

Still, the OW20 prototype was not the product of a development project aimed solely at the GP500 class. At that time, in the latter half of the 1960s, Yamaha was looking at the U.S. market as one with big potential and since 1968 had been competing in America's biggest motorcycle race event, the Daytona 200. At that time the Yamaha race machine was based on the 350cc TR2, but as rival makers began introducing 750cc machines Yamaha began developing a liquid-cooled in-line four-cylinder 700cc two-stroke machine dubbed the YZ648 from May of 1971.

For the power unit, a "V" type engine had been an option, but taking into consideration the race competitiveness and reliability, the Yamaha engineers had decided to go with an in-line four-cylinder configuration that basically lined up two of the 350cc TR2 racer's engines in one block. At the same time, development work was begun on the 500cc OW20 prototype that would eventually become the '73 YZR500. Due to the regulation requiring that 700cc class machines for the Daytona race be production machines manufactured in a lot of at least 200 units, the YZ648 adopted a die-cast crank case. In contrast, the OW20 was built with a sand-cast crankcase.

Retired Yamaha engineer Takashi Matsui recalls the start of the development project: "We were intending to eventually participate in the GP500 class so we started work on a 500cc model, but at the same time we had our eyes on Daytona. So we started developing a 700cc models simultaneously, calling the 700cc the "YZ648" and the 500cc the "YZ648A." By October of 1971 we had completed three prototypes to the point where they were running fairly well. Then we spent all of 1972 developing the finer details of the machines, which were now being referred to by their development code names "OW19" (700cc) and "OW20" (500cc)."

A number of innovative features were developed during that year. "In order to get a much slimmer engine design, we grouped the four cylinders into units of two each and then introduced an "idle shaft" between the right and left crank shafts as the drive force shaft. The enabled an engine width about the same as a V-4. And to make the overall design even more compact, we fitted the ignition system in the space on the non-drive end of the idle shaft axis," adds Mr. Matsui.

"From our experience developing the RD05 (the 1964 250cc GP machine) we learned that the ignition order has a big effect on the idle gear, and so we decided to use a well-balanced firing order in which the two inside cylinders and the two outside cylinders fired at the same time. In our efforts to increase the machine's potential banking angle, we designed triangular cross-section mufflers and grouped all four exhaust pipes together and ran them under the crank cases," recalls Mr. Matsui. These models also adopted a new chromium molybdenum steel frame with a wide-type side rail and disc brakes were added front and rear.

"At the time, we were told that the three-cylinder engines of the rival makers put out about 102 hp, while our OW20 put out 95 hp. So, the reason we were able to win our debut race was surely a result of the machine's good overall balance rather than its acceleration," concludes Masakazu Shiohara, head of the testing team at the time. As we see, the Yamaha development philosophy of pursuing overall performance and handling rather than depending simply on power output was already present from the first YZR500. Yamaha's commitment to racing was also made evident the following year, 1974, when a contract was signed with Italian hero and greatest GP rider of the day, Giacomo Agostini.