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 The machine that Johnny Cecotto rode to victory in the Finnish GP in July of 1977 mounted a secret new device. The machine's development name was the OW35K and it was the first YZR500 to
mount what would come to be known as the Yamaha Power Valve System (YPVS). In February of the next year, 1978, when Yamaha announced its GP race program for the year, it also officially introduced its
YPVS technology. The YZR500 mounting the new YPVS would be ridden that season in the 500cc class by Kenny Roberts and Cecotto, who had also been competing in the new World Championships Formula 750
class since 1977 on the YZR750 without YPVS.
At the time, the GP500 class was dominated by the Suzuki RG500, while the Formula 750 class was dominated by Yamaha bikes (three straight
titles form 77-79). The YPVS technology on the new Yamaha 500cc machines quickly won attention, however, because it clearly gave them a major advantage over the rival machines in the Grand Prix.
The roots of the YPVS technology actually went back three years to know-how developed for emissions measures in 1974. Since the new device also improved performance, it was first applied in racing on
Yamaha's motocrossers and then on road racers and finally production models.
Compared to a normal four-stroke, a two-stroke only emits about 1/10th the amount of NOx, but due to the blow-by
phenomenon, the two-stroke emits larger amounts of HC. Reducing these HC emissions was the development theme. Efforts to increase two-stroke power output by speeding up the exhaust timing and using the
exhaust interference wave to force gas into the combustion chamber proved successful in the desired high-speed range but also resulted in an increase in blow-by gas in the low speed range, thus creating
an emissions problem. It also caused a torque valley that was undesirable on a performance race machine. In order to solve this problem, a valve that enabled variable exhaust timing was added. Operating
in accordance with engine rpm, this YPVS device made it possible to optimize exhaust timing at each speed range.
The exhaust pattern on a two-stroke engine is a cyclical one that corresponds to
the combustion in the cylinders, and there are certain rpm zones where the pressure in the exhaust pipes is subject to either a cancelling out or augmentation of the pulse due to the overlapping of the
successive exhausts. When negative pressure resulting from the mutual cancelling out of pulses occurs in the area of the exhaust pipe at the moment of exhaust, it increases exhaust efficiency and thus
engine performance. "The history of trying to improve this unique two-stroke characteristic, called the Kadenacy effect, is virtually the history to two-stroke engine development itself. 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 Mr. Shiohara.
Looking at the YZR500's performance figures over the years we see that horsepower climbed from 80PS for the first model (1973) to 180PS for the 1996 model, while the maximum rpm climbed from 10,000
for the first model to 12,500 for the 2002 model. Clearly the rise in power output has been much greater than that in rpm, and the reason for this is largely the results of Yamaha engineers' efforts to
improve the "Kadenacy effect" through measures like the YPVS.
Bringing the YPVS to competition models actually involved a lot of trial and error. "The decision was made to introduce the YPVS
device in GP road racing from March of 1977 and the machine chosen to mount it on was the '77 GP machine OW35. It was given a new project code, OW35K, and work got under way. Development work had
already been done for mounting the system on single-cylinder motocrossers, but this was the first time for a four-cylinder machine, and it brought a number of unique problems, like how to attach the
cylinder to the crankcase and whether to make the four YPVS valves jointly operable," says Mr. Taichi Ito, one of the engineer staff.
"The most common way to fasten the cylinder to the crankcase
was to use stud bolts and nuts to bolt it down together with the cylinder head. However, due to the addition of the drum-shaped YPVS valves to the cylinders, a normal through-type stud bolt could not be
used on the front side of the cylinder. So, at first we adopted a system where a flange was added to the front side or the cylinder for fastening, while the rear side was fastened in the usual method
along with the cylinder head with stud bolts and nuts. But, the fact that the front and rear sides of the cylinders were fastened differently, thus creating imbalances in load, led to problems like
cracking of the cylinders or crankcases. So, eventually we had to use a flange system on the rear side as well," recalls Mr. Ito.
"What's more, the addition of the YPVS valve caused greater heat
distortion in the cylinders that led to problems like piston seizure, so we had to try a lot of new methods to alleviate this problem, like changing the shape to the piston cam model and the honing of
the cylinder," adds Mr. Ito. But these development efforts paid off, and eventually contributed to Roberts winning his first world title on the YZR500 in 1978. This returned the title to Yamaha after
successive wins by Barry Sheene on his Suzuki in '76 and '77. As the YPVS continued to evolve, Roberts went on to win three successive titles.
Today, a wide range of two-stroke powered Yamaha
products from production Enduro and motocross machines to production road racers, snowmobiles and personal watercraft mount the YPVS technology. Among these, Yamaha's Wave Runner PWCs use the YPVS as a
device to boost the effectiveness of the exhaust-cleaning catalyzer by functioning to control exhaust temperature. This technological know-how would play a vital roll in the YZR500s revolution after the
OW35K. |
