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Engine

4-Stroke Motocrossers
The Pioneering “4-Stroke”

Supercross takes place in stadiums with specially constructed dirt tracks.

“Supercross” is an American-born and highly entertaining kind of motocross race that takes place inside specially prepared stadiums. The cheers of fans emanate from the stands as they gaze in wonder at the riders flying through the air off the gigantic jumps and blazing through the many challenging banked turns unique to this sport. Quick acceleration is required for the extremely short approaches leading to steep jumps and for swiftly navigating the turns before rocketing out of them; this is where the light weight and explosive power of 2-stroke engines truly excel. Until 1997, the superb balance of light weight, power and drive force of 250cc 2-stroke machines made them the fastest around the track, and 2-strokes dominated the Supercross grid. However, with the push for more environmentally friendly machines and stricter emissions standards being put into place, 4-stroke production motorcycles had become the mainstream in dealership showrooms. Predicting that the 4-stroke wave would soon reach the motocross world as well, Yamaha quickly got to work on developing a 4-stroke motocrosser.
The result was the birth of the YZM400F Yamaha factory machine (#1). At the final round of the 1997 AMA Supercross season in Las Vegas, it won the race, marking the first win ever by a 4-stroke machine in a Supercross event. Yamaha also applied its 4-stroke technology in developing the YZ400F production motocrosser, releasing it on the market in 1998 and instantly changing the face of the motocross scene worldwide. Today, 4-strokes have become the mainstream choice in the top categories of motocross all around the world. What ushered in the age of the 4-stroke motocrosser is without a doubt, that incredible night in Las Vegas and the enormous challenge that Yamaha undertook to bring 4-strokes to the dirt.

5-Valve Technology to Achieve Rapid Combustion The development aim for the factory-spec YZM400F was performance potential on par with the 2-stroke YZ250 but in an even lighter package. To achieve this goal, Yamaha applied its technology and expertise garnered in motorcycle engine development and automobile racing (#2) in the 1980s and 1990s and began development of a unique DOHC 5-valve single-cylinder engine. The engine used the same cylinder head design as an F1 engine and was lightweight, tipping the scales at only 500 grams more than that of the YZ250. The 5-valve layout made it easier to give the combustion chamber a lens-like form and also made it easier to achieve a high compression ratio. The resulting rapid combustion produced a lot of power and achieved the team’s goal. The area around the cylinder head also had a compact design, leading to a smaller overall chassis size. Furthermore, the wide powerband unique to a 4-stroke gave riders the added advantage of not needing to shift as frequently.
Incidentally, motorcycles were not the only place Yamaha took its 4-stroke challenge. The YZF-R1 (#3) supersport bike was released during the same period as the YZ400F, and its 1,000cc 20-valve (five valves per cylinder) engine was transplanted into the FX140 (#4) in 2002, creating the world’s first 4-stroke personal watercraft. In 2003, the same engine was used to power Yamaha’s first 4-stroke snowmobile, the RX-1 (#5).

(#1)The YZM400F 4-stroke factory motocrosser

(#2)The 0X66 automobile racing engine for the All Japan F2 series

(#3)The 1998 YZF-R1's 1,000cc 20-valve engine (five valves per cylinder)

(#4)The FX140 4-stroke personal watercraft

(#5)The RX-1 4-stroke snowmobile

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