The 4-cylinder: Yamaha's flagship high-performance engine
Left: YZF-R1 (1998 model)
Right: RX-1 (2002 model)
The RX-1 launched in 2002 as Yamaha's first 4-stroke snowmobile was in fact, an offspring of Yamaha's fourth development project undertaken in 1999. The engine it employed was the 998cc liquid-cooled 4-stroke, DOHC, in-line 4-cylinder, 5-valve mill developed specifically for that project. Its base engine was none other than the epoch-making power unit of the original supersport motorcycle, the YZF-R1. The engine of the RX-1 used the same pistons, connecting rods and intake/exhaust cam and valve systems as the YZF-R1 engine. In contrast, the crankcase, cylinder head, lubrication system (dry sump type adopted), cooling system, reduction (gear) shaft and other parts were redesigned specifically for the RX-1.
The original R1 engine had a reputation for high power output as a supersport engine, but the biggest challenge for the RX-1 development team was how to make the engine light and compact while retaining as much of the high power output as possible. Looking back, the RX-1 development team members recall thinking of it as quite a large and heavy engine for a snowmobile.
Two of the solutions they came up with to make the engine lighter and more compact were an innovative layout for the three shafts (optimized assembly designs for each shaft assembly and positioning of the water pump) and adoption of a dry sump lubrication system. These changes brought a reduction in engine height of approximately 30 mm compared to the YZF-R1 engine. Another factor in this was the specially designed crankshaft.
Furthermore, the adoption of a rear-facing exhaust system made it possible to position the engine closer to the center of the chassis, improving space efficiency and making room for the first double wishbone type suspension ever on a Yamaha snowmobile. Eventually, the know-how gained in the development of this engine would form the base for subsequent development of the 3-cylinder and 2-cylinder snowmobile engines.
This 4-cylinder engine underwent its next evolution in 2006 for the Apex model. The new engine gained fuel injection and a much lighter crankshaft design. This crankshaft was a full 1 kg lighter compared to that of the original model. With measures to reduce torque fluctuation produced by its revolution, this crankshaft helped achieve linear response characteristics and an improved feeling of acceleration. Combined with the newly adopted fuel injection, this helped produce a feeling of exhilarating response.
The engine underwent further evolution for the 2011 model Apex. The exhaust layout was changed from 4-2-1 to a new 4-1 design and the Exhaust Ultimate Power Valve (EXUP) system was adopted for the first time ever on a Yamaha snowmobile. Other additions like a 'knock control' system and a new intake funnel design helped increase power output by 5HP. Idling Speed Control (ISC) was also adopted for better starting performance and engine braking characteristics.
This 4-cylinder engine became the flagship engine of Yamaha's 4-stroke snowmobiles and pioneered the first steps into the uncharted realm of 4-stroke sport models. With its subsequent evolution, this engine would become the one that showed the world the potential of 4-stroke sport models.
The 3-cylinder: An extremely well-balanced engine
Yamaha developed its 3-cylinder engine with the mission of bringing the world of 4-stroke sport riding to a broader range of customers. The base engine for its development came from the liquid-cooled, DOHC, 4-cylinder engine of the FJR1300 motorcycle. Redesigned specifically for snowmobiles, it took form as a 973cc liquid-cooled, 4-stroke, DOHC, 4-valve engine with an in-line 3-cylinder layout. Around the same time, a new trend was emerging among riders in the North American market. With the evolution of suspension systems came a new riding style focused on rough trails and off-trail playgrounds. Since Yamaha didn't have a model geared toward this emerging category, a new development project began that would use the 3-cylinder engine. It was a project to develop a new flagship model for rough trail riding.
Employing the concept of a "snocross racer replica," the first 3-cylinder engine model was developed in a project that included participation in Snocross races in North America and Japan to achieve cutting edge performance and weight reduction to racer levels. The resulting engine was bored out from 79mm to 82mm to increase the displacement to 1,049cc. Revisions in the intake/exhaust systems, addition of fuel injection, changes in the crank web (-770 g) to optimize crankshaft inertial mass and other measures produced a dramatic improvement in response. Additionally, a weight reduction of 2 kg was achieved in the engine along with the new crankshaft and cylinder head designs. When you figure in the lighter-weight designs of the intake and exhaust systems, the power unit mounted on the 2008 model FXNytro was about 5 kg lighter overall.
Looking at the record of this 3-cylinder engine's development and evolution, it might appear to be mainly a pursuit of higher performance, but in fact, major progress was also being made in the engine's reliability from the 973cc era. Measures like the use of low-pressure casting in parts like the uppercase and the closed deck cylinder helped to achieve high levels of unit rigidity and more stable performance. At the same time, efforts were made to not only create high-performance, but excellent drivability by introducing advances like the Engine Braking Reduction System (EBRS). Efforts like this helped make the 3-cylinder engine outstanding in the kind of all-around "total performance" Yamaha engineering always strives for.
This 3-cylinder engine first appeared on the "light sport 4-stroke model" RSVector, which was pointed squarely at delivering 4-stroke powered snowmobiles to mainstream trail riders. The engine would later undergo further development and evolution on the FXNytro and became a motor of choice, capable of serving in a number of areas as an important pillar of Yamaha's 4-stroke performance.
The 2-cylinder: Proposing a new type of snowmobile
In profile, Yamaha's 2-cylinder snowmobile power unit was a 500cc liquid-cooled, 4-stroke, DOHC, parallel 2-cylinder, 5-valve engine. The Phazer series models showcasing this engine were developed as Yamaha's proposal for a new type of sport snowmobile. The concept was a snowmobile for X-treme sports. The message Yamaha Motor wanted to send to snowmobilers with the Phazer was that this was a model that would respond the way they wanted and help them enjoy the pure fun of snowmobiling with performance that surpassed the limits of conventional models geared for riding-for-fun or touring. The key to this new dimension of performance was the 2-cylinder engine.
The development targets for this new Yamaha engine were: "super lightweight, super compact and high power output." For the engine layout, there were a number of choices considered like a 660cc single-cylinder, a 600cc parallel 2-cylinder, a 600cc V-twin and others, but it was finally decided that a 500cc parallel 2-cylinder layout fit the development objectives best.
Although 3- and 4-cylinder motorcycle engines have been used successfully for developing snowmobile engines, the 2-cylinder would have the same bore and stroke specs as the engine of the YZ250F motorcrosser at the time. However as a 4-stroke snowmobile engine, it would be newly designed in basic dimensions such as the overall height of the cylinder head and connecting rod length.
One of the keys to achieving the objectives of super-light weight and compactness was the connecting rod length. Compared to the YZF-R1 engine which had the same displacement per cylinder, this connecting rod was 3 mm shorter at 100 mm, which reduced the weight by 25 g. Normally, for the ratio between the connecting rod and the radius of the crankshaft—know as the lambda—a ratio of 4 : 1 is considered standard, but the lambda ratio for this engine was 3.7 : 1. By shortening the connecting rod, the engine could be designed with less overall height, and by calculating in effects like that of a wider cam pitch, the distance between the crankshaft and cam shaft axis was reduced by slightly over 10 mm compared to that of the YZF-R1 at the time.
To produce higher power output, fuel injection was adopted and, with consideration for the fact that maximum power output would be reached at 11,250 rpm, a crank phase of 180 degrees was chosen to enable the use of a single-shaft, coupled-force balancer and thus contribute to a more compact engine design. This had advantages in terms of size and weight compared to a 270 or 360 degree crank phase, either of which would require a two-shaft balancer layout. Long hours of development work were also spent finding the optimum exhaust system settings for this design.
The 2-cylinder born of these efforts underwent further development for the 2011 model. It also incorporated a new "oil Pressure Sensing System" that regulates the ignition system and fuel supply system to limit engine rpm and control load when oil pressure in the engine drops drastically due to external forces. This helps improve engine reliability.