AmSnow.com is now SnoWest.com
Why so reliable?What makes 4-strokes reliable?
More than 100 years of development in cars and motorcycles pays dividends. For instance, motorcycles are now expected to last 100,000 miles, and the drivetrain "duty cycle" that bikes and cars must go through during development is much more severe than the standards for snowmobiles.
For example, the RX-1's engine had already seen 10 years of development in Yamaha's top-line road bike. Although a 2-stroke engine can produce more power per cc and with less weight, it's also higher stressed because the piston sees a combustion load on every revolution. A 4-stroke only fires on every other revolution, leaving the piston a full revolution to cool off with a fresh incoming mix of air and fuel.
While the 2-stroke crankcase is used as a pump, and only has a small amount of oil mist in it, the 4-stroke crankcase is full of oil that swirls around and lubricates the cylinder walls and cools the pistons. Most 4-stroke crank designs use automotive plain bearings, which float on an oil film and with split rods make it possible to use very strong one-piece forged cranks.
Performance 4-strokes also have over square cylinder dimensions. This means that the bore often is 50% larger than the strokes. This permits more and larger valves to improve breathing efficiency at higher rpm. Combine this with more and smaller cylinders, and you have a 4-cylinder 4-stroke RX-1 motor revving effortlessly at 10,000 rpm.
Extreme over square dimensions do not work well in 2-strokes, because it compromises the transfer flow and makes cylinder filling less efficient. 2-stroke engines produce more power when cylinder bores are close to the same as the stroke, and this means that a performance 2-stroke usually ends up with a longer stroke for the same displacement and number of cylinders, which puts increased load on the pistons, cranks and bearings. 2-strokes usually are quicker in throttle response and the engines are lighter per horsepower produced because of less rotational mass and a lack of valve train to power.
Yamaha is using its snocross racing effort to address these issues. Yamaha has proven that the 3-cylinder Nytro engine has adequate power to compete. Yamaha riders can usually be found out front during starts because of output thought to be in the mid-150s and with a wider power band than the 2-strokes. Reliability is not an issue either, although the triples are revving between 9,500 and 10,000 rpm.
According to Yamaha Race Manager Greg Marier, this year's engines have a lightened crank and the "pork chop" balancers on the balance shaft have been eliminated. The valves are titanium, and even the cases are CNC-machined to remove weight. In total, more than 10 lbs. have been cut from the cases, cranks, balance shafts and valve train.
The fuel injection system also is different this season. Six injectors, two per cylinder, are now used to improve mixture distribution and supply in all rpm and throttle ranges. This was done to improve throttle response out of corners and through mogul sections where timing jumps is critical. More information was not available from U.S. Yamaha sources, because they never see the inside of the engines. The race engines easily run the snocross season without a problem or teardown, and are then removed from the sleds and sent back to the factory in Japan for teardown and inspection.
The fuel-injected 3-cylinder 1000cc 4-stroke engine is emerging as a good compromise between power and weight in the top end of the performance market. Yamaha's racing involvement is improving and refining the concept, and is convincing a growing portion of consumers that 4-strokes are viable as a snowmobile power source. With Ski-Doo's 1200cc triple producing competitive power, and Arctic Cat's Z1 4-stroke turbo now being branded as the top dog in the horsepower department with 180+ hp, and Polaris' 140+ hp 4-stroke turbo, 4-strokes are emerging as serious contenders even in the upper level of the performance market.