In 2007, Yamaha became the only snowmobile manufacturer to use a four-stroke only across its line-up (in the United States only – the VK 540 model remained available as a 2-stroke in other markets). Yamaha had introduced 4-strokes to their line-up in 2003 with the release of the RX-1. This 4 cylinder model became the first performance-oriented 4-stroke snowmobile on the market (it was not the first modern 4-stroke snowmobile produced - that honor belongs to Arctic Cat for their Yellowstone Special (released in 2000), which was designed as a rental sled that could meet Yellowstone National Park's stringent emission requirement). However, Yamaha received much criticism for its weight disadvantage when compared to similar 2-strokes, despite its fuel economy and low-range torque. Yamaha further used 4-stroke technology to introduce the 80FI engine equipped in the Phazer and Venture Lite models in order to provide small displacement, lower horsepower models marketed towards smaller riders. This engine had one of the highest specific output of any 4-stroke in production, with 160 HP/L. Yamaha achieves this even without the use of a forced induction system. Yamaha is also a key player in the "4-Stroke Wars", which are a series of advertisements from opponent Ski-Doo, who claim their E-Tec-equipped 2-strokes are still cleaner and more efficient than 4-strokes, while Yamaha claims the 4-strokes are cleaner and more reliable.