The 2004 NINJA® ZX-6R is Kawasaki's middleweight contender for the 600cc sportbike class, the most popular displacement of sport motorcycles in the United States. With 37 more cubic centimeters of engine displacement than its sibling, the competition-oriented ZX-6RR, it is perfect for motorcycle enthusiasts who want more torque for riding the twisty backroads or racing in modified 600cc sportbikes classes. The extra displacement also gives the NINJA ZX-6R an edge over the competition for everyday, real-world riding. As CycleWorld.com said, "Those extra 37cc make for a bigger difference than you'd expect, as the engine produces more midrange torque and the most horsepower compared to its displacement-starved competition." Powering the ZX-6R is a liquid cooled, DOHC, 16-valve, 636cc, four-cylinder engine tuned for optimum performance, especially in the high rpm ranges. Digital fuel injection and large-bore 38mm throttle bodies with sub-throttle valves, plus a centrally located Ram Air duct system works in conjunction with a digital ignition system to deliver precise throttle response through the entire rpm range.