Honda, Yamaha, Kawasaki and Suzuki rule the 600 sportbike class, but it was Kawasaki that rocked the 600 world in 2003, with the completely new ultra-light and ultra-powerful ZX-6R.
With 107.8 dyno-tested rear-wheel hp, it's the most powerful production 600 the world has ever seen. Combine that with a dry weight of just 389 pounds, and the ZX-6R has an incredible power-to-weight ratio of 5.2 pounds per horsepower with a 170-pound rider aboard.
The resulting performance is breathtaking. The ZX-6R squirts to 60 mph in 3.27 seconds and through the quarter mile in 10.87 seconds at 128.4 mph.
Powering this beast is a new all-aluminum DOHC inline four-cylinder and a six-speed transmission. For '03, the engine received a reduced overall cylinder head height for a more compact package, shorter valve stems, new single intake valve springs with oval cross section wire to save a few grams and improve valve control at high rpm. There is also better camshaft oiling for durability. The compression ratio is an unreal 13.0:1, and the cams feature slotted sprocket-mounting holes, so inveterate tinkerers can piss away their Saturdays looking for the killer cam timing.
The cherry on top is Kawasaki's new intake tract, which works with beer-bong efficiency. It jams big gulps of atmosphere into a quartet of new 38mm fuel-injection throttle bodies via a centrally located NACA ram-air duct system that blows right through the frame's steering head for the straightest possible shot into the engine. Basically, it's a hoodscoop.
The ZX-6R's chassis bits are every bit as impressive. The new aluminum perimeter frame is made of alloy pressings that are welded together. This puts the strength where it's needed most, while keeping weight to an absolute minimum. Flex hurts steering precision and stability, so Kawasaki fights back with a stiff, new inverted 41mm-diameter front fork. In back there's a single gas-pressurized Kayaba shock working through a UNI-TRAK linkage. Naturally, the suspension at both ends is fully adjustable. The dual 280mm front disc brakes are things of beauty, with radial-mount, four-piston, four-pad, dual calipers that deliver single-finger braking power and track-ready fade resistance.
The ZX-6R is really two cosmetically indistinguishable motorcycles: the ZX-6R and the limited-production ZX-6RR. What's the diff?
The $7,999 ZX-6R has a displacement of 636cc, while the $8,399 RR has a displacement of 599cc.
Why? Racing.
The big-engined version can race in 600 class club-level events just about anywhere in the country, since most organizations allow a small amount of extra displacement to accommodate tired cylinders that have been rebored and fitted with slightly oversize pistons. For AMA and FIM 600 Supersport racers, the ZX-6RR motor meets the smaller displacement rules of these classes.
The two engines have different bore, stroke and connecting rod dimensions. The RR further justifies its $400 higher price with strong forged (instead of cast) pistons and a back-torque-limiting clutch to help keep the rear wheel from hopping and chattering while downshifting at the limits of traction. The RR also has an adjustable swing-arm pivot to let you fine-tune the handling to suit a specific track, and a handy threaded frame boss so you can bolt on an aftermarket steering damper to calm head-shake under acceleration. Kawasaki also offers a comprehensive racing kit for the RR.
You might expect the mere 37cc difference between the R and RR spec engines wouldn't much matter, but on the road, the small engine's 6 less peak horsepower and decidedly weaker midrange torque characteristics are noticeable. We use the term "midrange" advisedly, since redline for both engines is a lofty 15,500 rpm.
Both bikes also feature a programmable shift light and a handlebar-mounted lap timer that recalls your last 99 laps. By then, you'll probably want to stop for a while anyway, don't you think?