Kawasaki Zx-6
KAWASAKI ZX-6
A STEAMROLLING 600 THAT CAN OUTRUN ANYTHING IN ITS CLASS. AND MOST 750s.
CYCLE WORLD TEST
SINCE 1991, HONDA HAS OCCUPIED high ground in the 600cc class with its CBR600F2, a machine that blends power, comfort and handling more successfully than perhaps any sportbike in recent history. The key to the CBR’s success is balance-it’s a bike that can be roadraced one weekend and ridden cross-state with a set of soft saddlebags lashed to its flanks the next.
For 1993, Kawasaki is out to give Honda a hard time.
Tired of sucking CBR600 exhaust fumes, Kawasaki designed its all-new ZX-6 with one goal in mind: to beat the multi-talented CBR at its own game and offer the most competent 600cc sportbike on the market. Like the CBR-but unlike the Suzuki GSXR600 and Yamaha FZR600, which take more of a race-replica approach-Kawasaki’s 1993 ZX-6 was intended to offer a wide range of capabilities rather than just expertise in one particular area.
Kawasaki’s previous ZX-6-winner of Cycle World's Best 600cc Streetbike award in 1990-offered a solid foundation from which to build. Three years later, the completely revised middleweight ZX gets a redone chassis along with a host of power-boosting changes to its engine.
Revisions within the ZX-6’s dohc, 599cc, liquid-cooled inline-Four include a higher compression ratio of 12.0:1, and lighter-weight pistons, piston pins, connecting rods and crankshaft-these combining to reduce reciprocating mass for a quicker-revving engine. The connecting rods are not only lighter and more compact, but have a 3mm-smaller big-end diameter, which reduces power-robbing friction between the rod and crank journal. The cylinder liners have been shortened 8mm to allow better crankcase windage between cylinders, reducing parasitic power loss.
PHOTOS BY RICH CHENET
Valve diameter remains unchanged at 26mm on the intake and 22mm for the exhaust, but the valves are now lighter, due to a .5mm reduction in stem diameter. Exhaust-valve tappet weight has also been reduced by 40 percent. New camshafts have 8.8mm lift and 308 degrees duration on the intake, and 8.2mm and 292 degrees on the exhaust side. While lift has only been increased .1mm on the intake and remains unchanged on the exhaust as compared to last year’s engine, duration has increased 28 degrees for intake and 12 degrees for exhaust. Valve-spring pressure has been reduced as a result of the lighter components and new wide-duration lobe profiles, which open and close the valves more gently.
Kawasaki’s Twin Ram Air induction is the most noteworthy feature outside the engine. Pioneered on the ZX-11 in 1990, this system improves volumetric efficiency as speed increases, and with it comes increased power output. Another unique feature is the 4-into-2-into-l-into-2 exhaust system, which uses crossover tubes connecting cylinders two and three, and cylinders one and four, designed to strengthen midrange power. Capping off engine changes is a digital ignition system, which has a new ignition-timing data map to complement the engine’s higher compression ratio and altered valve durations.
What sort of power gain has all this work rendered? It should first be noted that our dynamometer runs were performed in a static air-flow environment, nullifying the slight-yet-significant power gain offered by the ZX’s ramair system when the bike is running at high speed. Even so, the ZX-6’s power readings were impressive. A horsepower comparison of the new ZX-6 and the old-style model (which, like the old-old-style Ninja 600, remains in the lineup for ’93) shows the new motor pumping out 13 more peak horsepower, with as much as 10 more horsepower in the midrange. At 88 horsepower, the ZX-6 also outpowers 1992 versions of the CBR600F2 (85 horsepower), GSXR600 (80) and FZR600 (80), none of which has received any major internal alterations for 1993.
How does the new engine feel from the saddle, out on the road? Pin the throttle open at 3000 rpm in second gear and you’re answered with instantaneous throttle response and hard acceleration. There’s a noticeable increase in acceleration as the tach needle flashes past 6000 rpm and again at 8500. A very minor soft spot resides around 10,500 rpm, then it’s pure unbridled horsepower the rest of the way to the rev limiter, which tugs on the reins at an indicated 14,600 rpm. As verified by our dyno, the ZX’s tach proved to be accurate only at lower revs, becoming increasingly optimistic as revs increase, reading about 1000 rpm over actual engine speed at the upper end of its sweep. This would explain why our seat-of-the-jeans observations don’t quite match the power graph that you see in the data panel.
Rolling-on the throttle in a tall gear masks the peaks and valleys in the power curve, resulting in a seemingly linear and seamless flow of power from the motor. A hard run through the ZX’s six-speed gearbox propels it to threedigit speeds quicker than many larger-displacement machines, and if sheer speed is your vice, the more powerful, more aerodynamic ZX-6 has your fix.
Benefiting from a tailwind during top-speed testing, the ZX registered an astonishing 161 mph on the radar gun. A return pass into the wind produced 145 mph on the gun. Averaging the two runs gave us 153 mph, easily the fastest production 600 we’ve ever tested. In fact, the ZX would have run away from all 10 machines in Cycle World’s 750cc shootout last April. Suzuki’s new liquid-cooled GSX-R750 and Kawasaki’s own revised, ram-air ZX-7 should outrun the 6, but not by much.
Chassis stability at top velocity was nearly on par with that of its rock-solid, larger sibling, the ZX-11, but our first impressions of the new 6’s handling fell short of total satisfaction. It exhibited vague feel and feedback from the front end along with a hint of chassis weave when cornered hard. It was later discovered that the steering-head bearings had been overtorqued during set-up procedures.
Backing off the spanner ring a half-turn transformed our ZX-6 into a magnificently well-balanced sport-riding tool.
Bend the Kawasaki into a top-gear sweeper, and it tracks as if on rails. Steering remains neutral, with only a minimal amount of effort required to make line adjustments if needed. A change to Michelin A59X front and M59X rear radial tires along with a 1.5-inch reduction in wheelbase as compared to last year’s ZX-6 deliver quicker steering response at all speeds. Steering into slow comers is practically an extension of thought; minimal input through the handlebars produces an immediate and precise transition into the turn.
In the past, we have cited middleweight Kawasakis for being softly spmng and having too much compression damping, a combination that could jar the rider out of the saddle over sharp bumps. This has been rectified on the new machine. Although spring rates-both front and rear-are still a bit too soft for super-serious flogging, the Showa fork and shock perform admirably over a wide variety of conditions, soaking up road irregularities and freeway joints, and maintaining chassis composure right up to the limits dictated by cornering clearance and tire grip.
There’s no side-stepping the importance that full-tilt cornering capability plays in the ultra-competitive middleweight sportbike class. Fact is, it ranks right up there alongside acceleration and top speed-performance, and if the ZX-6 gives up anything to its competition, it’s cornering clearance. We immediately found the new 6’s limit at the Kawasaki press intro held at Laguna Seca Raceway. The footpeg feelers are first to touch down, giving advanced warning of the ensuing shower of sparks soon to follow. Without too much difficulty our 150-pound test rider managed to bevel both sides of the ZX’s exhaust system, centerstand, sidestand, fairing lowers, footpegs and shift lever. But while this will be a concern of anyone contemplating roadracing a ZX-6, brisk riding on the street seldom saw as much as a footpeg skimming the deck, with an occasional centerstand scrape when slamming through a depression bump while heeled over.
Fast laps on a road course also work the Kawasaki's front brakes-unchanged from last year with the exception of .5mm-thinner rotors-to their limit, bringing on fade and a soft, spongy feeling at the lever when used hard. But, again, for most riders this won't be a problem; on the street, the ZX-6's brakes work exceptionally well, providing strong stopping power and excellent feedback with firm lever feel.
In the past, another demon that tarnished the ZX-6’s otherwise clean reputation was excessive drivetrain lash. Thanks to a redesigned rear-wheel-coupling damper, the problem seems to have been exorcised once and for all. Without drive lash mucking up the works, shifting the new ZX is a much smoother and more pleasing experience. Gear changes from first to second are still quite noisy, accompanied by a solid mechanical thunk. Changing up into third is noticeably quieter, and rowing through the top three cogs is poetry in motion. Shifting throughout the entire gearbox, whether up or down, requires short lever travel and light effort. Clutch engagement is smooth and linear, with average effort needed to pull in the lever, and the clutch pack stood up to the abuse doled out at the dragstrip with nary a squawk.
Smooth also accurately describes the ZX’s engine, which produces only a moderate level of vibration. The minor degree of tingle that does sneak through to the bars, pegs and tank, rises to a small crescendo at 4500 rpm, where an audible buzz buried somewhere within the ZX’s bodywork sings out over the next 100 rpm, then vanishes without encore until the next pass through that rpm. Engine speeds between 5000 and 6000 rpm (60 to 70 mph in top gear) provide the state of greatest calm, while revving higher, especially above 8000 rpm, brings back a minor amount of tingle. Overall, though, the ZX-6 powerplant is one of the smoothest inline-Fours in recent memory.
This engine smoothness, combined with a well-padded saddle, generous bar and peg placement, and ample wind protection, makes the ZX-6 a very comfortable place to spend a full day of sport-touring. Convenience items like swing-out bungee hooks, dual tripmeters, a small in-fairing storage pocket and two under-seat stash areas add to the bike’s usefulness. Available in two color schemes-red and blue-purple as worn by our coverbike, or our testbike’s teal and black-the ZX-6 is priced at $6199. Not cheap for a 600, but not bad for a 750, which is what the ZX-6 acts like.
Kawasaski intended to set new middleweight standards with the 6, and succeeded on most counts. Certainly, the ZX-6’s handling, comfort and overall balance are worldclass. And its performance numbers speak for themselves. □
KAWASAKI ZX-6
$6199
