Kawasaki ZRX1100: Project Z-Rex

Transplant some of the ZX-11's heart into the ZRX1100 and watch this bad boy eat sportbikes for breakfast.

We'll be the first to admit the stock ZRX1100 is a great bike-lots of low-end torque, good handling, stellar looks. What more could you possibly want? Um...more power? Even better handling? This is a superbike replica after all, it should act like one. We wanted to pump up our ZRX using some ZX-11 parts and aftermarket goodies, but we didn't want to spend a pile of money or take the motor apart. We were pleasantly surprised at how easy it was to turn the ZRX into something more appropriate for our sporting tastes.

A look at the torque curve will explain what's going on inside the ZRX's mill; peak torque of 71.5 foot-pounds occurs at just 5750 rpm. The ZX-11 (which the ZRX engine is based on) makes only slightly more torque, but with bigger carburetors, different cams and more compression, it does so at higher rpm to give it 131 horsepower-35 more than the ZRX. If we could move the ZRX's curve up in the rev range to be more like the ZX-11's, its power would go up accordingly. It may not reach the peak levels of the 11, but if the green bike could hold, say 60 foot-pounds of torque to 10,000 rpm, that would work out to about 115 horsepower. And that is nothing to sneeze at.

Initially, we wanted to graft the 11's cams and carburetors onto the ZRX, but we realized the ZX-11 carbs are downdraft and the ZRX's are sidedraft-a no-go. We settled for using the cams, along with a jet kit and pipe. The ZX-11 cams offer approximately 1mm of additional lift and 30 degrees more duration than the ZRX bumpsticks. Ideally, the ZX-11 pistons would be nice to use also, but more compression does not necessarily translate to more top-end power, and changing pistons is a big job. Deciding to keep it simple and cost-effective, a Factory jet kit and a D&D; all-black pipe were ordered up along with the cams, and then we got to work.

To keep pace with the anticipated extra ponies, the suspension was sent off to Jason Lewis at Race-Tech for a once-over. Lewis installed Gold Valves in the cartridge-style fork, along with stiffer springs and fresh oil. The rear shocks were rebuilt and thicker oil was used to increase high-speed damping. The springs were stiffened slightly by cutting off part of a coil. For looks and comfort, we ordered an AFAM superbike-bend handlebar from Lockhart Phillips, a Corbin seat and a Targa tank bra.

Installation of the pipe and jet kit was simple enough, and the fact that the Kawasaki's ZX series utilizes followers for valve actuation vs. shim-under-bucket eased the cam switch. (The cams don't have to be removed to change shims.) We swapped the cams ourselves, but took the bike to Alex White's Motorcycle Performance shop for a valve adjustment afterward. Only three shims needed replacement. We didn't want to get too involved with cam timing, so the ZX-11's bumpsticks were bolted to the ZRX sprockets with no slotting.

The results of the engine modifications are incredible. With the pipe, jet kit and cams installed, our Z-ReX has an amazing 124 horsepower-a whopping 27 horsepower more than stock. Just as we had hoped, the torque curve matches the stocker's up to approximately 6000 rpm. However, where the stock torque falls off at that point, the Z-ReX's torque curve continues on, peaking at 7500 rpm and not falling away as rapidly. This makes the modified bike's power curve tower above the mild-mannered ZRX, while retaining the stock bike's ridability and torque characteristics.

The action of the modified suspension matches the increase in the engine's performance. The Z-ReX doesn't have the midcorner wallow of the stock bike and it offers a more confidence-inspiring ride. The bike moves around much less on its fork and shocks, and corners can be attacked with aplomb. We've surprised a number of sportbikes on the canyon roads around our Los Angeles office. The low, wide AFAM handlebar gives lots of steering leverage and the flat Corbin seat puts you in a good riding position, as well as letting you go all day without suffering from monkey butt.

Total cost for the cams, jet kit, pipe and suspension is less than 1400 bucks. This is a smokin' deal for the huge increase in performance our Z-ReX has over the already-great ZRX.

Quarter-mile: 11.02 @ 125.11 mph
Top speed: 150.40 mph
Roll-on, 60-80 mph: 3.39 sec.
Roll-on, 80-100 mph: 3.90 sec.

This article was originally published in the October, 1999 issue of Sport Rider.