Blown Away--Supercharged Kawasaki ZRX1200

When most people decide to build a big-horsepower project bike, they always go for the easy candidates: Hayabusa, ZX-12R, GSX-R1000. Starting with a proven base platform is the logical way to go since it keeps major modifications to a minimum, and the chances of coming up with those big numbers (which is not always a sure thing) are much higher. But then you have guys like John Voter. Some of you may remember his last project bike, a nitrous-injected GSX-R600 that topped out at 193 mph ("600s Gone Wild," April 2001)--not exactly your typical high-horsepower monster. "I like machines that are different than all the others you usually see at the local bike hangout," says Voter, adding that "this ZRX project came out of a dinner conversation Richard [Sims] and I had one night. I told him I wanted to build something out of the ordinary, and when we were throwing around ideas on what bike to use, I suddenly thought of the Kawasaki ZRX1200 I had sitting in my garage."

Sims should be familiar to longtime SR readers as well. His unreal creations have been featured in our pages since the beginning of SR (and even before--the original "Superbikes From Hell" comparos in Motorcyclist saw several of Sims' speed machines compete). He's built everything from 1400cc normally aspirated beasts to supercharged 750s, all of which left us slack-jawed at the sheer engineering work that went into them. As Doug Meyer of Muzzys Performance said recently, "Sims probably has one of the best `true' engineering minds in the country. And he still does all his designing on a drafting table with pencil and drafting tools, not a CAD program on a computer."

In fact, Rob Muzzy recognized Sims' talents enough to hire him as one of the company's chief product designers. Adding him to the Muzzys' R&D team resulted in many of the latest trick pieces that have come out of the Muzzys' facility in Bend, Oregon (and many you haven't seen, including a totally hidden/self-contained nitrous system for a ZX-12R). It was the combination of Sims' engineering mind and the considerable resources of Muzzy's Performance that persuaded Voter to commission Rob Muzzy and company to build this latest one-of-a-kind pavement rippler.

A MAJOR ENGINEERING TASK
The concept of bolting a supercharger onto an '03 Kawasaki ZRX1200 is fairly audacious, especially considering this would be designed and built completely from scratch; there aren't any bolt-on "supercharger kits" available. Take a closer look at all the details and you can see just what a considerable engineering feat this bike really is. "There were a lot of people [who] said this couldn't be done, waiting for the project to fail," recalls Sims.

Why a supercharger and not a turbo? Other than the "different" factor, Voter says he "wanted a bike that I could ride on the street that would haul ass without having to put up with the low-speed rideability and ground clearance problems that usually come with bikes like this." A supercharger's main advantage is that since it is driven off the crank (instead of the exhaust flow, like a turbo), the response is instantaneous--no "turbo lag" as you wait for the turbo to spool up. "John doesn't ride in the high rpms all the time, so he wanted something that would give him major power the instant he twists the throttle," says Sims.

At the heart of all that power is an Eaton 45-cubic-inch "high helix" Roots-type blower sourced through noted supercharger proprietor Jerry Magnuson that features an internal bypass valve which eliminates the normal parasitic power losses associated with superchargers at cruising speeds. Since a supercharger is driven directly off the crankshaft, it is always pressurizing the intake tract; the bypass valve works by opening during the aforementioned cruising situations and allowing boost pressure to bleed off. Once the throttle opening is increased for more acceleration, the valve shuts and permits the blower to instantaneously provide boost.

The big problem was providing the proper amount of fuel and intake airflow for the blower unit, but in a way that allowed its power production to be hospitable enough for street use. This required junking the ZRX's stock carburetor setup, as it would be far from adequate for this application. It also meant designing and fabricating a throttle body that could provide a progressive intake flow from low rpm (superchargers provide instantaneous throttle response and boost, which can be a handful if improperly tuned), and adapting a fuel injection system to work with both this custom piece and the supercharger itself--a huge undertaking.

"Blowers don't like restrictions in the intake," says Sims. "If you notice the [top fuel dragsters], they have huge butterfly intake valves since they don't have to worry about coming off the line smoothly. That's where I really had some driveability problems in designing this thing; getting big-enough throttle bodies that would supply enough air to the blower, and yet not too big to make it unrideable." Sims initially designed and fabricated a complicated progressive linkage that would open one butterfly valve before the other, "but it took a muscleman to open the throttle. So I tied the two butterfly valves together and did some calculations on size. It works better because the blower is producing seven pounds of boost like it was designed without working too hard due to intake restriction and heating up the intake charge, or becoming too difficult to get off the line smoothly because the butterfly valves are too large."

Grafting the EFI system onto the ZRX was a serious chore. Due to the blower's large size, available space between the engine and frame was pretty scarce. "It was difficult to get the injectors close enough to the intake ports without running afoul of the blower unit," recalls Sims, forcing him to make what he thought was a compromise in the injector angle positioning. "I was able to get them angled at 40 degrees, but I really wanted to have them closer to 45-50 degrees. Still, it doesn't seem like it affected their performance much." Sims used injectors pirated from a Kawasaki ZX-12R because their flow rates are well within the needs of this particular application, and a Muzzys engine management system controls the works.