PREVIEW: Kawasaki’s H2 Superbike

A technical look at Kawasaki’s upcoming supercharged superbike (based on what we know so far). Bonus: Kawasaki teaser video!

Kawasaki Ninja H2 lead artwork

In 1969, Kawasaki decided to disrupt what it saw as a sleeping motorcycle market. In quick succession, the company introduced the two-stroke 500cc H1 triple, then in 1972 the two-stroke 750 H2 triple, and in 1973, the epoch-making 903cc Z1 four-stroke. Each of these machines in succession raised the performance bar, daring the other makers to wake up and compete in a high-performance market.

Kawasaki is poised to repeat this slap with a supercharged machine that may pack the punch of an unlimited machine like the ZX-14R or Suzuki Hayabusa in the size and weight of a liter-bike. Just as in 1969, the competition is resting comfortably, offering sportbike models that have been in production for years, updated only with the usual "bold new graphics." Time for an attention-getting bonk on the nose with a length of two-by-four!

Kawasaki has teased us with sound-bites of rev-and-squeak acceleration, it’s shown a supercharged “idea bike” at the 2013 Intermot show, and it’s tantalized us with patent drawings of gear-driven blower-bikes.

So let’s consider the various strategies Kawasaki might employ in such a bike. Remember–a supercharger is driven mechanically from the crankshaft. A turbocharger is just a supercharger that is driven by an exhaust-gas turbine instead of by mechanical means.

As an engine is given more and more boost, a level is soon reached beyond which a lower compression ratio becomes necessary to prevent detonation. This lower compression was responsible for the rather dull street performance and throttle response of some early supercharged bikes.

Also, the more boost you apply, the more you need a charge air cooler (aka "intercooler") to remove the heat of compression from air going to engine inlets (the hotter the intake air, the more likely deto becomes). The charge cooler is bulky, so the patent language suggests that the fuel could be injected—upstream of the supercharger, using the cooling effect of fuel evaporation to reduce charge temperature (Most WWII supercharged aircraft piston engines, except for German designs, used this technique).

Kawasaki patent engine side view

As boost increases, more knock-resistant fuel (higher octane number) may be required as an added defense against detonation (combustion knock). Some engines are given knock sensors and a system that retards the ignition spark if knock is detected.

The above factors work against using more than 5 to 10 psi of boost. More can certainly be made workable, but as boost rises, techniques for its management grow in complexity.

If Kawasaki is using a centrifugal supercharger (everything in its patent drawings says it is), that choice fits with the above because a centrifugal blower, being the most efficient type, has the smallest temperature rise. Kawasaki does use a less efficient Roots blower on its 300-horsepower, 1500cc watercraft engine, but with a boat there is unlimited cold water available for the charge air cooler.

Let’s see roughly what various possible boost levels would produce in three Kawasaki engines; the ZX-14R, ZX10R, and ZX6R.

Boost Horsepower
A 180-hp ZX-14R 5 psi 240
7.5 psi 272
10 psi 302

The above numbers certainly would be exciting, but realistically are too much for street use.

Boost Horsepower
A 153-hp ZX-10R 5 psi 203
7.5 psi 231
10 psi 257
Boost Horsepower
A 107-hp ZX-6R 5 psi 143
7.5 psi 162
10 psi 180

The one set of figures that stands out to me is taking the ZX-10R to 203 hp on 5 psi of boost. Why boost the 1000 rather than the ZX-14R or the 600? Right now, liter-bikes get the cream of chassis and suspension development, and get all the development in racing. This makes them the center of the high-performance market.

Normally, the big problem with a centrifugal supercharger is this: In a single- ratio drive, it boosts power in proportion to the square of rpm, giving a steep, almost unrideable, power curve (which is why turbo bikes are, to put it mildly, tricky to ride off corners). Patent drawings show a two-speed drive, but the text also mentions continuously variable drives. The point of this ratio changing is to keep the supercharger impeller’s top speed close to 1000 feet per second as engine rpm increases. Continuously variable drives are not cheap, which probably explains why the “preferred embodiment” is the simple two-speed drive actually shown.

One drawing also shows a one-way roller clutch in the drive to the impeller so that sudden reductions in engine rpm (throttle blips) are not transmitted to the blower. The dimensions implied in the drawing (scaled from crankshaft stroke) suggest unreal impeller drive ratios.

Kawasaki patent side view

All of this equipment—the gear drive from the rear of the balancer gear on the crank, the two-speed or variable drive, the planetary step-up gearing and the centrifugal blower—is packaged behind the cylinders and above the gearbox. The output from the blower’s scroll housing is directed upward, pressurizing the intake airbox that in turn feeds air to the four intake throttle bodies.

World War II fighter aircraft also had multi-speed superchargers, but for a quite different purpose. As the aircraft climbed, turning the blower faster and faster compensated for the loss of atmospheric pressure at altitude (mountain snowmobilers love turbos—they just keep on pulling, even up at 12,000 feet!). Kawasaki built the supercharged Ha-40 V-12 aircraft piston engine in 1941-45 with the same efficient centrifugal supercharger type shown in the H2 patent drawings.

With a blower, an engine has less need of the longer cam timings that normally deliver strong top-end performance—because the blower can force in airflow that would normally be let in by longer timing. Without need for such longer timing, low-end and midrange performance would benefit. The result could be a nice rideable engine whose torque output could very easily be smoothed by use of electronics.

Why take this step now? No one knows what lies ahead, but one way to find out is to give the future a poke with something like this supercharged H2. If it’s based on an existing bike, the H2 will be much less expensive to develop than a whole new model. Either way, it’s clear to us that Kawasaki will soon plant the flag on the mountaintop and challenge others to follow.