There is no explicit answer other than efficiency...
Which then contributes to the lower 8.5:1 compression ratio, it takes into account what happens upstream in the airbox...When Kohei Yamada, designer of Kawasaki’s original Ninja 900 of 1984, planned this supercharged ogre-bike that started as a skunkworks project, he asked the aftermarket for a blower that could do everything he wanted it to do and work well without a bulky intercooler (no room!).
“No way,” came the reply. Compressing air makes it hot, and if that heat is not removed, it pushes combustion toward knock or detonation. So Yamada went to Kawasaki’s own Gas Turbine & Machinery Co., which designs and manufactures jet engines. They designed a fast-spinning centrifugal compressor whose high efficiency would heat the airflow least.
That compressor’s 2.7-inch impeller is the heart of this motorcycle and has the beauty of natural law. Manufactured from a forging, it spins at up to 140,000 rpm. Air is flung outward from its six vanes at up to 1,500 feet per second, and then this velocity energy becomes pressure peaking at 38.4 psi in the blower’s scroll housing. This pressure is ducted upward into the “highly rigid” aluminum airbox, containing the engine’s four steeply downdraft 50mm Mikuni intake throttle bodies. The airbox has to be rigid for the same reason 747 fuselages are: They are pressurized.
So yea no intercooling but Kawasaki has mitigated the heat through small efficient applications throughout the powertrain. Sounds like a bunch of brilliance if you ask me...Because so much compression takes place in the supercharger, engine compression ratio must be reduced to 8.5:1. The cast pistons (as in two-strokes and in gas turbines, materials with highest hot strength cannot be forged, only cast) have flat, featureless tops and no squish: In supercharged engines there is intense charge motion from their higher intake velocity, and the open combustion chamber preserves flame-accelerating turbulence all the way to TDC. Each piston’s heavy thermal load is handled from below by two oil jets. For reduced friction, there are two compression rings and an oil scraper. The special connecting rods have larger big-end bearings and larger (than ZX-10R) rod cap bolts.