Air is a compressible fluid and increased compression and air flow into the
motorcycle engine is what a ram-air induction system is attempting to do.
However, air behaves more like an incompressible fluid at speeds less than Mach 0.3. At standard temperatures and pressures on the earth’s surface, the speed of sound is 768 miles per hour. So an air velocity greater than 230 mph would be required to begin to observe air compression in an air intake duct. To accomplish this in a ram air system requires that the air intake duct be tapered down to a smaller diameter to increase the velocity of the airflow to be greater than Mach 0.3 into the engine. This is similar to the intake of a jet engine and the design of these type ducts is highly dependent on the inlet pressure of the engine and the ram air compression at 12 to 14 thousand RPM for a YAMAHA YZF-R6 bike where they measured a 5 horsepower increase using the ram-air system. Also, compressing air increases its temperature, and this changes the Mach number as the air passes through the various stages of the intake system making the design problem even more complex. If you have read this far you are a nerd.