The centrifugal blower of a diesel engine super-charger operates under pulsating flow at 10 to 70 cycles per second, therefore if the scavenging system is not properly designed, the flow through the blower decreases remarkably and so does the power of the engine. The pulsating flow test with the rotary valve has proved that the characteristics of a centrifugal blower under steady flow condition are also available in pulsating condition. And we find that the graphical methods, taking into account the boundary conditions using steady flow centrifugal blower characteristics can be conveniently used. The equivalent length of the centrifugal blower is also determined by this test.