Right now also in centrifugal because of high rpm since it is directly coupled to engine the pump creates partial vacuum this means the liquid will certainly flow through vacuum therefore the liquid will be recieved at the centrifugal pumps suction and as the impeller rotates the fluid will be displaced aside forward with a particular Centrifugal Vacuum Pump energy.
A vacuum pump has the inlet hooked up to 1 or both valve covers, sometimes the valley pan. 
It SUCKS the air flow from the engine, therefore reducing the atmosphere pressure build-up created by blow because of combustion gases going past the piston rings into the pan.
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