A road draft tube develops a suction at the end that hangs down into the airflow under the car, at speeds over 30 MPH. To get airflow through the engine, there must be an inlet somewhere. Early valve covers had slots, which also let dust into the engine. Later model valve covers without the vent slots need a vented oil filler cap with a dust filter built in. Engines with a PCV setup instead of a road draft tube had a tube from the "clean" side of the carburetor air cleaner to a nipple in the valve cover. That's called a "closed" ventilating system. California was one of the first places where the tree huggers demanded that no crankcase vapors could be vented to the atmosphere.
Jerry
