Excess Crankcase Pressure

Without the PCV valve introducing vacuum to the crankcase, the rings will not seal quite as good. They need that added vacuum on the oil pan side of the pistons to aid in complete ring sealing. This is why there were road draft tubes before the PCV systems were introduced. There was so much crankcase vapor, that it had to "exit" the engine compartment, or it would appear the engine was on fire.