PCV ("POSITIVE crankcase ventilation") introduces a filtered, metered airflow from the "clean" side of the air filter through the crankcase and into the intake manifold. The normal intake manifold pulsations take small sips of air from the crankcase through a spring loaded valve instead of a constant flow. The flow rate and frequency varies according to engine speed, throttle position, and manifold vacuum. The vapors and gases from the crankcase are burned in the combustion chambers. The fuel system is calibrated to compensate for the volume of air flow that bypasses the carburetor. It's a much more effective means of clearing out the blowby and condensation from the crankcase than relying on vacuum produced by air flow past the vent tube and introducing unfiltered, dusty air into slots in the valve cover. When the tree huggers in California in the mid-1970's decreed that ALL vehicles regardless of age had to be converted to PCV, my shop sold and installed hundreds of retro-fit kits mandated by the state and enforced by roadside "smog check" roadblocks. One unintended consequence was a major increase in engine life from less abrasive wear because the dusty central San Joaquin valley air got filtered before being sucked into the crankcase.
Jerry
