a PCV system ventilates the engine at all speeds, even idle. A road draft tube is ineffective below 30 MPH because it needs airflow past the end of the draft tube to create any flow through the crankcase. The biggest advantage is the introduction of clean air into the crankcase, if the inlet air is drawn from the "clean" side of the air filter. Road draft systems that rely on slots in the valve cover, or other unfiltered sources for the vent air introduce dust into the engine and increase abrasive wear of the engine parts. Even the vented oil fill caps had a very ineffective horsehair-type filter media in them.
When PCV systems became mandatory in the late 1960's, there was a dramatic improvement in engine life due to the reduced contamination of the lubricating oil.
When Califronia mandated that all engines should be equipped with "sealed" vent systems in the mid-1970's, the shop I owned retro-fitted hundreds of older engines with a special kit the Commiefornia legislature required. We plugged up the road draft tube, installed a PCV valve, and introduced clean air into the engine by installing a vent nipple from the filtered-air side of the air cleaner to the valve cover or crankcase. A sealed oil fill cap was part of the conversion to prevent any fumes from getting to the atmosphere. I can't recall ever having a problem when that was done.
Jerry
Last edited by Hotrod Lincoln; 09/29/2011 7:23 PM.
