OK Guys, I know this will cause some trouble -but - we have cars other than Vettes in our stable. One is a 56 Ford with a 312 engine. While doing some research on this motor, I came across the engine ventilation section. Ford did ALOT of testing to perfect the crankcase ventilation on their Y-Block engines especially pre-PCV. The book I was reading even has a diagram of the air flow through the engine. Anyway, Ford could not be alone here - GM had to do the same testing. The vented cap was an intregal part of the Ford system along with [are you ready] the cooling fan. They described the small amount of vaccuum created [road draft] as the car was driven and how that "helped" with the ventilation. As I stated in a previous post, the closed cap was put on in response to complaints by customers, who had the oil slick problem. Maybe GM didn't care if the crankcase was vented properly so long as the complaints went away. PCV stands for Positive Crankcase Ventilation right? With that you must have an air intake of some kind. PCV valves are designed to work with the engine's vaccuum and the different amounts of vaccuum at different engine speeds and loads. Because the PCV valve's vaccuum source is the intake manifold, except at WOT or heavy load there is always vaccuum, therefore to ventilate the crankcase and provide for "cleaning" of contaminates, not suck gaskets "in" AND keep the feds happy, fresh air is a must.