Originally Posted by 1971mach1 (Post 7480079)

The inside of the engine is constantly changing air pressure so this air mixed with toxic gasses has to go somewhere so it gets vented thru the valve cover. A PCV hooked to manifold vacuum will pull the bad gasses out of the (passenger side) valve cover and burn them. Clean air gets pulled into the engine from the breather on the driver side valve cover (also used as the oil cap). If the breather is clogged with oil or gunk, it won't work properly and you'll get a nasty oily/gas/exhaust fumey smell. Likewise if the PCV has no vacuum, or you don't have a PCV, same smell.
Some of the early cars vented these toxic gases by hose under the car. Unless it's a trailer queen, get a PCV, it is so much better when it works right and it doesn't affect performance at all.