First off, let’s go over the purpose of a PCV (positive crankcase ventilation) valve. The engine compresses the air and fuel mixture in the combustion chamber using gaskets, valve seals, and piston rings to ensure pressure is not lost. The seal is not perfect though, and it gets worse as the parts wear. This phenomenon is called blow-by. The pressure that squeezes past the seals builds up in the crankcase and can leak out other parts of the engine. In the ‘20s, before emission laws, it was acceptable to allow this high hydrocarbon content vapor leak into the atmosphere. Of course, this didn’t last long, and the crankcase fumes were redirected into the engine using a PCV valve. It’s usually located at the top of the engine’s valve cover. It’s a one-way valve that allows the blow-by gases to be drawn back into the intake while not letting the intake breathe into the valve cover. This traditional setup can be upgraded with better-looking valves, but it hasn’t fundamentally changed. The down side to using a PCV valve is that along with the fuel-rich blow-by gases, oil vapors are trapped in the same spaces. This means that they both get sent back into the engine to be consumed with the regular air and fuel mixture. The oil can leave residue on the back of the valves and in the combustion chamber, inhibiting flow, and generally being dirty. Many engine builders use a catch can without a valve to allow free exchange of gases to the crankcase. These catch cans aren’t as environmentally friendly since the vapors are sill released into he the air, but they help keep the engine clean.
Another option is to use PCV valves, but re-introduce the crankcase vapors into the exhaust, that way the engine is not affected by the oil contamination. It’s also common for builders to incorporate a vacuum pump. This literally pulls the blow-by gases out and helps prevent the oil leaks caused by high-compression engine blow-by.