Mostly used in Racing situations but not limited to, an oil catch can is used where excessive blow-by of air and fuel vapor occurs (including humidity). This creates a positive pressure in the crankcase. Engine manufactures have placed a valve on the engine block which releases this pressure. This valve is known as a PCV (Positive Crankcase Ventilation) valve. During engine operation, blow-by gases, as well as oil mist from the rotating components of the engine, pass through the PCV valve and are routed back into the intake for the engine to burn off. However, some of the oil mist and other products settle along the engine intake and over time form a "gunk." The oil catch can collects the oil mist and condenses the fuel vapors while allowing "cleaner" gases to be passed back into the intake. Typically the blow-by gasses are passed through a wire mesh, which give the vapor droplets something to adhere to. Since the oil catch cans condense the vapor portion of the gasses, they will need to be drained periodically of all the oil, fuel and other contaminants.
Here is something I wrote on another thread about Catch can's and tuners.
"There does seem to be a lot of confusion on pretty much all the forums in regards to catch cans and what they do and why when one has a tune on there engine it is a wise idea to have a catch can ( if your engine can take one). Not only will it help keep your intake clean which is a good thing, but it will also help you burn your fuel cleaner.
So Here is some Information on What it is and why its a good Idea to have one with a tuner.
For anyone that does not know what a catch can is its a container that catches fumes and vapors from the valve assembly or crank case prior to being introduced back into the intake system for emissions purposes.
From what I have been gathering the main reason to install a catch can with a tuner is to catch the fumes and vapors and to prevent them from re-entering the intake system thus contaminating the Air/Fuel mixture Quality being used by the engine during its burn (also can help keep the intake cleaner than without one). The fumes and vapors that are re-introduced into the intake will carry oil, fuel vapor, and can also carry condensation from the valve assembly or crank case. These fumes reduce performance, longevity and can actually reduce the octane level in your combustion which in turn can steal fuel economy and could cause harmful detonation. There are many factors that can contribute to the detriment of Octane levels in a fuel being burned within an engine, some of these are air quality, humidity, age of the Gas used, Station mix ratio's and also Oxygen levels due to the level the truck is to the sea. Tuners modify the engine mapping, fuel delivery and other adjustments to try and extract the maximum power the engine can provide without it being detrimental to it, these adjustments can demand a very precise Fuel/air mixture, any contaminants introduced into the burn cycle can contribute to the Fuel mixture being less than adequate for the tune's new configuration of the engine. The Catch can will eliminate most contaminants from this process allowing for less or no contamination of the mixture being burned. People who install tuners can be in area's where one or many of these influences could create for an Octane mess which can then cause harmful detonation. Also Grime and sludge build up on the valves and cylinder head further make a mess of your engine over time so the catch can now catching these harmful vapors will help keep your engine cleaner and running better. Also I would like to point out, in my experience I have found that using a slightly Higher Octane fuel than that recommend by the Tuner company can alleviate many issues with engine detonation due to external influences on the Octane level of fuel used in said Vehicle.
I am in no way an engine engineer and don't consider myself one, over the years of racing and talking to these said people of knowledge the above information is an interpretation of what I have learned over the years as a Driver and vehicle user, the information is my opinion on said information and is to be taken as such. I don't condone nor sell these products. I do own 2 vehicles with Tuners and catch cans and have had them in all my race cars over the years. Please use this information for what it is, information."