Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community - FT86CLUB

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An oil catch can is used in turbo applications, or high-performance race applications where excessive blow-by (leakage past the piston rings) of air and fuel vapor occurs. 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.

In general the same reasons that it would normally be on a FI engine would be valid reasons for putting it on an NA engine. Keeping gunk out of your intake runners mainly.