This adapter, from a 2.0 liter gasoline VW, fits in the oil fill cap location on the valve cover assembly. The oil cap attaches to the top of the breather, and it allows a second breather location for the cylinder head. Additional cylinder head breathing can be beneficial in highly modified TDIs, as it will reduce pressure in the cylinder head and allow oil to more easily flow to the head and the turbo. Most people use this modification with some sort of a catch can.