Actually since this is an oil-catch can, it has 2 top connectors, since its purpose is to separate the blown oil from the valve cover ventilation, the bottom I assume is to return the oil back to the engine if wanted. But you will still use both connectors. One will be hooked to the radiator and the other will be your overflow hose, you can leave it open or hook the hose and aim it to the ground (thats what I did).