If you have a turbo on your car or you want to put a bigger turbo on than your current turbo engine. Once you have fitted the turbo, your engine will also need to breathe more out of the crankshaft vent, which knows there will also be more powerful oil vapors and these must be collected by an oil catch tank instead of being sent back into the engine. When you have not fitted an oil catch tank, the octane number is lowered and thereby you lose horsepower, as the engine will more easily ignite. The gasoline catch tank holds the oil there and has been sent out of the crankshaft vent. The oil catch tank is an important part of the tuning once you have mounted the turbo on the car.