GrimmSpeed has a unique design for the Subaru WRX, STI and Legacy GT which allows you to keep your PCV valve while separating the oil out of the crankcase and head ventilation air. None of the other competitors on the market are able to vent both. The unit is very compact and efficient, it bolts directly on to the factory oil cap location. You will no longer have to keep tabs on your oil level and or empty the catch can like on most standard cans as this unit is self maintained which requires no maintance at all. GrimmSpeed's AOS uses a baffling system that is unique to their unit only and does not allow oil back out of the can. You will only see clean air exit and never have to worry about oil blow by again! This unit prevents reduced fuel octane, engine knock, and worse emmisions that are caused by oil in the system. This item is a winner in every aspect and is perfect for completely stock cars to full out race cars.