Many Subaru owners find themselves in a deep debate about the best STi air-oil separator. Some argue the Grimmspeed AOS is the best, while others profess that the Crawford Air-Oil separator is the only choice. Truth is, the best STi air-oil separator is one that will be used for it's intended purpose. The aftermarket is chock-full of performance parts. Some tout features such as "PCV valve retention" and "large volumetric swirl pot". Sure, you can read the product description and find yourself in awe about select features, but do you know why it's beneficial to your car?

• GrimmSpeed WRX/STi Air-Oil Separator
• Moroso AOS
• Perrin Air-Oil Separator
• Crawford Air-Oil Separator

First, lets look at the GrimmSpeed WRX/STi Air-Oil Separator. Most importantly, the AOS is self-maintained and coalesces oil without a mesh media. This means you no longer have to check or empty your catch can. It's also very compact and slim, which is nice. You're also permitted to retain OEM PCV system, which is great for daily driven cars. It's priced around $260 which is relatively inexpensive. Could this be a contender for the best STi air-oil separator?

Next, lets take a look at the Moroso AOS. Ringing up at $416, it is the most expensive air-oil separator, but offers a larger capacity due to it's size. You're still allowed to keep your factory Subaru PCV system, which is great news for all types of cars, be it Autocross or Daily Driver. One major drawback of the Moroso air-oil Separator is that it does need to be drained.

Third, the Perrin Air-Oil Separator gives you increases intercooler efficiency while allowing you to keep the factory PCV system. It's attractive powdercoated finish gives you durability over time, but does require cleaning and inspection. While not being as inexpensive as the GrimmSpeed unit, it's powdercoating finish is much cleaner and more durable. It's also water heated, which is great for cold weather locations and it prevents condensation.

Lastly, we have the Crawford Air-Oil Separator. This AOS employs a relatively new concept in it's chambered design. This chambering of the canister results in a gravity-like pull that separates the Oil. It is also water cooled, removing any chance of condensation which can be harmful to Air-Oil separators. This unit is compatible with both TMIC and FMIC, so that a major advantage.