Modern Automotive Performance
Modern Automotive Performance

Best STi Air-Oil Separator | How-To Weed Out the Bad from the Good

Posted on 31 March 2014

Searching for the best STi air-oil separator ? Search no further as we dig deep for you!

best sti air-oil separator


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?

best sti air-oil separator GS

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?

best sti air-oil separator moroso

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.

best sti air-oil separator perrin

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.

best sti air-oil separator crawford

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.

In summary, let's review what we've read about the best STi air-oil separator. The GrimmSpeed AOS is great for daily driven vehicles with light modifications that wont see high boost levels. Consider it an entry level Air-Oil Separator. Either the Crawford or Perrin AOS would be ideal for track driven or highly-modified, high boost Subaru WRX/STi's. While the Moroso separator is a step above the GrimmSpeed unit, it is not water heated, making it a bad choice for people living in cold-weather locations due to the risk of condensation.

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  • ConQuezzy: November 10, 2015

    What condensation does

  • Alex Studenroth: November 10, 2015

    Dig deep? You just copy and pasted their product descriptions. Where’s the Mythbusters tests? Where’s the data? I feel like this is just a forum post. IAG threw their hat into the aos ring too. Any opinions on that?

  • InstaGib: November 10, 2015

    Good summary of the options. What’s the primary benefit of an AOS which retains the OEM PCV plumbing? Much appreciated, S

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