Navigating AOS choices: Performance Vs. Pitfalls

If you have a Subaru WRX, STI, or BRZ, there is a good chance that you have at least thought about putting an Air Oil Separator (AOS) on your car to help the engine work better.

You may have already installed one on your car.

There are so many brands and designs of Air Oil Separators, it can be difficult to pick one, and if you have installed one, is it really working as well as it should?

We get asked these types of questions a lot and we decided to put together a video to tackle these questions, and to give you the best information so you can make sure that you get the best AOS for your car, and that you can make sure that it is working properly after you installed it.

In this episode:

00:00 - Introduction - What AOS is Best
00:36 - Two things you need to know before you pick an AOS
02:00 - Do you need an AOS? Or would a Catch Can do?
03:28 - Remember and AOS is just a Band Aid
04:10 - Red Flags that your AOS isn't working properly
05:41 - Should you pick an AOS for easy install? Grimmspeed AOS
06:54 - Looking at crankcase pressure can indicate if your AOS is working properly
07:35 - Ideally you want to preserve the factory PCV circuits
08:41 - The fittings used on your AOS do matter. What is best?
09:51 - Where you are going to be able to mount your AOS is really important
10:22 - Proper routing of the drain hose is key to AOS function
11:42 - How the 3MI Racing AOS Drain hose makes your AOS work better
13:15 - Which AOS is best for which application?
15:31 - Let's look at the RCM AOS. What makes it different?
17:39 - Final Thoughts and Wrap up

In this episode, we take a look at:

3MI AOS Drain Hose

Grimmspeed Air Oil Spearator

Perrin Air Oil Separator

Radium Engineering Air Oil Separator

Killer-B Air Oil Separator

IAG AOS

Roger Clark Motorsport Track Attack Catch Tank

Hopefully this video is helpful, and here are links to the videos we mention in the video if you would like to research further:

Crankcase Pressure video timed to the Oil Cap AOS data:

Is an Air Oil Separator just a Band-Aid?

Taking a look at the Subaru EJ25 Turbo PCV System

Thanks for watching and Stay Tuned!
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[bmoore]

Thanks for the great content as usual. You caught my attention when discussing the loss of oil even after AOS installation. My original motor 04 Forester XT has an IAG Street AOS on it, 267K miles, and I'm still losing about a quart of oil every 3500 miles. VF39, 17.5psi max, conservatively tuned street-only car, used to do autox events, now my wife's daily driver.

I have noticed that the oil drain hose from AOS to PCV port is not quite restriction free, there is a slight up bend in order to accommodate the height of the nipple on the PCV side. I hadn't heard of this 3mi oil drain hose, I'll give that a try! Thanks for the info!

[jeremyjackson]

Great review thanks! I already had the IAG AOS V3.

[jonnyturbo]

Great vid once again…funny, I bought the GS aos without really thinking, and partly because of the price, but once I got it and I looked at how it worked it just didn’t make sense…so now I have the IAG unit and will be using the competition set up as per my tuner..also now after this vid I’ll possibly add the 3MI hose set up as well. Thanks again

[kevin-jmqb]

Love your videos. Picked up some oil brands you suggested from an older video. Video is great for my Gr Supra

[kevinoneill]

After I installed a larger multiple baffled and SS pot scrubbers as screening with 1"ID tubing to the inlet and the same on the exit Catchcan on my daily driver. I noticed improved cold starts and my turbos sounded like they were spinning up better. No, I did not return the exhaust side of the catch can to my intake. I will not interfere with my highly metered air-fuel ratio to give me the best and cleanest combustion. That is insane. In my opinion. And yes I have bypassed the EGR going back to the air intake manifold. That is also insane. PS I have also increased the MPG by 1/3. Go figure. Do not restrict the blowby going through the catch can.

[tdro]

Hi, I have a slightly weird one. But I haven't had the chance to put anything over the turbo as that spot has already been used.My idea is to initially use a Gremspeed AOS on the circuit that moves the most oil (I'm guessing the one from the AOS/case) and use the last port to monitor the pressure/vacuum . Also plan to get valve covers with improved oil separating plates in the future and then continue with the grimspeed one on the other circuit .

[MorrisseyOner]

I am thinking about buying an Outback Wilderness - 2025 in the next few months. I am only interested in upgrades that improve efficiency and keep up the health of my engine. My current car is a VW rabbit - 17 years old, just starting to break down. is there any downside to getting an IAG, mainly just to keep down the carbon buildup? Also I live in Texas with 100 degree days in the summer, traffic jams, etc. so I might get/upgrade a transmission fluid cooler. Is it worth it? Thanks,

[atinlove]

As always awesome vid! Will you ever touch on accusump or any updated information on an accusump system?

[cruzinUSA_]

2020 Forester with direct injection: Could this possibly help reduce carbon buildup in the top end?

Great video!

[cesarbermudez]

Found a V3 Crawford Performance AOS for cheap for a 09 WRX. Any opinion on that model AOS, I dont see much literature/usage online about the Crawford brand

[kiangloongng]

What about the Provent solutions? Yes its a filter system but a very effective and low restriction AOS nevertheless right? Also, one can always keep the dual PCV circuit by having dual catch cans.

[TheCarTakerGarage]

Hey there J, and Flatirons team :) Thank you so much for your continous awesome videos. I just wantd to say thank you once again for great help last time when i got my question answered...the one about " reconecting the PCV system, after it has been disconected and only venting to air. So i have another question today ...The two return hoses that goes into the turbo inlet...one closer to the turbo..and one further away from the turbo...( aka block and camcover vents....)...My turbo inlet is a huge 70mm APS one..and it only has ONE port available to connect return vapour gasses into...My question is..CAN i use this port, for both crankcase vent, and the camcover vent...meaning making a T piece...and leading these two into this one port? I hope you understand my question.hehe. Greetings from Norway, Stefan :)

[zeusgallo]

Using the hose to relocate the drain, what do you do about the original drain just plug it off with rubber cap?

[CarsOverPeople]

Any thoughts or experience with the Verus Engineering AOS system (with their drain-back + coolant kit)? More specifically for the GR86 & GR Supra.

[barilbaril]

is this also a good thing to do on a stock wrx blobeye? or is this only for tuned cars?

[--wrx-]

Catch cans for me. I notice the backside can (2016wrx) catches like a shot glass worth of fluid. Where the front can catches just what kinda coats the walls of the can. Vert little. Only running 21lbs of boost.

[CPE-ju]

In non-turbo engines: just don't return it into intake - simply have a plastic bottle with breather somewhere in the engine compartment - that's how the racing non-turbo cars are equipped.

[jminkvihubyb]

My engine is procharged so I have no need to route anything to a turbo... That's what's throwing me off when I look for a separator to buy. So where would I route that third tube? A breather? I'll have a shop install it but I want to understand what's going on.. I definitely have too much oil going into my catch can & I know what's causing it. It's a design flaw of the intake manifold I bought (Edelbrock 7179)

[anthonymarquez]

Curious on your opinion of the Crawford Performance AOS V2?

[ragemonikers]

My truck uses the carburator to pull oil vapers into the combustion. I wonder if oil vapers and fuel vapers bond?