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
www.flatironstuning.com/3mi-r...
Grimmspeed Air Oil Spearator
www.flatironstuning.com/grimm...
Perrin Air Oil Separator
www.flatironstuning.com/perri...
Radium Engineering Air Oil Separator
www.flatironstuning.com/radiu...
Killer-B Air Oil Separator
www.flatironstuning.com/kille...
IAG AOS
www.flatironstuning.com/iag-v...
Roger Clark Motorsport Track Attack Catch Tank
www.flatironstuning.com/rcm-r...
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:
• An In depth look at wh...
Is an Air Oil Separator just a Band-Aid?
• Is an Air Oil Separato...
Taking a look at the Subaru EJ25 Turbo PCV System
• Taking a look at the S...
Thanks for watching and Stay Tuned!
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[@jeremyjackson8643]

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

[@FlatironsTuning]

Thanks for watching and glad that is working well for you. Stay Tuned!

[@kevin-jm3qb]

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

[@FlatironsTuning]

Thanks for watching, and very glad that the videos have been helpful! Stay Tuned!

[@bmoore8853]

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!

[@FlatironsTuning]

Thanks for watching and thanks for the question. With what you are saying about your drain hose, I would start by installing the 3MI hose for sure. It would be worth looking at the hoses that connect to the turbo inlet to check for signs of oil. Most likely you will find that which is a good indicator that the AOS isn't draining. Glad the video was helpful and Stay Tuned!

[@tdro98]

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 .

[@jonnyturbo6142]

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

[@FlatironsTuning]

Thanks very much! Stay Tuned!

[@1atinlove]

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

[@FlatironsTuning]

Thanks for your question. We have talked about the Accusump a couple of times. Here is the most recent video: kzbin.info/www/bejne/a3-xaXqJncimoa8 And here is an older video we did talking about the Accusump as well: kzbin.info/www/bejne/mZa4qKiZadqVgZI Hope that helps and Stay Tuned!

[@1atinlove]

@@FlatironsTuning I've nee following you for a while professor...I've seen these thanks. Just wanted to see if anything new popped up on your radar or a better system to prime

[@matthewdaniels7462]

Again thank you sir!!!

[@FlatironsTuning]

Absolutely! Thanks for watching and spread the word if you can. Stay Tuned!

[@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 :)

[@FlatironsTuning]

Thanks for your question Stefan! You can absolutely do this as a start. The cans that remove one of the connection are effectively doing the same thing. The down side of course is that if you do start to get into positive pressure in the crankcase, that single fitting will reduce the engine's ability to breathe. Since you do have a hard inlet, it wouldn't be too difficult to drill an additional fitting up at the front to add the second circuit. I would say that would be the ideal long-term solution. Hope that helps and thanks for watching the video! Stay Tuned!

[@kevinoneill41]

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.

[@uglyandattractive]

The perrin aos on the vb wrx fa24 uses both hoses

[@kiangloongng1322]

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.

[@FlatironsTuning]

Thanks for your question. I'd recommend going through our crankcase pressure video here: kzbin.info/www/bejne/fnLZqZ2Jmbamjqs That has clear examples of the systems that work and don't work. But yes, dual catch cans are also a good option to be sure. Thanks for watching and Stay Tuned!

[@ragemonikers]

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

[@goatg2747]

Between Crawford AOS and an IAG AOS for 2002 wrx which would you recommend ?

[@cruzinUSA_]

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

[@FlatironsTuning]

Thanks for your question. We actually have a video where we talk about this specifically: kzbin.info/www/bejne/jZ64kGyGe9RjiLs In short, yes, a catch can or AOS is a good way to make sure that oil vapor isn't getting into your intake path where it can cause those carbon deposits over time. If you aren't using much oil (and I would not suspect that you are), then a catch can may be a better solution than an AOS in that case. Hope that helps and Stay Tuned!

[@niallrampersad9124]

Great vid, as a rhd owner I appreciate the rcm content. More pls! Also I believe the rcm eliminates the stock vaccum system of the pcv and just routes the 3 breathers to the can, a drain to the sump and the top fitting is just left to secure to vent to atmosphere using the hose. Is that correct or am I wrong because I’m considering it for my daily driver.

[@TheZohanLohan]

Much better to have it recirculated on a street car

[@niallrampersad9124]

@@TheZohanLohan I agree fully,currently looking for a rhd aos option or to adapt an iag setup.

[@FlatironsTuning]

Thanks for the question. I believe you are correct that all of the RCM Catch Tank's disconnect the PCV system from the inlet of the engine. If I hear any different I'll let you know. Thanks for watching and Stay Tuned!

[@satoshibautista7766]

So is the threads different as to why the grimspeed aos doesn't fit a GD or 02 wrx?

[@CarsOverPeople]

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

[@FlatironsTuning]

Thanks for watching and thanks for the question. We don't have any first hand experience with the Verus AOS. If we get our hands an a GR86/BRZ project car, we'd definitely like to test it. Thanks again and Stay Tuned!

[@user-zh4vc3nf1g]

I drive short distances like 4 times a week and 2-3 further distances. I always try at least to warm up the car for 1 minute minimum, but what other tips you have when I drive short distance ? I drive a stock sti 2013. Weird question, When it's very cold outside and I start my car it makes a very high metal scratching sound for 1 sec, do you know what this could be ? I think it;s the starter, I have read that this is also a comon subi problem?? Thanks for the videos very informative!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Almost bought the grimmspeed haahhah

[@FlatironsTuning]

Thanks for your question. Warming the car up definitely can help. If you do a lot of short drives, taking the car for at least 1 - 2 30 minute drives can be helpful, especially in the winter. For very cold temps, I'd recommend looking at a block heater to keep the engine a bit warmer which will make that very cold start easier on the engine. I'd guess that noise you are hearing is either the PS pump or one of the pulleys. Thanks for watching and I'm glad the video was helpful. Stay Tuned!

[@-16-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.

[@FlatironsTuning]

They can be a solid option for sure. Thanks for watching and Stay Tuned!

[@goatg2747]

@@FlatironsTuningare catch cans just as effective as an AOS?

[@FlatironsTuning]

@@goatg2747 Yep. They do the exact thing. Only difference is that the AOS returns the oil captured to the sump. Stay Tuned!

[@goatg2747]

@@FlatironsTuning many people recommend to run 2 catch cans , what’s your opinion on that ? Would one suffice like the Cusco catch can or would 2 better ?

[@JasonLeung-4roller]

Do you have any thoughts on NA applications and the breather port as opposed to only PCV?

[@FlatironsTuning]

Generally speaking, NA engines should have less crankcase pressure. Is this for a bolt on turbo application? Or strictly an NA engine application?

[@JasonLeung-4roller]

@@FlatironsTuning strictly NA. FB20. I use engine braking and high RPMs quite a bit, it's a tiny engine so I try to use 100% of it's power. I see a lot of oil going through the breather port. Where I have installed a catch can. I recently modified the brass filter and replaced it with wads of steel wool to act as baffling. I found that the brass filter was quite restricted in letting crank case pressure back into the intake tract. I am ultimately trying to create a baffle like the OEM intake tube baffling. Why does more oil go through the breather than my PCV? I am using a Crawford Baja AOS on the PCV side. I am guessing turbo applications are ported a little differently when it comes to PCV and CCV

[@cesarbermudez1461]

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

[@FlatironsTuning]

Thanks for your question. Crawford Performance was actually one of the first companies to release an AOS for Subaru's and Quirt absolutely knows what he is doing when it comes to building engines. (Quirt Crawford is the founder of Crawford Performance). They are good designs for sure, just make sure that you plumb it in correctly. Hope that helps and Stay Tuned!

[@KennyWilson-nh9fs]

Car I tuned recently had a Crawford and I didn't like it all that much, the connections on it aren't bead rolled and seem to pop off really easy

[@stressed_garage]

so if im jdm would you suggest the rcm? i have been looking at the iag but still on the fence (my old mishi catch can does nothing!)

[@scarp400]

I have installed the RCM, on my 06WRX, being RHD here in Australia. If you have a cylinder 4 cooling mod you will run into problems with the drain location on the AOS, if the motor is still in the car it is pretty much impossible to drill the holes in the rails to be able to mount it. I had a bracket made that I mounted to two factory holes that are on top of the rail where I installed rivnuts, this also solved the clearance issue for the cylinder 4 cooling mod. The drain line is way too long, and I would say that using a pan that has an additional bung hole (as the RCM pan does) is a better option than running it to the factory drain hole. It is however the only AOS made specifically for RHD vehicles that is why I went with it.

[@SonikJesus]

Can it also be normal for a new car to consume 0.75 qts of oil in a 3k OCI? Would you recommend switching to 5w40 to reduce consumption? My STi has 16,000 miles, is stock, and has used 5w30 since I got it new.

[@FlatironsTuning]

Thanks for watching and thanks for the question. I would say that is in the realm of normal. Does the consumption increase toward the end of the interval? If so, that might indicate that some of the additives are breaking down, etc. I'd look at your intercooler, or turbo inlet for signs of oil. It is still possible that some of the vapor from the crankcase is moving through the PCV. Hope that helps and Stay Tuned!

[@anthonymarquez4563]

Curious on your opinion of the Crawford Performance AOS V2?

[@FlatironsTuning]

Thanks for your question. The Crawford AOS is a very good design. They were largely the first to see the benefits of putting an AOS on a Subaru. Hope that helps and Stay Tuned!

[@anthonymarquez4563]

@@FlatironsTuning Yes it does! Thank you for your prompt response. I have one currently on my 06 wrx and wanted to make sure it was a good one as it came with the car when purchased.

[@sohchx]

That Killer B unit is soooooooo sexy.....the price though!!!

[@FlatironsTuning]

It works really well for the right applications. Thanks for watching and Stay Tuned!

[@barilbaril1]

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

[@FlatironsTuning]

It could be. It all depends on if/how much oil you are using between changes. I touch on that in this video, but here is a more detailed conversation about that: kzbin.info/www/bejne/iHvbnXV8lJKaias Thanks for watching and Stay Tuned!

[@zeusgallo9485]

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

[@jesussepulveda5181]

I believe you have to order the NA breather hose that doesn’t have the they smaller y

[@zeusgallo9485]

@@jesussepulveda5181do you happen to have part number? And in regards to plumbing would it still just go to the back port of inlet ?

[@FlatironsTuning]

The top of the factory AOS is typically a vent, not a drain. So if you install the 3MI hose for the drain, you then just use the fitting on the top of the block as a vent only. You would still attach it to your AOS. Hope that helps and Stay Tuned!

[@jesussepulveda5181]

@@zeusgallo9485 Sorry, I misspoke. Just pulled it out of my parts bin and it comes with a straight adapter for the block vent aka "the original drain" so you dont have to cap or get anything extra. If you search through the Smeedia GR WRX videos I believe he installs it on the car.

[@user-ix9hl4uw5i]

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.

[@user-nf3oq2ge3g]

Main difference in an aos is the the engine coolant heating it up so it ev aos the crap and sends oil back to your motor that’s someone clean

[@VerusEngineering]

Why no Verus AOS? :(

[@FlatironsTuning]

Honestly, we haven't had one in the shop yet. We will have to take a look at it :-) Stay Tuned!

[@itZ.aProject]

Thoughts on radium's dual catch can on a stock ej motor?

[@FlatironsTuning]

Thanks for your question. That is what I am now running, and the details about that are in this video: kzbin.info/www/bejne/n3vWYoaJdrSMn7c If you aren't using much oil, a dual catch can is a great option. The one thing I would say is that you may not want to plumb it the way that Radium specs it. I put one can on each PCV circuit and so far it is working well. Hope that helps and Stay Tuned!

[@blueridge8992]

@@FlatironsTuningI have the Radium dual cans on my ‘22 WRX. Is their suggested plumbing routing okay for the FA24, or would you make a change? The car has under 10k mikes, running 330whp/360ft-lbs on e30. Just my daily driver. The PCV can is barely catching a few drops every 2k miles, and the CCV can is catching maybe an ounce over the same period (which is the opposite of how they performed on my ‘17 WRX. The CCV can stayed essentially dry on that car).

[@FlatironsTuning]

@@blueridge8992 Thanks for your question. For a street driven car, it is likely all right. You are doing the correct thing for sure by monitoring how much fluid you are catching. Keep an eye on it, and pay close attention if you do decide to take the car to the track, as things will change most likely. Thanks for watching and Stay Tuned!

[@MaddMattsGarage]

So an AOS would be a good idea for my EG33ETT build even though it is only a 700hp "Street" build?

[@maaaxxd372]

Catch can probably better at that level or the iag Comp with the dump going to a separate container so you don't oil your tires

[@MaddMattsGarage]

@@maaaxxd372 I thought feeding it back into the turbo drain was a better idea... I hate catch cans I had to use them for racing my old Escort and Sierra cosworth...

[@FlatironsTuning]

Thanks for your question. The starting point for that type build would be to measure your crankcase pressure. If you are going positive at all, then you would need something in place to catch or prevent the movement of oil through the PCV system. At that power level, I'd look at a dry-sump oiling system. That is not something that I would recommend for a street car, but that will be your ultimate solution, and at that power level (or higher) it really is your best bet in terms of both dealing with crankcase pressure, and for ensuring proper oil pressure. I'd recommend taking a look at what ASF Machine did with their build: kzbin.info/www/bejne/oZqlm6lvqp2ghMU Hope that helps and Stay Tuned!

[@coltonobriant6307]

What about the NA models? Say a ej253 with high oil consumption

[@tomop5603]

Is it used for motosport? If not then then probably do more regular oil checks

[@FlatironsTuning]

Thanks for your question. The first thing you need to do is to try to get a sense of where the oil is going. If you do have oil in your intake, I'd recommend trying to figure out which line it is coming from as best you can. If you aren't getting oil in your air inlet, then you are most likely leaking from somewhere else. Hope that helps and Stay Tuned!

[@jacobbivens8302]

Does anyone know if the 3MI racing drains hose is compatible with the COBB AOS??

[@FlatironsTuning]

Thanks for your question. The Cobb AOS is their version of the IAG AOS, so yes, the 3MI hose would work with it. Thanks for watching and Stay Tuned!

[@jacobbivens8302]

Wow thanks for the quick response! Just for that, I live in Loveland and I want to use you guys to 6 speed swap my 02 bugeye in the future cheers!

[@FlatironsTuning]

@@jacobbivens8302 Excellent! We'll be here to help whenever you are ready.

[@jacobbivens8302]

@@FlatironsTuningdo you have these in stock in person I will run down there this weekend

[@FlatironsTuning]

@@jacobbivens8302 Yes, we do stock the 3MI hoses. Unfortunately, we aren't open on the weekends though. If you can send an order through today though, you would have it tomorrow if you are in Colorado. www.flatironstuning.com/3mi-racing-aos-hose-kit

[@user-nf3oq2ge3g]

I made my own aos that fits perfectly in my car. But the concept is always the same. Clean air to your engine. And the aos is mandatory for a Subaru

[@michaelblades917]

Seems like all vehicles could benefit from an aos. ??

[@yowie0889]

It would certainly give a Tesla more "bling".

[@FlatironsTuning]

Absolutely. That is why the OE Manufactures include them with their engines. However, as discussed in the video, the capacity of the OE AOS can be over-run as you increase power, or start tracking the car. That is where it can be advantageous to add in a larger AOS, etc. Hope that helps and Stay Tuned!