What an AOS or Catch Can is for and the differences

ZachTTLM

New member
Wasnt sure where to put this, so I put this in here.

After I posted the sale offer for the Grimmspeed AOS, I had a few people message me asking me what these are and what they do. So I am going to try to explain the system and how it came about.

First of all it started back with the EPA required the automakers to re-route the crankcase vent from venting to atmosphere to venting back into the cars engine to burn off the "harmful" gases. To do this they had to route the vent into the intake system. As you can imagine when venting the gases from the crankcase you are also going to be getting oil mixed into that which is also getting sent back into the engine through the intake. This is called Blow By, what happens is the blow by goes into the combustion chamber via the intake system. When oil enters the combustion chamber it lowers the performance of the car by effectively lowering the octane of the fuel, leading to detonation.

There are two solutions for this problem.
1. Air Oil Seperator
2. Catch Can

What are these two solutions?

1. Air/Oil Separators effectively removes the oil from the air coming out of the engine?s ventilation system via a two stage process using chambers in the system. The first chamber is designed to use centrifugal force to remove the oil from the air, the clean air is then pushed into a second chamber and extracted from the AOS to the engine while the oil is recirculated back to the crankcase where it belongs.

A Catch can is a tank that is put inline of the system to capture the blow by and hold onto it. Once this gets full, you are required to empty the can.

Ok now you know what these are, now lets tell you features and benefits.

1.The Air Oil Seperators are more expensive than your average catch can
2.The Catch Can needs to be monitored to ensure that it does not get too full and stops doing it's job, where as the AOS seperates the two and sends them to their rightful places.
3. A lot of catch cans on the market dont do a whole lot for the high boost cars due to the air forcing through so fast, however ones with offset ports or baffling fix that problem by giving the air containing oil to run into first creating seperation.
4. Catch cans not only require you to monitor the tank, but you need to monitor the oil level of the car as well being that it does not return the oil to the crank case.
5. Catch cans build more moisture specially during climate changes. This can also get into the air.
6. Some AOS systems that incorporate the tanks also can sweat and build moisture but much less than the catch can and there are companies out there that have designed their systems to minimize/ eliminate this such as Crawford Performance and Grimmspeed.

You will hear a lot of argument on the web on which is better and so on but our professional opinion it comes down to more of which is more dependable, requires less maintenance, and performs the best. We believe that goes to the AOS because of the design to remove the blow by oil and send it back to the system crank case.​
 

Vermont

New member
Great write up on the whole AOS issue. Moved to Engine section so more people would have a chance to look at it, and since it also falls more inline with the general engine technology than vendor specific. :tup:
 
Ok im going to necrothread instead of creating a new thread about this discussion.

So I know I need to get a air oil separator or catch can system but refuse to spend a few hundred dollars on one of these subaru specific systems.

So my question to everyone is do any of you guys have any suggestions on a good baffled oil catch can along with do I need to have two catch cans or will one be sufficient?

Another topic would be has anyone considered using another car manufacturers stock air oil separator for their Subaru I know there's several boosted cars that have them stock and you can get them for around a 100 dollars on eBay?



Sent from my ONEPLUS A3000 using Tapatalk
 
Ok im going to necrothread instead of creating a new thread about this discussion.

So I know I need to get a air oil separator or catch can system but refuse to spend a few hundred dollars on one of these subaru specific systems.

So my question to everyone is do any of you guys have any suggestions on a good baffled oil catch can along with do I need to have two catch cans or will one be sufficient?

Another topic would be has anyone considered using another car manufacturers stock air oil separator for their Subaru I know there's several boosted cars that have them stock and you can get them for around a 100 dollars on eBay?



Sent from my ONEPLUS A3000 using Tapatalk

I had a cusco catch can on the car when the second motor went on the track. I have upgraded to the IAG and suggest you bite the bullet and do so as well. There are so many benefits in doing so. But if not that cusco is just sitting here and I think it might have your name on it.
 
I had a cusco catch can on the car when the second motor went on the track. I have upgraded to the IAG and suggest you bite the bullet and do so as well. There are so many benefits in doing so. But if not that cusco is just sitting here and I think it might have your name on it.
I've actually been considering 3 inlet port catch can with one outlet how many ports does yours have?

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If it means anything, I decided to go with the IAG AOS Street set up. Since I'm running E85 this configuration should help with oil life and keep the system clean better than others on the market now. I looked at the GrimmSpeed, Cusco and Crawford units and this one has the most features and there is a good explanation of how it works on their site. Yes its expensive but the engineering of the system is spot on and makes total sense. Check it out and see if it will work for your purposes. Hope that helps.

http://www.iagperformance.com/IAG-Comp-Air-Oil-Separator-AOS-Subaru-WRX-STI-p/iag-eng-7250.htm
 

SudoSTI

Member
I have a Saikou Michi dual catch can setup. One for the heads, one for the crank. I'd like to get the IAG unit eventually though.
 
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