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While perusing Subaru materials for a potential vehicle for my SO, I came across the concept of Air-Oil Separators like those made by IAG. Seems Subie enthusiasts swear by them. This seems like a more user-friendly solution than a catch-can as you do not need to empty the thing. On these forums I've only seen folks discuss catch-cans. Is there any reason why? Are there no AOSes available for our cars? Is the functionality of our engines somehow different?
 

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By definition, a catch can and an AOS are the same thing. You might be thinking of a serviceable AOS that you have to empty (our typical "catch can") vs one you don't (IAG/Subaru "AOS").

Looking at the IAG offering, their particular AOS/catchcan is heated (by the cooling system) so that water and fuel vapors vent to atmosphere (bad for emissions/environment) and the remaining condensed oil drains back into the engine/oil pan.

You can definitely do the same for our cars, but there is definitely some added complication. TBH, from the amount I empty from my catch can, I see very little reason to do this on the RS. Not sure about the modern Subaru FAs, but I didn't get that much when I had to empty my "catch can" installed on my old EJ (05 WRX wagon).
 

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IMHO, changing oil has its process. It’s not hard, but I wouldn’t call it a bada bing bada boom either, so saving time by not having to turn a valve, doesn’t seem worth the extra money.
The can also let’s you see the results. My friend (yes, I have one) logs his separated oil quantity.
 

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IMHO, changing oil has its process. It’s not hard, but I wouldn’t call it a bada bing bada boom either, so saving time by not having to turn a valve, doesn’t seem worth the extra money.
The can also let’s you see the results. My friend (yes, I have one) logs his separated oil quantity.

Does he also logs his MPG?
 

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Does he also logs his MPG?
No, he logs the oil from the separator, captured in a measuring cup, in photos (?)
I gave up trying to figure him out along time ago. ( I’m sure he’d say, back atcha!)
edit: something just occurred to me. I do....I have every gas receipt since day 1, neatly filed, by year, in my RSs family album.
That makes ME the craziest one, and don’t you forget it!!

Sorry OP, you can have your thread back.
 

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Discussion Starter #6
By definition, a catch can and an AOS are the same thing. You might be thinking of a serviceable AOS that you have to empty (our typical "catch can") vs one you don't (IAG/Subaru "AOS").

Looking at the IAG offering, their particular AOS/catchcan is heated (by the cooling system) so that water and fuel vapors vent to atmosphere (bad for emissions/environment) and the remaining condensed oil drains back into the engine/oil pan.

You can definitely do the same for our cars, but there is definitely some added complication. TBH, from the amount I empty from my catch can, I see very little reason to do this on the RS. Not sure about the modern Subaru FAs, but I didn't get that much when I had to empty my "catch can" installed on my old EJ (05 WRX wagon).
Regarding IAG, they have two kinds as far as I understand it: Street and Competition series. The former does not vent to atmosphere, the latter does. I'll poke the WRX folks for more info. The few Subaru-owner videos I did find were very insistent on differentiating AOS and catch-cans, but I think what you are saying makes sense - they both separate air and oil, but what they do with the oil differs in that the "AOS" flushes the oil back down into the crankcase

IMHO, changing oil has its process. It’s not hard, but I wouldn’t call it a bada bing bada boom either, so saving time by not having to turn a valve, doesn’t seem worth the extra money.
The can also let’s you see the results. My friend (yes, I have one) logs his separated oil quantity.
I don't have a garage right now, so I got the maintenance package - don't even do my own oil. This means I don't have an oil drain pan, nor anywhere to store it. I do have access to a garage, but I hate lifting the car every time due to a lack of appropriate 3rd jacking points front or rear (like other cars I've owned). It feels so awkward. I avoid lifting the car up for that reason, and only really do wheel swaps, and smaller stuff. That's where the appeal of something I don't have to actively maintain - especially in terms of dealing with liquids - comes in.

Myself, I don't actually use an oil catch-can on the RS. Seeing so many Subie owners on the other hand proselytize about AOS made me wonder if Subies have an inherent problem that needs working around.
 
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Gotcha! ⬆
Tonight, we’re going next door. There son will be there (WRX sti) I’ll see what he says. He’s pretty savvy and actually helped me install mine.
 

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Whatever the cause (stock tune, weak pistons, etc) the STI is prone to ringland failure. One of the potential causes is detonation. Oil in the intake reduces the octane rating and makes the car prone to detonation, and then engine failure.

An AOS/catchcan/whatever reduces the oil in the intake so it's some engine insurance.
 

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Discussion Starter #9
Whatever the cause (stock tune, weak pistons, etc) the STI is prone to ringland failure. One of the potential causes is detonation. Oil in the intake reduces the octane rating and makes the car prone to detonation, and then engine failure.

An AOS/catchcan/whatever reduces the oil in the intake so it's some engine insurance.
Interesting, interesting. This was one of the top hits when I started searching ringland failures. I've heard of those before, but never paid attention to it. Seems to postulate it might be a fundamental design choice that causes this. Also more applicable to tuned cars.

 
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