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Old 06-26-2016, 07:06 PM   #1
jlambert533
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Default Why doesn't Subaru offer an OEM air oil separator?

I've been shopping around for an AOS for my 2016 STI and a question occurred to me; if our cars are notorious for consuming oil, I mean to the point that there are hundreds of aftermarket AOS and oil catch cans, why wouldn't Subaru offer this as an option? Are there any drawbacks to having an AOS? It just seems like they could solve the oil consumption issue that our cars are plagued with. Maybe I'm missing something and I'm sorry if this is posted in the wrong place or of it's been asked before.
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Old 06-26-2016, 08:22 PM   #2
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Quote:
Originally Posted by jlambert533 View Post
I've been shopping around for an AOS for my 2016 STI and a question occurred to me; if our cars are notorious for consuming oil,

I don't think they are "notorious" for consuming oil. As the rings wear down blow by becomes more and more prevalent. This is especially true in forced induction vehicles.

I mean to the point that there are hundreds of aftermarket AOS and oil catch cans, why wouldn't Subaru offer this as an option?

There is hundreds of aftermarket parts for everything on all vehicles from all makes. If there is a market someone is going to slide into that niche.

Why doesn't every manufacturer period ship cars with AOS systems? Well because in 99% of applications the PCV valve is fully capable to the task. If it wasn't AOS would be a global manufacturing standard.

Are there any drawbacks to having an AOS? It just seems like they could solve the oil consumption issue that our cars are plagued with.

I am on forged pistons and don't burn a drop.

Maybe I'm missing something and I'm sorry if this is posted in the wrong place or of it's been asked before.

Yeah this is the wrong sub forum but I don't think anyone is gonna come headhunting for ya lol
That said, I think an AOS is still a good buy. If I had a brand new STI that would be on the very short list of mods I would do with the vehicle.
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Old 06-26-2016, 09:10 PM   #3
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Thanks for the thorough answers! I have yet to burn any oil on mine but from what I have read on "the Internet" it's just a matter of time lol. Thanks again and I appreciate the help. I think I've read so much and have done so much research on my car that I've become a hypochondriac for my car. Every little chirp or clank I'm ready to start looking for something broken lol.
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Old 06-26-2016, 10:01 PM   #4
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You're welcome. Yeah that is the power of teh interwebs lol. I know what you mean about being a hypochondriac. The first time I heard injector tick I thought there was something wrong with my valves haha. Reading is good, I try to read and acquire more knowledge every day.

It is pretty commonly accepted you're going to have some blow by regardless. Perfectly healthy motors over time will leave a bit of oil residue in your intake tract. If you are having serious issues with blow by its rebuild time anyways, not buy an aos time.

Like I said I think its a worthwhile investment. But not necessarily something you should go pawn your family heirlooms for and install ASAP.
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Old 06-27-2016, 12:44 PM   #5
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Originally Posted by jlambert533 View Post
...why wouldn't Subaru offer this as an option?
A few reasons - the biggest being the term "AOS" is something that caught fire with the Subaru community but has been used on racing vehicles forever as oil catch cans (AOS = a [usually] well-designed catch can w/drain-back).

AOS is a generic term, which is why each and every manufacturer of these kits decided to route their system entirely differently.

You can take this a step further and ask why Subaru doesn't install a dry-sump oiling system on the OEM cars; the answer to this is obviously cost. As mentioned, the OEM PCV system is more than enough to handle this task on OEM cars.

Now - since the oil catch can is a compromise between the OEM PCV and a full dry sump, why wouldn't Subaru offer a catch can system as an option? The big reason is emissions. The better question to ask, though, is if the OEM PCV is sufficient and shares parts commonality with most of their other engines, why would they bother to change anything?

Don't forget that the people recommending AOS the most happen to either be companies that sell a kit or internet fanboys that are regurgitating the marketing hype without understanding the crankcase ventilation system. Most AOS systems available are severely overpriced and over-complicated for what they are actually doing. A few hoses and a vent accomplishes the same task for probably 95% or better of the engines people are running.

Don't take this to mean the AOS systems out there are 'bad'. I've looked at the design of most, and the majority work just fine! Personally, I'd rather accomplish the same goal without making my engine more cluttered and difficult to work on, while saving probably $200+ in the process.
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Old 06-27-2016, 05:42 PM   #6
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Let's not forget that there already is an air/oil separator on the rear of the passenger side of the engine block.
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Old 06-30-2016, 06:05 PM   #7
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And in the PCV ports of the heads!
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Old 06-30-2016, 08:33 PM   #8
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Quote:
Originally Posted by monkeyposeur View Post
Let's not forget that there already is an air/oil separator on the rear of the passenger side of the engine block.
There is? Where exactly, near the turbo? If there is one it doesn't do the greatest job to be honest.

If you guys have ever had the intercooler off and discovered the little pool of oil sitting in your throttle body hose; or changed your turbo inlet tube, you'll see it's not pretty.

The stock PCV system is doing nothing more than sucking the crank vapor into your turbo inlet.

The downside of eating oil especially on these cars, is you are effectively lowering the octane of your fuel, potentially causing knock. Turbo cars are even more sensitive to this and while yes, the ECU will retard timing, knock events are not good for the long term health of your motor. That's the main reason why people buy an AOS.

Is the factory system sufficient? The bean counters say so; and it satisfies emissions requirements. But for most people who start adding power, making sure your octane stays true, keeping knock events at bay, is pretty important.
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Old 07-01-2016, 12:06 AM   #9
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Default Why doesn't Subaru offer an OEM air oil separator?

this is a generalized statement... but AOS are mainly for forged motors that produce alot of extra blow-by due to the larger internal clearances.

subaru doesnt offer one because a stock motor with cast pistons doesnt really need an AOS, but that doesnt mean they dont work. get one for piece of mind if you wish. there are no downsides to a properly installed AOS
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Old 07-01-2016, 01:02 AM   #10
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Quote:
Originally Posted by meebs View Post
There is? Where exactly, near the turbo? If there is one it doesn't do the greatest job to be honest.

If you guys have ever had the intercooler off and discovered the little pool of oil sitting in your throttle body hose; or changed your turbo inlet tube, you'll see it's not pretty.

The stock PCV system is doing nothing more than sucking the crank vapor into your turbo inlet.

The downside of eating oil especially on these cars, is you are effectively lowering the octane of your fuel, potentially causing knock. Turbo cars are even more sensitive to this and while yes, the ECU will retard timing, knock events are not good for the long term health of your motor. That's the main reason why people buy an AOS.

Is the factory system sufficient? The bean counters say so; and it satisfies emissions requirements. But for most people who start adding power, making sure your octane stays true, keeping knock events at bay, is pretty important.
And there you go.
Subaru doesn't put extra equipment on because the stock stuff is sufficient for the cars as designed, for most people. Once you start driving outside the norm, or increasing power levels, or otherwise screwing with the way the vehicle functions, you may discover that every other system may need to be upgraded to remain fully functional. Add a thousand HP, you'll probably want to beef up your trans, driveshaft, axles, brakes, fuel system, exhaust, etc.
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Old 07-01-2016, 01:19 AM   #11
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Subaru doesn't offer one because it is a completely unnecessary component.
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Old 07-01-2016, 12:19 PM   #12
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Quote:
Originally Posted by meebs View Post
The stock PCV system is doing nothing more than sucking the crank vapor into your turbo inlet.
OEM PCV pulls vapor into the intake manifold under low load, and turbo under high load.

Quote:
Originally Posted by meebs View Post
The downside of eating oil especially on these cars, is you are effectively lowering the octane of your fuel, potentially causing knock. Turbo cars are even more sensitive to this and while yes, the ECU will retard timing, knock events are not good for the long term health of your motor. That's the main reason why people buy an AOS.
This is a very true, very textbook reason why the OEM PCV system has a few downsides (at high load, ONLY).

No, people buy an AOS because the companies that sell them have effective marketing teams. A catch can or even a straight vent and some re-routed lines are as good as an AOS in most situations.

Quote:
Originally Posted by meebs View Post
Is the factory system sufficient? The bean counters say so; and it satisfies emissions requirements. But for most people who start adding power, making sure your octane stays true, keeping knock events at bay, is pretty important.
As mentioned above, a few re-routed lines is all most people will need to eliminate the high-load oil ingestion into the intake. You can also eliminate the PCV and dump the oil vapor at low load... but all you'll do is fill the catch can sooner or create a bigger mess (assuming no catch can).

Here is my setup, which is a direct copy of the GpN modifications made to the GD-series cars; neither I, nor Prodrive used the catch can (just a vent), but the routing is the same:



At low load, oil goes through the OEM PCV on the intake manifold. At high load, it goes out of the filtered vent. That's it.

Quote:
Originally Posted by Titter View Post
...there are no downsides to a properly installed AOS
Unless you're the mechanic that has to wade through the sea of hoses every time the car needs service.
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Old 07-01-2016, 01:56 PM   #13
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Default Why doesn't Subaru offer an OEM air oil separator?

Thanks for all of the information on this. I've got a Perrin AOS sitting in the box ready to instal but my lord it looks like I need a degree in rocket surgery to install with all of the fittings and hoses etc. I just have a Borla cat back exhaust and a tune by Clark tuning and I have yet to see my oil level drop at all after a couple thousand miles on the tune so I'm still debating if I need to carve out a full day to install this thing. Thanks again for the support on this. Anyone wanna volunteer to show me how to instal this by installing it on my car? Jk
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Old 07-01-2016, 02:09 PM   #14
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If they built it and advertised as a "must buy" as most vendors do, they're admitting there's a problem with the car out of the factory. I would argue that, if it's such a big deal (it's not for a stock car), there would be a recall.
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Old 07-02-2016, 03:01 PM   #15
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Quote:
Originally Posted by mrsaturn7085 View Post
OEM PCV pulls vapor into the intake manifold under low load, and turbo under high load.



This is a very true, very textbook reason why the OEM PCV system has a few downsides (at high load, ONLY).

No, people buy an AOS because the companies that sell them have effective marketing teams. A catch can or even a straight vent and some re-routed lines are as good as an AOS in most situations.



As mentioned above, a few re-routed lines is all most people will need to eliminate the high-load oil ingestion into the intake. You can also eliminate the PCV and dump the oil vapor at low load... but all you'll do is fill the catch can sooner or create a bigger mess (assuming no catch can).

Here is my setup, which is a direct copy of the GpN modifications made to the GD-series cars; neither I, nor Prodrive used the catch can (just a vent), but the routing is the same:



At low load, oil goes through the OEM PCV on the intake manifold. At high load, it goes out of the filtered vent. That's it.



Unless you're the mechanic that has to wade through the sea of hoses every time the car needs service.
Yep good info. I was using "AOS" in general terms for the act of separating air from oil; like kleenex for tissues. Catch can/vent/AOS; their goal is the same.

I actually went the same DIY route as you apart from the external vent on my old 04 wagon, and put together my own "catch can" using one of those air compressor filters you can get at Lowes/Home Depot. It caught a fair amount of gunk but ultimately it wasn't worth it.

Like I said above, if you're already pouring thousands of dollars in upgrades; adding air/oil separation is going to be part of your plans whether you go el cheapo or high quality. Stock cars just aren't going to see a major benefit though it would be nice to pull the IC off and not see any oil. Marketing or not; big builds need to keep knock in check.

I've clocked 116,000+ miles on my 2009 ("Stg1" reflash since 45K) and do not feel the need for one. I accept that the car eats a quarter of a quart over 6,000 miles, and that's OK with me.
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Old 07-05-2016, 09:15 PM   #16
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I use a grim speed AOS with a catch can as backup. It's very easy to install.
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Old 07-06-2016, 02:57 PM   #17
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I use a grim speed AOS with a catch can as backup. It's very easy to install.
Probably overkill... but doubt that you're hurting anything. Is the catch can vented or are you retaining a closed system?
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Old 07-06-2016, 04:18 PM   #18
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I had a forester xt that consumed oil from brand new!!
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Old 07-06-2016, 09:00 PM   #19
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I've got a Perrin AOS that ill be installing tomorrow. It has coolant lines routed to it to keep the temperature inside the can up so that it separates the oil more efficiently I guess. Probably just a marketing ploy but it sold me on it. I'll post pics of the install and let you all know what I think. My tuner also says I should retune after installation because it will have an effect on the intake air. Just so everyone knows I have a 2016 sti with a Borla atak cat back, beatrush pitch stop mount, TWM performance short shifter and bushings, Cobb AP v3 with a tune by Clark Turner. I gotta say that the tune on this thing is amazing. Props to Clark.
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Old 07-07-2016, 12:28 PM   #20
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Originally Posted by jlambert533 View Post
I've got a Perrin AOS that ill be installing tomorrow. It has coolant lines routed to it to keep the temperature inside the can up so that it separates the oil more efficiently I guess. Probably just a marketing ploy but it sold me on it.
It will help condense the oil vapor in the air, but honestly, it isn't worth the added complexity and potential for coolant system leaks IMHO.

Quote:
Originally Posted by jlambert533 View Post
My tuner also says I should retune after installation because it will have an effect on the intake air.
While it won't hurt... I wouldn't make this a priority until you have other modifications. It will not be a large change to the intake charge.
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Old 07-12-2016, 06:50 PM   #21
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Has anyone on here by chance installed the Perrin AOS on their 2016sti? If so, would you be able to post some pics of how you routed everything? This is the first AOS I have ever installed and this one seems pretty complex and I don't want to screw something up by installing it incorrectly. I would really appreciate any help with this.
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Old 07-13-2016, 01:28 PM   #22
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That little plate you have to make sure that goes back on, on the back rear side of the short block....yeah.
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Old 07-14-2016, 01:36 AM   #23
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So an catch can, is not the same and AOS?

Also, Why are there there 2 port and 3 ports catch cans (mishimoto offerings for instance)
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Old 07-14-2016, 01:28 PM   #24
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A catch can is just that... a can to catch condensing oil vapor rather than blowing it out below the firewall, or returning it to the engine (crankcase or cylinder).

Once upon a time someone decided that they wanted to market a baffled (or otherwise 'improved') catch can in a way that gave them a sales advantage over other companies. This is where the term 'AOS' was born (technically, this term was borrowed from existing compressed air inline water traps).

2- and 3-port (and other) offerings are all just different opinions on the 'best' way to do the same job. Obviously there is some manufacturing/cost benefit to making a universal can rather than an application-specific can. Boxer engines have three distinct breather ports, one on each head and one on the block. A vent system using three ports is probably best, though a tee between the head ports, etc. can make a 2-port system work just as well.

This last piece is just given as an example showing how much 'motor' you can run with a relatively simple system:

WRC EJ20 engines (literally, for decades) used three breather hoses, one from each head, and one larger hose from the crankcase. All three hoses entered a baffled catch can w/o drain-back (i.e. you had to drain the oil during maintenance). The catch can was located on the firewall and was vented through the fender with a filter at the very base of the car (near the sills).

The ONLY functional difference between this system and the pictured system above, is the PCV on my drawing is not present on the WRC car. The PCV only functions at low load, and is a good way to prevent the catch can from filling too fast (especially on 'loose' motors). The pictured system also tee's the lines together externally, while the WRC system used individual inlets on the catch can.

Most of the AOS systems on the market clutter up the engine bay without good reason. Running coolant lines to the tank *will* increase the efficiency of oil condensation at the cost of additional lines and potential coolant system leaks. Terrible trade-off, in my opinion.

The biggest benefit many AOS systems add is drain-back. This means the system will require less maintenance as there is no tank to drain. Run an open system w/o catch can and you end up with the same result... you just dump the tiny (for a healthy motor) amount of oil vapor out of the car. Prova was probably the earliest system I can think of that had a nifty drain-back (using the oil filler neck) - you're probably more familiar with this as a Grimmspeed product, as they knocked-off the design and added a few minor improvements.

Keep it simple and save your mechanic the headache of swimming through tons of useless lines.
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Old 07-14-2016, 02:20 PM   #25
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Mrstaturn,

Great explanation.
I know you were talking about something like the Grimmspeed AOS before you mentioned it.

So let me ask you this, there is 3 options

1- STOCK. Crank and heads (motor) vents into intake?

2- CATCH CAN. Motor vents into intake / Oil is retained in can for dispossal
A. 2 ports with T
B. 3 ports

3- NON CAN. Motor vents int intake / Oil drains back to oil sum
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