Today’s topic of discussion is Air Oil Separators. Aka, AOS when seen in a list of mods when window shopping used Subarus in case you didn’t know what the abbreviation meant (we get it, you knew). Although common in the list of mods you may have, we talk about why it is one of the most important upgrades that you MUST perform to your Subaru’s engine no matter the model. If there is an aftermarket air/oil separator available for your year/make/model for your Subaru, then you should make the investment and purchase one if your Subaru does not have one already installed.

 

WHAT DOES IT DO?

As the name of the upgrade suggests, it separates oil from the air so that only air travels through your intake manifold and not oil, as well as oil mist and vapors. In the long run, air/oil separators are designed to save your Subaru’s engine from a slow death that is the result of what happens when too much oil and oil mist can reach your intake manifold.

 

THE ORIGINS (OR ORANGES) OF AN AIR/OIL SEPARATOR

Just about every vehicle has what is called a PCV valve:

PVC stands for POSITIVE CRANKCASE VENTILATION. This is a $5 check valve that you can literally get at any auto parts store for your vehicle. Why is it so important? Because it is a part of your overall emissions control devices. It’s main purpose of the PCV valve is to collect any of the exhaust gases that escape your piston rings, and route them back into your intake manifold to get burned off.

So, when cars start smoking, or if you are blowing oil seals, the cheapest part to replace the process of elimination when trying to narrow down the culprit of your problem is by replacing the PCV valve because it is the cheapest component. And yes, a bad PCV valve can cause your car to smoke or push oil seals because the exhaust gases that can’t escape your cylinders will cause too much crankcase pressure to make oil blow past your piston rings and into the combustion process which would cause smoking issues because you’re burning oil. The excessive crankcase pressure would also simultaneously try to push your oil seals out.

Now in a boxer or horizontally opposed engine such as a Porsche 911 or Subaru this can be an issue mainly because of the design of oiling system. With Subarus, oil mist and vapors like to travel pass the factory’s PCV system where oil will make it into the intake system.

 

OH OKAY. DEN WHAT HAPPENS?

The oil will start to build up on the intake valves, and the oil mist and vapors will form carbon buildup on the top of your pistons. Now here’s where things begin to get hairy. If your pistons are coated with carbon, as well as your intake valves, it effects how the fuel that is burned and combusted within the cylinders. The major effect is a lowering of your fuel’s octane rating. If you have a COBB Accessport or a tune where you are only running 91 octane at minimum, you will experience detonation issues. Detonation is where the timing of your combustion is too soon, where the internal explosion happens when piston is near the top of the stroke and near the valves. What is another common Subaru engine fail point? Ring land failure.

Ring land failure is common in modified Subaru engines due to many irresponsible variables such as a bad tune, too much boost, driving too hard, but detonation is one of the leading causations to ring land failure. Now if you have an Impreza and it’s not modded, then you can get away with replacing your PCV valve during any major service interval. But if you have a WRX, STI, or any of the turbocharged XT models that has mods, then getting an air oil separator is crucial so that you are not getting those harmful oil vapors and contaminants in your intake manifold that could lead to detonation problems. Because not only are you destroying your pistons, but you are also destroying the valvetrain in the head as well. So, when a mechanic takes apart an engine that has experienced detonation issues, there will be chipping marks all over the pistons, but also in the head where your valves are.

 

HOW DOES THE AIR OIL SEPARATOR WORK?

Any air/oil separator that we carry here at RSD will include hoses, and many examples will include a catch can system. Air oil separators work by collecting any oil and oil vapors and it will reroute them back into your oil system and not through your intake manifold. But will it contaminate your oil? Not if you are performing regular oil changes which you should be doing anyway with fully synthetic oil if you have a modded engine. There are many designs available, but what is important is that you at least get something. Let’s look at some of the different designs. Remember all will work, there is no bad choice as long as you have one.

 

IAG Performance Air Oil Separator 2008-2014 WRX / 2008-2021 STI

 

 

 

GrimmSpeed Air Oil Separator 2002-2007 WRX/2004-2021 STI

 

 

 

Killer B Motorsport Air Oil Separator 2008-2014 WRX / 2015-2021 STI

 

 

Radium AOS-R 2015-2021WRX/STI

 

FINAL WORD – WANT TO KEEP YOUR SUBARU ON THE ROAD?

Because you want to maintain the highest level of performance as well as the longevity of the engine in your Subaru is why you MUST get an air/oil separator. Is it the most attractive upgrade? Of course it’s not. Does it add power? No, it doesn’t but it maintains your power levels. Having upgrades when you could be dealing with unknown detonation issues will only accelerate your engine’s wear and tear. Detonation doesn’t only cause destroyed ring lands, but it also destroys your heads and your valvetrain as well. If you haven’t got yourself an air/oil separator yet, get one, have yourself or your mechanic install one, and run a few cycles of a good-rated fuel system cleaner to help remove any existing carbon buildup on your intake valves, as well as your pistons to renew and protect the engine in your Subaru.

 

 

CLICK HERE FOR ALL AIR OIL SEPARATORS