Air Oil Seperator Explained?

I keep seeing references to the air oil separator but can't find anything that describes where it is located or why it is there or a description of why one is needed. Can anyone enlighten me? I have an '07 987.1 Cayman S .

A good article from a well known Porsche expert: Porsche Air Oil Separator (AOS) for Porsche Boxster, Cayman, and 911 Models

Layman’s terms, Porsche over-engineered a PCV valve .

Thanks for the very informative article, Schwinn, as well as the layman summary Racer1735! Only very occasionally do I have exhaust smoke at cold start, and my oil consumption tracks at about a quart/10,000 miles, so I think I am probably fine. At least now I have a good working knowledge about the issue. Thank you both!

Well, I think the AOS does more than a PCV. PCV just vents to the intake... the problem is that if there is a lot of oil, then this causes all sorts of issues on the intake tract. So, the idea is to separate out the oil from the air, so it doesn't make a mess in the intake. In that sense, it's better than a PCV, which can't do any of that. So, I think it's more than "over-engineering" of a PCV valve ... but it is rather wacky. I saw a user replaced their AOS with a custom setup using a circular vortex... this slings the oil to the outside, and is likely more effective than the Porsche design. It seemed to work better for them, at least. In that sense, I'd say the OEM AOS may be under-designed? I actually don't know how it works, but I hear there's a membrane which can fail if you overfill the oil... the implication is that it may be passing air through a mesh, hoping to catch droplets and drop them out...?

I'd say the OEM AOS may be under-designed? As I'm sure you are already aware, they make a deluxe Motorsport unit sir, you just need dental repair money to purchase it.

The Porsche 987 Motorsports AOS is recommended by many others for the past 5-6 years due to a couple of items. We are generally running stickier tires , So, thiese tires create more G loading in the curves, which points to an oil starvation issue of rod bearing #6. So, a deeper LN sump with x-51 baffles and windage plate to control the oil around the oil pickup nossle. All good modes but the windage plate is flat and allows the returned oil to slosh up the sump sidewalls into the vacuum hole at the top of the sump. This hole is the bottom of the AOS so you can wash down your OEM AOS just by taking long high G corners rapidly. Here is a video of a comparison using the new RE71RS on a gentleman sports car for less than $34,000. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwj135bc86v8AhVMJDQIHQkMAbYQtwJ6BAgSEAI&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DRNMBFGCWR1E&usg=AOvVaw2HG-BzobGKAcqAix6PgPqM So if you fit this criteria, then do yourself a big favor and get the mods that will protect your $25,000 engine. That's the cost of a rebuild from oil starvation without any upgrades, $35,000 for a 400 hp LN or flat6innovations motor. This episode outlined a few other mods that you should consider. 1. Steel Numeric shifter cables and short shifter. 2, GT3 Cup front and rear bake ducts. 3. R brake pads 4. Sporting alignment with -1.6 camber f and -1.8 camber r. 4 minutes of toe out in f. The OEM adjustments will allow these adjustment with out changing the LCA's. 5. Very few cars in this price range will stay on your backend with this setup using the new Bridgestone gentleman's race tire. Its a cheater road tire that works great in the wet as well and will last at least 10k miles as long as your tract time is limited. I have the newest version that replaced their older softer RE71R's tire which gave me 8k miles until replacement. I still had some rubber to the wear bars, but I did flip these tires once. This video opens a whole new price point $25-35k for having more fun than you can imagine. Just to first protect your engine, then upgrade your brakes.

I have read where the 981 series suffer less from the AOS issues than the 987 model years , is this true ?

James, OEM yes, 987.1S with a Porsche Motorsports AOS nope. That's my point here. The 987.1S AOS plays a bigger role than the 987.2 or the 981 AOS . The 987.1s Porsche MotorSports makes up the difference.

Sorry if I don’t understand the system well enough but wouldn’t a tall oil catch can with the drain open to the original AOS bottom attachment work just as well and have no moving parts to fail? I guess I just don’t understand how the membrane works.

First, you need vacuum to suck out the sump various smoke, oil droplets and other air born particules from combustion. Second, in a 987 Porsche there is a significant amount of oil that gets released into the two heads from various systems which needs removal. Porsche has 2 oil pumps (one at each end of the head) to suck out much of this oil, however there is also a significant amount of oil that is mixed with air causing a froth to form (some oils make more froth than others). Third, the AOS vacuum sucks out this froth, As Porsche figured out, our M97 engines need more oil return than our 2 pumps and the existing AOS can handle as oil is known to pool in the heads as our heads are flat as a flat 6 with just a little gravity down flow to the sump. Fourth, It pools in the heads to a point where with a normal OEM 8 qt. sump, the engine with long high G curves using high RPM can run out of oil at the pickup point in the sump. This loss of oil pressure causes rod bearing scaring usually at rod bearing #6. Fifth, At the same time being occasionally being washed down with oil being sucked out of the top of the sump while destroying the membrane or the real name, flow control diaphragm. If that occurs, your engine may become oil locked to a point of causing a bent rod and/or a lot of oil smoke. So the AOS needs a very consistent vacuum to do these functions. The 987.1 M97 engine AOS does a lot of different functions, some better than others, which is where the Porsche MotorSports AOS arrives to save the day. Hope this info helps.

Very much so! Thank you

Hi I am thinking of changing my AOS , what parts are needed other than AOS itself? Is there any other parts I should change while I am changing the AOS ?

Probably should replace the 90 degree elbow breather hose at the bottom. I also needed one of those hose clamp pliers with the cable to undo the lower hose clamp. It’s not that hard once you figure out how to undo the hose clamp.

would you happen to have a part number for this elbow?

Thank you