PF

Hi.

I am finishing a full rebuild of my -89 Cup car and would like to know your experiences and which way to go concerning the AOS and how it is set up.

The hole has been drilled out in the AOS so either I attach a hose to it and then to a breather tank like lindsey etc.

Or I route it back like stock to before the turbo and put in a restriction like the stock opening in the AOS.

Is there any disadvantages of connecting to a breather tank?

And with the stock routing I have heard of people with higher output engines having issues with the turbo "sucking" oil out through the AOS.

Please tell me your experiences

Thom

On an engine running the stock turbo and airbox good for a shade above 300 €hp at the crank I would not redesign the wheel - just plug it all like stock.

PF

Ok thanks but if maf and bigger turbo would be used later is it better too do it differently then or still same setup?

Thom

Same set up, with perhaps an additional oil catch tank before the turbo inlet.

PF

Is there any risk of a bigger turbo sucking oil like that with the bigger opening in the AOS?
Why did Porsche restrict the opening and later on added that valve in the line too the j-boot?

Thom

By the time liquid oil will be sucked out of the AOS, some other major problems will have occured.
I suppose the opening is that small because the engine designers probably estimated that blow-by would remain minimal with a K26 turbo running a mere 0.8 bar of boost (will take time to wear out the bores of such a big engine with such a remarkably low specific HP output), and they made the fitting large enough to fit a "standard" size hose. I guess the hole is the same size as on the other 944s, as perhaps they weren't even bothered thinking about making a larger hole for the 944 turbo.
The function of the valve on later cars is to release boost from the AOS line when closing down the throttle when upchanging gears under WOT.

V2Rocket

The AOS is a type of "cyclonic separator".
The smaller the orifice at the exit the higher the "filtration" efficiency can be, all else being equal.

Drilling the hole out is something only to be done when fitting a breather/catch can, as you are then basically transferring the AOS's job to the catch can, and using the AOS as a very overbuilt oil filler.

PF

Ok thanks for the clarification.

PF

Ok but why do some drill the AOS? What is the point? Since mine was (not by me) I wonder.

So with my setup best would be to hook it up to a breather tank.
Or if routing it back like stock attach a Catch can in between

V2Rocket

A breather/catch can does the same type of job that the AOS does...separate oil from the air in the crank case, and route fine vapor into the intake.

If you install a breather you don't really need to use the AOS anymore as it was designed - all it serves to do is fill the oil holes in the engine block and act as a "pipe" to feed your breather tank. So since it isn't needed to separate air/oil the cyclonic function isn't needed so the opening can be enlarged.

PF

Ok sorry but I still dont see the reason why the opening is enlarged? Is there a specific reason as too why that is done? Is there a gain in some way? If there isnt I can't see why some do it if it means you just move the oil separation to an additional tank!?

V2Rocket

The point of using a breather is that you are venting crankcase pressure to the atmosphere rather than circulating it into the intake tract (which is under vacuum which would help the flow).

A larger opening lets the pressure "breathe out" faster.

PF

Ok thanks! So that means using a breather is not going to affect anything in a bad way exept maybe smog laws?

Is the only gain routing it the stock way that you rid the bad smell and fumes in the enginebay or is there other reasons.

V2Rocket

I guess potential for some oily mist in the engine bay.
Definitely not emissions-friendly.
The idea is to keep the oil vapors out of the intake manifold/cylinders.

Tom M'Guinn

I drilled my AOS because at high-output, my motor would produce excessive crankcase pressure, which would cause oil leaks, push the dip stick up, etc. Drilling the AOS lowers the crankcase pressure under heavy boost, and eliminates the oil leaks, dip stick issues, etc. As V2 says, however, it also reduces the effectiveness of the Air-Oil-Separator (AOS), allowing more oily mist to escape out the top of the AOS. Because of that, most people who drill the AOS also install a closed-loop catch can, where the hose from the top of the AOS is routed to the catch can, to trap the oil, and another hose from the catch can is routed to the j-boot to vent the pressure. If you use a breather that is not routed back to the j-boot, you'll get a burning oil smell that can be annoying for a street car.

Not sure what you are referring to when you say the turbo might "suck oil" through the AOS, but there are two related issues there. First, when you alter the pressure/vacuum in the crankcase, the oil lubricating the turbo sometimes can't drain fast enough back into the oil pan. That's usually solved by controlling the oil feed to the turbo and reducing pressure in the crank. The other issue is when too much oil vapor spits out the top of the AOS and gets sent to the j-boot, resulting in noticeable amounts of oil in the j-boot and turbo inlet. That is why people use a catch can -- to trap the oil in the can instead of sending it straight into the turbo. A breather vented to atmosphere will also prevent oil from getting to the j-boot, but makes for a smelly car as all that oily pressure fills the air.