Subarus like all flat engines, blow a lot of oil out of the crankcase in their blowby gasses. This is always a problem with performance flat engines like Porsche or Subaru. The fact that the WRX and STI are turbocharged just makes the issue worse because turbo engines produce more blowby. When oil is ingested into the engine via the crank vents and PCV system, it contributes to detonation which will damage your engine, especially if is turbocharged. Oil can accumulate in your intercooler, reducing its effectiveness. Subarus blow so much oil that they can run the crankcase dry in one track session which obviously causes engine damage. Because of this, there is a big market for Subaru air oil separators. At first, we thought that the Nightmotorsport valve covers were just another pretty billet part but we soon found that they are more than that! Nightmotorsport has another solution to help remove the oil from blowby gases internally by incorporating an improved air-oil separator into their billet valve covers.
The Nightmotorsport valve cover is CNC machined from 6061 aluminum billet. It is available in many different colors but we chose the raw billet finish to match our Process West intake manifold. When looking inside the valve cover, you can see the big hump of the new air-oil separator inside the valve cover right in the middle. You can easily tell that the Nightmotorsport part has way more interior volume than the stock part and is much longer internally.
The OEM separator is a flat plate covering a passage in the valve cover. In this picture, you can see just how much smaller it is than the Nightmotorsport part from the outside.
You can see the raised gasket compression stop at the lower bolt hole in this stock valve cover. Nightmotorsports does not have these so you can control the gasket compression to reduce the likelihood of valve cover oil leaks. If you own a Subaru, you know this is a problem!
When you take the plate off you can see how simple it is internally. Blowby gasses come in at the left top of the chamber, make a short U-turn and go right out. There is a trap for the oil at the bottom and when it fills to about an inch, it can drip out of a hole. This is sort of odd because the blowby oil and condensation can accumulate and sludge can form in the trap with no way to exit. You can see the sludge residue in this lightly used valve cover.