Nightmotorsport Air Oil Separating Billet Valve Covers for Subaru EJ Engines

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.


    1. Initially, I thought so too but the owner of Nightmotosport came by and walked me through the features and I was very impressed with how well thought out they were, especially being a Subie owner and living through all the typical Subie issues.

  1. Curious to see an update at some point of accumulation in the IAG AOS after a track day. Love the content as always.

    1. Well the IAG AOS continually drains back into the crankcase which is a great feature, this way no oil is going to blow out of the engine!

  2. Wouldn’t such a part actually have the ability to free up power considering that the intake charge is no longer being contaminated with oil?

    Although, I have heard that Ferrari (and Mercedes?) were purposefully using oil consumption to make more power, although I am certain that they were tuning for the oil and using some pretty special oil additives.

    “Oil burns when it’s introduced into a gas-powered engine’s combustion chamber, which helps generate more power at the expense of, well, oil. In F1 today, fuel is heavily regulated by the FIA, but there’s a lot more flexibility with oil used. In practice, this means that teams can use certain additives to their advantage and create a more volatile fluid, helping with combustion.”

    1. Just reduce the likelihood of detonation, it’s not going to make more power, just let the engine make its intended power. Buring oil doesn’t make power, it has less BTU than fuel.

      1. Couldn’t you then run more aggressive timing and make your power that way? Similar to running higher octane fuel.

        I imagine that they only did it in F1 to get around the regulations. Not the way anyone would prefer to make power. Makes a mess.

  3. Is this item available in the 2.0 XT in my 2016 Forester? I am also curious best build for track day, (Road America Excludes 2016 Forester XT being a SUV, from running on their track, due to roll over by their experience), Brakes, Engine oil pump, oil pick up tube, oil pan, intake hosing, ect?

  4. So does this mean this could be installed instead of using an AOS like something like IAG AOS systems?

    1. We are still going to run an AOS on our motor with these valve covers. Night Motorsports says you dont have to.

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