A better drain option for your AOS from 3MI Racing

Air Oil Separators are pretty common these days on modified Subaru's.  There are a number of different options out there, but when installing them, there is one common factor that you have to take into consideration.

That is the drain hose.

Every AOS that we have seen as of writing this uses the top of the factory separator system (the 3/4" fitting just under the turbo ) as the drain for any oil that is captured.  In order to make sure that your AOS can drain that oil back to the block without any restrictions, you have to make sure that your drain hose has a downward slope.  If you have any bumps or inclines, that can limit the ability of the AOS to drain that oil back.

And there can also be the factor where that drain is also being used in part to vent pressure from the block.  It is a large fitting, but depending on circumstances, it may not be ideal for all applications to use that fitting as both a drain and a vent.

Without a better option, we have all just learned to keep an eye on the drain hose, and made this work as best we could.  As a side note, this is particularly difficult on a car that is keeping a top-mounted intercooler.

A while back, 3MI Racing approached us because they were aware of this problem, and they had come up with a solution that they wanted us to help test.  This was to take the drain of the AOS away from the top port on the block, and instead move it to join with the turbo oil return.

We immediately wanted to try this out, and we put the first test hose on Scotty's STI race car which ran the Killer-B Air Oil Separator.

The advantage to this design is that the drain hose now connects to the engine much lower, so maintaining that downward slope is much easier and a steeper slope for the drain helps as well.   This is especially true for any car that runs a top-mount intercooler because that drain hose is not competing for space with the intercooler piping.  And beyond that, the vent for the factory air oil separator goes back to being only a vent which ensures that it can function properly.

We were very happy with the results running this hose, and we are glad that we can finally make this solution available!

There is one note about the install of this hose.  Depending on routing, it may be necessary to modify or remove the inner turbo support bracket to give the 3MI Racing hose enough room to up to the AOS that you are running.  In our case, we ended up simply removing that bracket, and have not had any issues so far.  Ideally it does help to have two mounting points for the turbo, so long term we may look at fabricating a bracket, etc.

If you are wondering why this hasn't been available up until now, our best guess is because this does add a fair amount of time and complexity to the installation of an Air Oil Separator.  Especially since the turbo will need to be removed and re-installed.  It is a lot of extra work, but it really does help the AOS function better, and could well ensure more reliable function long term.

Thanks for reading and Stay Tuned!

- Jon Cooley