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Building An(other) Oil/Air Separator

A DIY oil/air separator that looks professional but costs little.

By Dusko Mackoski

Click on pics to view larger images

Oil vapour mixed with the intake charge will reduce the octane rating of the fuel/air mix, encouraging detonation or pre-ignition due to carbon deposits. A layer of oily film on the inside of intake piping - and most importantly inside the intercooler - will also reduce its ability to cool the intake. So if your engine is breathing excessively, you will be losing power and in some cases reliability. And while every motor has some blow-by, running high boost, a worn out engine or loose build tolerances can cause an excessive amount.

So, what to do about it?

My Car

I had noticed a film of oil in the intake plumbing of my 170,000 km Subaru Liberty RS. I was unsure if this was affecting the performance or reliability of the motor, so I decided to get rid of it anyway. An oil/air separator was the answer.

I have seen many after market products that perform a similar function - oil/air separators or catch cans are the two common types. (A catch can doesn't return the cleaned vapour back to the intake - instead, you have to periodically empty it of the accumulated oil). But all the commercially available ones seemed way too expensive for my liking - one workshop wanted $300 (including fitting) and others wanted even more!

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Although the AutoSpeed oil/air separator ["DIY: An Oil/Air Separator"] was a cheap DIY alternative, it didn't appeal to me, as I wanted something that would look a bit better in the engine bay and be more compact to boot.

The obvious thing was to make my own. I was very particular as to what the device should do and how it should do it. It had to separate the oil from the air and not simply release it to the atmosphere. The clean air should then be returned to the intake, thus enabling the factory breather mechanism to still function in a basically unaltered fashion. A final criterion was that the device should be easily serviced if required.

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At first I planned on using a round 3 or 4-inch pipe that could be obtained as an off-cut at an exhaust shop. The design would incorporate two vertically mounted plates within it, giving three totally isolated chambers. The middle chamber could then be filled with a filtering medium (steel wool or foam rubber, perhaps?) and the air could be forced through the filter via the drilled holes.

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But as the sketch shows, it looked like a difficult prospect to weld the vertical plates in place - this was to be a genuine DIY job. So it was with a bit of luck that I stumbled onto some square U-shaped pieces of metal at a local metals supplier. I played around with them and realised that the smaller ones, when put together and placed inside the bigger ones, would hold together perfectly. And it would also be easy to weld them together using an oxy (or as I found out later, with a simple blow torch and solder). The picture shows how the parts stay together perfectly even before any welding is performed.

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The logic behind using square shapes rather than a 3 or 4 inch round pipe is that the internal filtering system (ie comprising the baffles and filtering medium) is much simpler to construct. As can be seen in the diagram of the round separator, it would have been more difficult to insert the two flat plates to create the baffles.

The Build-Up

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The process begins by first preparing and cutting the metal to the correct size. There are three separate parts to it: the two U-sections of the outside casing, the two U-sections of the inside casing, and the two square bits for the top and bottom. The internal filter casing and the outside casing need to be the same length, so that once the top and bottom are welded on, the only way through to the exit is via the drilled holes. This will ensure that the dirty air passes through the drilled holes and the filtering medium, rather than on another path.

A good result can be obtained by using a hacksaw and a hell of a lot of effort. Alternatively, an angle grinder makes cutting through the steel a breeze.

The holes were drilled with a power drill. Start with a smaller pilot hole first, and then use successively larger drill-bits. There are two large holes drilled on the outside casing on either side - two for the exit of clean air, and two for the entry of the dirty oil/air mix. There are also small holes drilled in the two middle separating sections. The best location for these is not directly opposite the main entry holes. Rather, the small holes leading to the filter section are on the bottom while the oil/air mix entry holes are on the top. This is all shown in the diagram of the round separator and ensures that the dirty oil/air mix travels through the whole of the filtering medium.

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Once the separator casing and filtering casing are cut to size and slotted together, the top and bottom lids can be marked out on the remaining U-shaped metal. I simply placed the metal plates on top of the separator and used a pencil to mark around the edges. The angle grinder made light work of these bits as well.

One of the flat bits will need to be drilled using a large size drill in order to accommodate a 1-inch fitting. This hole will go on the bottom and is used for filling the separator with its filtering substance. The removal of the filtering substance is also possible through this large hole, by the use of some tweezers.

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The burrs and rough edges can be cleaned up using a hand file but it will take time. I made light work of it using a belt sander. To prepare the metal for welding, I had to remove the galvanised material from the edges. It is also a good idea to sand a V-shaped groove where the weld will go, as it will provide for a better weld. The copper fittings were also prepared by being sanded and cleaned.

This picture shows all the components ready for welding.


If you are at all unsure of the procedures required to safely operate welding equipment, we strongly suggest that you have the welding carried out by a workshop.

The welding process proved to be a learning experience. I had never done any welding before and chose to use a friend's oxy-acetylene welder. The oxy welder gives a pretty good finish and is fairly easy to do. But this was short lived as it ran out of gas. The project sat in the shed for a few days until I decided to try a simple blowtorch on it. It's just a normal bottle of LPG used for barbeques, equipped with a blowtorch fitting. Although the blowtorch had quite a big orifice, it turned out to be just as good as the oxy. It does have some drawbacks, though. It takes a slightly longer time to heat up the metal and I don't think it is suitable for small items. It also tends to warp large flat surfaces.

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Luckily I achieved some pretty good results using the blowtorch with 5% silver welding sticks (similar to solder), and welding flux. I tried using normal welding sticks, which were supposed to have flux on them, but they proved to be no good when combined with a blowtorch. A touch up with the belt sander after each weld cleaned up the excess solder and also the black deposits that had built up after heating the metal.

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The copper fittings were slightly more difficult to silver solder. This was due to the steel requiring at least three times as much heat before it started to glow red, whereas the copper was glowing red in a mater of seconds! In the end, it was no major drama and I am quite impressed with the results (for a first attempt, anyway!). The trick is to heat the surfaces until they glow red. Then you add the flux that will magically melt away and completely clean the surface of the metal. Then you slowly feed the welding stick along the join while still heating. The stick melts and fills the V-groves.

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When all the welding is completed, sanding the box smooth and removed any dirt - vital if the paint is to take. To make sure of this, I cleaned it in a bath of mineral turpentine overnight. To be safe I used a high temperature paint. Giving it many coats will ensure small scratches and imperfections are smoothed out.

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Once it is painted, it is straightforward to fill the filter section with sponge material. I was initially going to fill it with steel wool, but a friend of mine who was using sponge to filter blow-by on his XF Falcon recommended I try it. The sponge is less permeable than steel wool and will theoretically trap more oil in. Anyway, if it turns out to be no good, I can always remove it and refill it with something else. I used a generic dish washing sponge/scourer.

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That's the type that has a green scourer stuck onto the back of a sponge. Simply remove the scourer part and shove the spongy bit into the separator. You will need more than one sponge, so get the multi-packs.

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The bottom was closed off by a copper plug and some white plumbing tape ensured a perfect seal.


I wasn't too interested in showing the separator off, so I found a nice discreet place that wasn't too far away from the original breather fittings - on the Liberty RS, that was just below and to the right of the intercooler. This location is on the opposite side to the turbo and hence in one of the coolest underbonnet areas. In order to fit the separator, a few things had to temporarily come off - firstly the intercooler, the turbo heat shield and the intake piping before the turbo.

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The Liberty - and most likely the WRX engine - has four places that the blow-by gasses can escape from. The PCV valve mounted on the intake manifold, the crankcase breather directly on top of the crankcase and one on each cam cover. These four hoses join into two and then go into the intake plumbing before the turbo. So, all that has to be done is to tap in to the two hoses just before they enter the intake. Divert them to the separator, and then from the separator back into the intake plumbing.

The intake piping that is just before the turbo has three small hoses plugged into it. The BOV connection is the obvious one, the combined PCV and crank breather line is the one closest to the turbo, and the combined cam cover breather line is the last one. You unplug both of these and use the 5/8-inch hose to take them to the dirty side of the separator. I ended up using only one return hose from the separator. The other opening was simply plugged up.

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I have had the oil/air separator fitted now for a year. The intake pipes before the turbocharger have noticeably less oil film build up on their insides. Although, examining the state of the foam filtering substance, it does not have as much oil build up as expected. I guess the car wasn't breathing that much oil to begin with!


In general you will have to figure out what parts you need for your engine. The separator I made was for a Subaru Liberty RS and most likely will suit most Subarus. Use the following as a guide to what you may need:

  • 5/8-inch hose. The length will depend on the chosen mounting position
  • 4 x 5/8-inch copper fittings. 3 will actually do as I ended up blocking the second return fitting. I used 2 x 90 deg bends due to the location of my separator. (Note: It may be better to use steel fittings rather than copper. The main reason for this is that hose clamps are strong enough to distort and buckle the copper. I did not realise this until I installed the separator.)
  • 2 x U-shaped galvanised steel pieces of metal (70mm x 32.5 mm, length was about 20 cm)
  • 2 x U-shaped galvanised steel pieces of metal (68-70mm x 20 mm roughly)
  • 1 x 1-inch or bigger fitting for the bottom
  • 1 x 1-inch or bigger plug for the bottom fitting
  • At least 5 x 5/8-inch hose clamps
  • 3 x household scourer sponges. (remove the harsh scourer part)
  • 2 x solder sticks, 5% Silver or similar will do the trick
  • Blow torch

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