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The Purpose and Benefits of an Air/Oil Separator

Matthew Rogers  septiembre 23 2020


Performance driving, especially with certain engines, can cause oil vapors to get into your air intake. Many vehicles prevent this with a catch can. However, this leads to oil loss. The solution may be an air oil separator. This is especially popular with turbocharged Subarus such as the WRX and STI. Learn what this component is, how it works and why you should use one.


What Is an Air Oil Separator?

Oil from the crankcase can get into the blow-by gases escaping from the engine cylinders. These blow-by gases need to be recirculated back into the cylinders to reduce pressure (street-legal vehicles are not allowed to vent them into the atmosphere).

To let off the pressure and recirculate the blow-by gases, many vehicles have a positive crankcase ventilation system. This reroutes those gases to the inlet system of the car. However, the gases pick up oil vapor as they pass through the crankcase. This can cause a build-up of oil in the engine and can even cause improper detonation in the cylinder (this can be very damaging).

Therefore, some vehicles use either a catch-can or a modern advanced air oil separator to remove the oils from the recirculating gases. Essentially, they are there to act as a filter for the air passing through the system.


How Does an Air Oil Separator Work?

The basic concept of an air oil separator or a catch can is very simple. The oil-infused air passes through a narrow hose into the filter. The air then exits the filter through an outlet that is at a hard angle turn from the inlet. The air can make this turn, but the oil cannot, causing it to drop into the filter. Add to that the lower pressure of the filter vessel and a large portion of the oil is effectively removed.

Some catch cans and most air oil separators have more elaborate arrangements with additional chambers and baffles inside the vessel. This helps to filter even more oil from the air. Nonetheless, the basic concept is the same: pass the oil-infused gases through a path that is restrictive for oil but not air.

The key difference between a catch can and an air oil separator is how they deal with the filtered oil. The former is just a receptacle that must be emptied manually. The latter has a drain that returns the oil to the engine’s oil supply.


What Are the Benefits of an Air Oil Separator?

An air oil separator can be a valuable addition to many vehicles, especially those that are prone to oil build-up in blow-by gases. These are some of the main advantages of using this component:

  • Avoid Oil Build-Up: The primary reason for using an air oil separator is to avoid recirculating oil into the cylinders. This can coat the air intake with oil and slowly clog the air flow. That translates to reduced maintenance and more consistent performance over time.
  • Protect Against Detonation: Another major benefit of using a separator in the PCV system is that it prevents excess combustible oil from getting to the cylinder. Too much oil can cause premature combustion in improper parts of the engine. These detonations can cause significant damage, especially if they are allowed to continue.
  • Minimize Oil Loss: One of the main drawbacks of catch cans is that they remove oil from the system. For certain vehicles, particularly those with horizontally opposed engines, this can cause significant loss of oil. An air oil separator fixes this issue by draining the filtered oil back to the oil system.


How Do I Install an Air Oil Separator?

Although an air oil separator may sound very complex, the installation process is surprisingly simple. Any vehicle that has a positive crankcase ventilation system can have an air oil separator installed easily. This is especially true for a vehicle equipped with an existing catch can.

You should follow the specific instructions for your air oil separator. Nonetheless, the following steps are the general process for any version of this component:

  1. Locate the PCV hoses.
  2. Mount the bracket for the air oil separator in an appropriate location near the PCV lines.
  3. Disconnect the PCV hoses.
  4. Remove the existing catch can if there is one.
  5. Connect the air oil separator to the PCV hoses. If there was no existing catch can, you may need to add a second hose.
  6. Connect the drain hose to the oil system.
  7. Mount the air oil separator on the bracket.
  8. Reconnect the OCV hoses.

After following these steps, your air oil separator should be ready to clean your blow-by gases. This is an easy but worthwhile upgrade for almost any project car, especially a turbocharged Subaru (they are prone to oil loss). At MAPerformance, we have a selection of top air oil separators including IAG Air Oil Separator for Subaru WRX. This could be your next upgrade.

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