If you are looking for an Air Oil Separator (AOS) or a Catch-Can, and you aren't sure which one to get, or which one you need, we have put together a lot of information to help you understand how the PCV system works, what an AOS or Catch Can does, and which one you might need. 

We created this option to group all of the videos and blogs that we have on our site on this subject into one place, and you can find all of that information below.

In the journey to figure out what is going on in the Subaru EJ PCV System, and what the best solution is for a street or track car, Jon installs an IAG AOS with a 3MI Drain Hose in this latest installment.

If you want to improve the function of your PCV (positive crankcase ventilation) system, it helps to have a good understanding of how that system works.  We put together a video detailing how the system works on the EJ255 and EJ257 turbo subaru engines, and that video is below.  The function of the EJ205 PCV system is the same, the only difference is that the EJ205 engines that came in the USDM 2002 - 2005 WRX don't have the additional ports that connect the valve covers to the center of the crankcase.

An Air Oil Separator or Catch Can work by diverting the flow of gasses through the PCV (Positive Crankcase Ventilation) System to their filtering systems to capture the OIl Vapor, etc.  If you are going to install one of these systems, it helps to have a clear picture of what the stock PCV system looks like, and where everything is connected.  We have a video going into detail for the Subaru EJ engines that can be a helpful reference point.

Next we created 2 different videos going into detail about the difference between an Air Oil separator and a catch can, and which one you might need.  If you aren't sure which one of these you need, these videos should help you make that determination.

One of the questions we get asked the most is, "Do I need a catch can or an air-oil separator?" This video tackles the differences between them, and in what cases they are useful.

We also made another video talking about signs that your PCV system is being over-run, which would indicate that you would want to install something like a Catch-Can or an Air Oil Separator, but those same indicators can help you determin if your Catch-Can or AOS is working properly as well.

The more that we looked at how the PCV system worked on a WRX or STI, the more it became clear that crankcase pressure was something that was worth looking at and monitoring to get a sense of what was going on with this system.  We worked with Harvey at The Boostcreep to log crankcase pressure on vacious cars that he tuned, and in the end, the date that he collected really helped to paint a more clear picture of what was going on in the engine when the PCV system, and the related Air Oil Separator or Catch Can were connected properly, or improperly.

This is a longer video, but the examples that we discuss here are very helpful when it comes to understanding your PCV system, especially when you are trying to trouble shoot an issue.

A few years ago, Jon got interested in trying to figure out what was going on in the WRX and STI PCV System, and in why some Catch-Cans and Air Oil Separators worked better than others.  The work and testing that he did is what largely lead to these videos above, but he also documented that process as well.  Here are those videos from our YouTube page in chronological order.

Cars that have Direct Injection can end up having a unique problem with carbon deposits on the intake valves because there isn't fuel running over them to keep them clean as there is with standard Port Injection.  But where does that carbon come from?  It turns out that this relates back to the oil vapor that flows through the PCV system, and that is a big reason that a Catch Can can be really important on a Direct Injection Engine.

Even if you are running an Air Oil Separator, there is a decision that you need to make about how or if you want to connect it back to the inlet side of the engine.  If you have a street car, you really do want to keep the AOS connected to the engine intake to keep the function of the AOS as close to factory.  A big reason for this is because then you don't need to tune the car when the AOS is installed.

But if you have a Race car, there are more things to consider.  Do you want to completely separate the PCV system from the inlet side so oil and fuel vapor can't get into the intake charge?  Do you need the vacuum that the inlet creates to help scavenge pressure from the crankcase?

In this video, we go into those details to help you make the best decision for your car.

When you install an Air Oil Separator, does this completely fix any issues that you might be having with your PCV system?  It would be awesome if the simple answer is that it does, but in reality, it is more complicated than that.  At this point in the testing of Catch Cans and Air Oil Separators, Jon ran into an issue with the Radium AOS that he installed related to the AOS drain.  The solution at this time was to go back to a Catch Can, but not a single one...

The change this time was to install a Dual Catch Can from Radium to see if that would be a better solution.

Each engine bay can present a different set of problems for installing an AOS or Catch Cans, and ultimately either option has to function properly to get the improvement that you are looking for.