Air Oil Separators and Catch Cans - Everything you need to know

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.

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If you still have any questions after going through this information, you can reach out to us though Live Chat, or you can submit a question for our Question of the Week series under any of those posts on our Instagram or Youtube pages.

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Details

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.

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.

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 Cooley 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, and as new videos come out (yes, this is still a work in progress) they will be added at the bottom of this page.

Part 1 - Getting a Baseline