Tag Archives: AOS

Air Oil Separators – Not All Created Equal

I’m thinking on adding an air oil separator (AOS) to my wishlist.  It’ll help keep the oil out of my IC and intake and keep the car from experiencing detonation by preventing any condensation from making it into the engine. There are arguments on if this item is even needed on a Subaru (or any blown) engine (see here). There’s quite a bit of AOS makers, too.  I’m going to link each one, but keep in mind that the links may break as the web designers move their products around their web pages:

  • Grimmspeed – this appears to quickly-installed product, but I’ve seen bad reviews of it; there are conditions that will cause blow-by, even with this AOS.
  • Moroso – this is a small dual can variant – $275; they have $350+ variants as well as variants that are $100 less.
  • Perrin – this is a nice unit; I like the fact that I could theoretically pull the drain hoses and see what the oil condition looks like after it has worked its way through the separation process.
  • Crawford – from their website: “Largest internal volume (.75 liters) on the market. This volume is imperative to the effective separation of the oil from the air.”
  • iAG – they’ve a LOT of science-like info on their site regarding this product, but I’m not sure that will be enough to sway me.

I may have missed a few, but those are the ones that jump out at me.  Plus, it seems like the aftermarket is flooded with tuner interpretations of the AOS concept. Note that almost all of them are in the $350-$375 range.  That’s a lot of cash, but apparently they aren’t all created equal. Grimmspeed’s doesn’t work all that well on engines that crank out 350-400 HP, but it’s cheaper than most of the others.  Apparently, they don’t take well to high G maneuvers and they also tend to be more of a street application (low HP and street with no hard street driving).  There have been reports of significant blow-by occurring with the Grimmspeed AOS.  I found a good NASIOC forum thread here that describes this issue and even has Grimmspeed feedback. Crawford’s and Perrin’s apparently work very well on high HP applications. The Moroso AOS appears to also be a good candidate and has many options.  They come in single and dual container configurations.  With the dual container option, you can opt for large or small containers.  The containers can be obtained with powder-coating too.  The thing I’m confused about with the Moroso is that they appear to be more of a catch-can solution.  They will not direct separated oil back into the oil supply.  They have to be periodically emptied and the from my understanding, the they don’t really separate the oil from condensation…the gunk is really something you don’t really want to reintroduce to your oil supply. There’s also the thought that with the general thought behind the AOS concept, you shouldn’t really be redirecting separated oil back into your engine’s supply.  The thing is, these aren’t considered cat cans.  Catch cans just catch any blow-by that occurs.  They have to be maintained…you don’t want them to be full and you have to throw out any gunk they catch.  With an AOS, there’s a separation mechanism and the idea is that once the oil is separated and de-condensed, it should be OK to put back into the oil supply, but I’ve yet to see vendors/makers provide evidence that the oil that is separated is clean and can be reintroduced to the engine’s oil supply.  So, I’m torn between buying an AOS and buying a catch can that I’ll have to maintain. I’m leaning toward the iAG (one of the most expensive) or the Moroso (because it looks good and because it’s cheaper…but it’s just a glorifed catch can…I really don’t think they are true AOSs).  iAG swears that they’ve stopped the issue of condensation, but they haven’t provided any hard evidence.  I’d hate to spend $380 on an AOS (any AOS) only to find that they’re putting gathered gunk back into the engine. Oh, and what’s the difference between an AOS and catch can?  The AOS separates the oil from any condensation and returns the oil back to the oil supply.  A catch can only catches it, it has to be periodically emptied, and there may be more than oil that’s captured.  And note that there are some $300 catch cans.  That’s a pretty ridiculous price for a catch can but apparently Mishimoto sells catch cans in that price range. Lastly, for those located in areas where there are strict emissions testing, an AOS might not be a good thing.  Remember, not only are there visual inspections that some states require (an AOS might not pass a visual), but there’s also the actual sniffing/testing…an AOS might cause a test failure.  I’m not 100% but I wanted to forewarn people, because the PCV is an emissions part.


UPDATE: Regarding the Moroso, I did a bit more research and found some clarification on the Moroso unit (regarding if it’s a catch can or actual AOS):

Q:  The title says it’s an Air Oil Separator, but the description sounds more like it’s an Oil Catch Can since it has to be drained. Which is it?

A:  Its a separator in terms of it breaks down the fumes much more efficiently compared to just a catch can.

If that’s the case, then the oil that it catches can be poured back into the oil supply (sounds like, at least).  I found the answer here, under Questions (it is the second question…the questions aren’t linkable).  And note that these do indeed require maintenace (ie, they have to be periodically drained/cleaned).  Some variants have drain petcocks, and others can be removed from the mount.