What s the main different between oil catch can & air oil separator?

oil catch tank seems able to catch more oil

but whats the main different between them and which one is better?

Do you have examples that you're interested in? As the names are often interchanged and probably talking about the same thing.

Sometimes the oil is drained back to the sump. Sometimes the air is vented to atmosphere and sometimes it'll go back to the intake.

It's going to get harder to pass a wof (or maybe cert) with a venting catch can as far as I'm aware.

There are a lot of subtle differences between the various kits available so it's best to judge a particular setup rather than trying to figure out what is "best" purely off what someone may call their kit.

One of these: http://www.grimmspeed.com/air-oil-separator-wrx-sti-lgt-fxt/

A catch can is an air/oil separator... it's function is to remove oil vapour from the air on it's way from the engine to the intake.

Some people (mainly yanks in n/a v8s) use a purely inline approach... but in a turbo, as soon as you increase boost, those things are

100% useless because you are going to have liquid oil flowing about, not just oil vapour.

 Scoobydoo said:

One of these: http://www.grimmspeed.com/air-oil-separator-wrx-sti-lgt-fxt/

Note the text is red that most likely means any car that is a few years old and/or has the boost cranked... so everyone in NZ

"Please note this product is designed to combat factory amounts of blow by. If you have a built motor or are experiencing abnormal amounts of oil consumption, this product may not work for you."

The difference between a "catch" can and a oil air seperate is exactly that - one catches the oil and stores it - and the other puts the oil back into the sump

Basically they're both the same thing / just plumbed differently

I don't think I'd run one that fed back into the sump. I had a small Cusco catch can on my high km BH engine, only connected to the head breathers. All that ever came out of it was oily coloured condensation, maybe 20ml per month. Still enough to contaminate sump oil though.

 Rosssub said:

I don't think I'd run one that fed back into the sump. I had a small Cusco catch can on my high km BH engine, only connected to the head breathers. All that ever came out of it was oily coloured condensation, maybe 20ml per month. Still enough to contaminate sump oil though.

That condensate is part of the reason that the operating temp of the oil is hot enough to boil off the water based contaminants.

Surely better extracted, collected and disposed of?

Depends, on your normal, modified road car I'd say yes.

If doing trackdays, some tracks tend to cause Subaru's to blow out a lot of oil from the heads. In that case, it's better off going back to the sump otherwise in extreme cases you can end up with more oil in the catch can than the motor.

So again, there's no simple answer as it all depends on the intended usage.