I plan on building a dual oil catch can setup for my car. I like the simplicity of the Campbell Hausfeld (or other air compressor water filter) method.

One thing I don't understand is why or why not to make the catch cans vented (i.e. - with a small filter on the top). Most of the name brand units don't appear to be vented as they don't have filters, but I have seen a few that do.

Is it better to mount the catch can above or below the lines coming from the valve cover. Some have said above while some have said below.

Could someone explain (please don't just say yes, no, above, below, etc....I am trying to learn.)?

 

They're vented because they don't understand why it shouldn't be vented. They're letting in unmetered air during vacuum. Arguable how much it really impacts during vacuum, but a vacuum leak is a vacuum leak to me. /shrug, but I'd rather leave the PCV system how it was designed.

 

That is what I was thinking. Why change the design of the factory system? I guess the only issue is to make sure any catch can can flow air both ways and that there are no one-way valves in the unit.

 

Just put it inline and don't vent it. Simple as that...