Ok in fully discussing this with my engine guru buddies, here is what we were able to come up with.
Background: In a typical engine, there is often a oily vapor that comes from the engine block. This vapor is oil blow by from tolerances such as the piston and cylinder interface. Since a typical combustion process is high in pressure and there isn't a perfect seal between the piston and cylinder wall, some air (with oil vapors and other combustion vapors) will leak around piston seals and into the engine block. All of this oily vapor is routed to a tube, and for emission purposes put back into your intake manifold where it can be reintroduced into the engine cylinders and used/burned off.
N/A and Boosted: In a N/A engine the intake manifold is normally under negative pressure as the pistons down stroke to "vacuum" intake air in the cylinder. The engine block is under positive pressure and will push the oily vapor blow-by into the vacuum created in the intake manifold. HOWEVER, in a boosted engine the intake manifold is under positive pressure (not negative). Also the combustion process is intensified, so this results in two things:
1) More blow-by around the pistons, increasing the amount of oily vapor being pushed into the block.
2) The intake manifold is now under positive pressure. Therefore the intake manifold check valve (PCV) prevents the positive intake manifold pressure coming from the super/turbocharger from pushing air/pressure back into the engine block.
As a result of these two items an oil catch can essentially becomes highly recommended, if not mandatory.
Setups: There are multiple setups for setting up a catch can. The most common is to route the block exhaust and intake manifold to a catch can with a vent. The SOS catch can, stevenlee's, EAC's catch can are all set up this way. Under boost, since the manifold is under positive pressure and the block is under positive pressure, AND the catch can is open to ambient, the flow of air will always go to the direction of the catch can. This will allow the catch can to catch all vapors off of the engine block. While not on boost, the vacuum will suck a tiny bit of air through the catch can while the block pressure will push the vapor exhaust to the catch can. Note, since the engine block vapor is NOT being returned back into the intake manifold, this car no longer technically passes emissions since the oily vapors are now open to the ambient.
My set up is different. My catch can is "Tee'd" off of the line instead of being inline. (See picture)
To keep the PCV or to not keep the PCV? That is the question. This is pretty simple once you understand how the oil catch can works and what kind of set up you have. If you have a catch can that is inline then it really doesn't matter if you have a PCV or not. The PCV is basically there to keep intake manifold air from pushing back on the oily block exhaust. Since the intake manifold air is now routed to an oil catch can and not the block, then it really doesn't matter since it cannot push back on the block anymore. The only reasoning I could possibly think of for not removing the PCV is that under positive boost, it would leak some boosted air out the catch can. But the pressure loss would be so minimal that it really wouldn't matter one way or another.
Now in my catch can setup, I would have to keep my PCV in. Otherwise, the oily block exhaust would flow right past my catch can and into my intake manifold OR the boosted intake manifold would blow past my catch can in the other direction, blowing into the block and prevent the oil blow-by to be relieved. By keeping the check valve in place the positive intake manifold pressure will keep the check valve closed, essentially dead ending that path. Therefore the oily engine exhaust has nowhere to go but into the catch can.
Route catch can oil back to pan?: Some people will say that the oil vapor leaving the exhaust will have products of combustion, water vapor and other contaminates in it and it will all be caught by the catch can. Others will say that being open to the ambient it will expose oil in the catch can to water vapor and the ability to condense water out as it cools in the catch can. All of these statements are somewhat true and if you've ever seen what's in the catch can, it is a pretty nasty frothy mess of watered down oil with god-knows-what else is in it. So does that mean you should absolutely not return this back to the oil pan? Not necessarily. What is caught in the catch can is what the engine would see normally anyway. And in a N/A engine, this is already returned right back into the intake manifold. While it's true there might be some water vapor introduced through the catch can due to condensing, it's not any more amount than the engine would typically see under normal operation. So ultimately, if you don't want to deal with the hassle of having to constantly empty an oil catch can, then the general consensus among my group of people seem to think it is ok. There are a number of installation where this is done with no long term affects that I know of. Although, however, if you don't want to take any chances and any risks of introducing any water and other nasties back into your system, then take the safe route and do not route the oil back into the pan. It's a peace of mind vs. convenience trade-off decision that you'll need to make.
