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From: Grew up in Manhattan Kansas, live in Saint Louis
PCV “clean” side
Can someone help me to understand the concept of a “clean” side in the PCV system? I think the picture I attached is how I need to route a catch can, but I do not understand how one side is clean and the other is dirty. The air is coming from the same spot - just passenger vs driver side head, so what makes one clean and the other dirty?
Thanks in advance
The PCV system draws fuel, water, and exhaust vapors out of the crankcase, through the valve on the "dirty" side, into the intake, downstream of the TB where there's vacuum. All that stuff passes through the combustion process and gets turned into carbon dioxide and water, just like regular fuel does.
That means air has to be let back into the crankcase from SOMEWHERE; it needs to be filtered somehow of course, so that dirt and whatnot doesn't get drawn in too. That's the "clean" side. Make-up air, to replace the dirty air that's drawn out.
BUTT... a car with a MAF can't deal with air coming into the intake that hasn't been through the MAF first. It has no way of "knowing" how much air that might be (how many oxygen molecules), so that it can include it in its fueling calculations. That's the reason why the make-up air comes from a point between the MAF and the TB: ALL of the air that makes it into the intake passes through the MAF first, just, a part of it gets diverted temporarily through the crankcase. Stock TBs have a port in that place, but some aftermarket ones don't, therefore with those a nipple has to be added somewhere to replicate it. If the make-up air doesn't come through the MAF, such as if it comes from "breathers", that causes all manner of idle problems etc. Which is one of the reasons people like S/D setups, without a MAF.
Your diagram is correct as to air flow. Should work fine.
Good explanation by RB.
Clean side= clean fresh metered air into crankcase.
Dirty side = contaminated air from out of crankcase and into intake through catch can and pcv valve.
Drawing is good. I would plumb the pcv valve between catch can and intake manifold in case the catch can is not fully sealed. Otherwise the can may be a vacuum leak.
From: Grew up in Manhattan Kansas, live in Saint Louis
I appreciate the responses - I think I need to clarify my question a little more though.
I understand the difference between clean and dirty, the part I don’t understand is why air flows from the Throttle body port to the crankcase. While air is flowing, isn’t there a vacuum at the throttle body? What causes the clean side to flow in opposite direction as the dirty side? I know it’s wrong, but I feel like both sides what naturally flow from the heads to the intake/or throttle body. I’d just like to know why. It’s the missing piece in that puzzle for me.
In front of the throttle blades, there is no vacuum. (or very very little anyway) Behind them of course, there is.
Vacuum is an artifact of the engine (pistons etc.) emptying the contents out of the intake faster than they can come in. When the throttles are wide open, there is essentially no vacuum. When they're mostly closed, as at idle and light-load cruising, the engine extracts the gases out of the intake MUCH faster than the throttle lets them in, which results in a pressure lower than atmospheric, aka vacuum, in the intake.
Air comes through the MAF, gets metered, and most of it continues on directly through the TB into the intake. A little takes a detour before the throttle blades, enjoys the scenic route into & through the crankcase picking up funky vapors along the way, and THEN makes it into the intake. All driven by the engine emptying out the intake faster than it can refill around the tiny crack at the edge of the throttle blades. Your catch can is designed to capture excess liquid oil in that "vapor" stream before it gets into the intake and either pools up inside it or disappears out the tailpipe.
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PCV “clean” side 
