The purple line represents the pressure/vacuum in the crank case. By design (or flaw depending on your viewpoint) there is a pressure differential because of the speed and location of the pistons. This is why the PCV was designed and implemented. It's intent is to relieve the positive pressure by using the natural pressure differential of the intake air.
If the intake manifold is at a lower state of pressure than the crank case, the intake can 'suck' (because it really isn't a suck, merely a descriptive term) the pressure out just as fast as it's being put in.
The reason there is a one way valve inside the PCV is due to WOT where intake vacuum is low or non existent. The valve can open and close to try and keep a slight vacuum in the crank case. But it can't open/close fast enough for this to actually happen. There is not a PCV in the world that can that I know of. So it's design is simply a compromise and you really don't need one.
But the valve does serve another purpose, along with it's outlet size.
To create a pressure differential you need an area (or flow) of high pressure and an area of low. Being that the intake flows more air than the crank case there is an area of lower pressure at most all times in the intake (speaking after the throttle plate).
So that's why a PCV line runs to the intake manifold post TB.
The valve and diameter act as a restrictor. In times of WOT where there is no pressure differential you can not equalize pressure by conventional means, simple high and low. In this case you have to rely on the amount of flow, intake being substantially higher than the CC. This creates a Venturi Effect and thus pressure is sucked out.
The reason for the extra breather port on the VC is to air in the Venturi Effect and just in case the bottom end becomes pressurized it has another path of escape.
*side note:
you can actually use CC flow data to find if the engine is in need of repair/rebuild. Volkswagen on newer cars has a sensor that monitors the flow though the PCV system. You can view this data and if it's out of spec (which I'm not sure what spec is) you'll know if the rings are shot or if a bore is worn, but not specifically just that you need to open it up and look. The sensor is also used with the MAF to calculate the exact amount of air being ingested. But in the case of the Focus the ingested air is only calculated by the MAF. Any extra air, such as worn rings and added blow-by, isn't accounted for, even though it has passed through the MAF once. If that's confusing, which I wouldn't doubt it is, you have to consider the time factor. Just because the air was measured doesn't mean it'll all get to the cylinder at the same time. So there may be times when more air enters and time when less does. Which will create slight rich or lean conditions.
And that is in part to why a narrow band O2 sensor 'switches'. It was an attempt to mimic the naturally occurring rich/lean conditions.
/side note
Hands on is the best teaching/learning tool. So, go out and pull your PCV out and put your finger over the hole with the engine running. Block the PCV with your other hand so the engine doesn't stall.
Feel the pulsation of air coming out?
That's exactly what the graph represents. There is a frequency to the pulsation that is very easy to figure out. It's simply the definition of frequency, Cycles Per Second. Since we know the engine Revolutions Per Minute all you have to do is divide RPM by 60 to get the frequency of that pulsation.
If your hand where more sensitive you'd be able to feel the same pulsation from the intake tube. The size of the tube, cam design and plenum design reduce the strength of the pulsations.
That's an older video but it is showing how my setup functions.
Basically I capped off the VC port, and that's it. I added another check valve, the GM one, and a catch can to filter and catch the oil. I also hardlined the PCV because I was having too many problems with rubber hoses collapsing. Used 5/8" soft Copper for the lines. Used a Aluminum water bottle for the catch can, just like the po'man thread.
It's by no means an ideal setup nor does it look pretty. It is functional though.
Time for more hands on.
Engine running, PCV back in it's place.
Try moving you boost gauge to a place you can measure the crank case pressure vacuum. The dipstick tube works well (also where the gauge in the vid is hooked to).
Pull the hose off the VC breather and put you finger on it.
You might hear a whistling. This will be air being sucked past the crank/cam seals. Won't hurt anything short term. Long term the lips of the seals can overheat and become brittle.
What's the boost gauge saying? Are you making vacuum in the crank case, you should be.
Give it a little RPM and as the engine accelerates vacuum will fall a bit. But once the RPM stabilizes vacuum should return.
Now unlike Tom, and anyone else things, a setup done this way does not 'suck' oil out of the pan. The setup is almost identical to you putting your hand over a vacuum cleaner hose. Air can be sucked out but none is allowed in. Your hand represents the crank case and the exit air represents the intake manifold.
There is no wind or pressure inside the vacuum (crank case). So the oil stays put.
Where sucking oil up comes from is air flowing through the system. More air being sucked out of the PCV than is allowed in through the VC port. It's the through air that brings the oil with it. Pass more air and you'll get more oil.
I'm kinda forgetting what I've covered and my mind just isn't concentrating on this anymore. I may of missed a few things. But I think I've got most everything covered now, the why's at least.
