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PCV/breather/catch can threads are always amusing b/c of how people love to ignore some simple basic facts.

1) Think about under what conditions the PCV opens. When there is positive crankcase pressure. This occurs when highly pressurized combustion gasses make their way past the rings.

2) Think about what the breather port does. Obviously, it is always open. It is a passage to the airspace under the valve cover. But guess what, the airspace under the valve cover is also linked to the airspace in the crankcase by virtue of the oil drain holes and the air space for the timing chain. It is all connected.

3) Think about the conditions in the intake pipe/manifold. Typically, there is vacuum in the intake manifold/intake pipe. At WOT, you can get close to neutral pressure if not a slight amount of positive pressure.

Now, you have to tie all those conditions together bearing in mind that you want:

a) neutral pressure or slight vacuum in the crankcase, you do not want excessive vacuum as it will create more blowby.

b) vacuum under the valve cover, this keeps excessive oil from seeping past valve stem seals.

c) you do not want oil to enter into the intake air.


To address concern a), you NEED the PCV valve in place if you are connecting the crankcase to a vacuum source. Because of how the PCV is sized, I would not recommend simply going to an open port to allow passive venting of crankcase pressure. Without the aid of vacuum, positive crankcase pressure may not be relieved quickly enough. In short, the PCV should be retained and it should be hooked up to a vacuum source. Unless you are planning to enlarge the PCV hole to allow for sufficient passive ventilation of the crankcase to atmosphere, don't mess with the PCV system.

To address concern b), you need to have the breather port on the valve cover hooked up to vacuum all the time. The closest you can get to that is to hook it up to the intake. Note the size of the breather port. It is fairly large because it is the PRIMARY way of controlling crankcase pressure. The PCV is essentially an assist to ensure pressure in the crankcase has a more direct outlet if needed. The breather indirectly vents crankcase pressure via the oil passages & air space around the timing chain.

To address concern c), you need a catch can to collect the oil vapor in the crankcase fumes. The catch can should have two inlets and one outlet with some type of baffling/filtration inbetween. Connect the outlet to the intake. Connect the PCV and Breather to the inlets. Make sure your ports are sized accordingly! If you have tiny little ports on your catch can and you hook up the 3/4" valve cover breather to it, you are severely affecting the vacuum applied to the air space in the crank case.

Now, what I've presented above isn't a complete discussion. To complete your understanding, you have to consider one more thing. What if you are engine braking? Then there is just massive vacuum in the cylinders. This happens because the throttle plate is closed and very little air can be drawn in via the idle air passages. This vacuum in the cylinders creates vacuum in the crank case b/c ring seal is not perfect and the valve stem seal is not perfect. In that specific situation, air flow will reverse through the breather because there is more vacuum in the crank case than there is in the intake pipe. Air will be drawn into the crank case via the breather. Surprise, surprise, this is why there is no one way valve on the breather like there is on the PCV. Airflow through the breather is bi-directional.

More or less, there is no need to re-work the stock system for 99% of all cars, stock or modded. The most beneficial thing you can do is add a catch can to capture oil vapors without altering the routing of airflow of the stock system.
So what do you recommend for a guy with an upgraded inlet manifold and it doesn't have the pipe on to connect the PCV valve back on to? I'm lost currently just venting to the floor
 
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