The PCV System Explained

[Legacy777]

I was typing this up as a reply, but feel that the PCV system, especially on the turbo engines may not be fully understood. I will start off discussing the PCV system from the center vent on the crankcase.

http://www.main.experiencetherave.com/s ... P_5113.JPG
http://www.main.experiencetherave.com/s ... P_5114.JPG

This hose should be routed to the valve covers on either side. This helps equalize the pressure difference between the center crankcase and the heads.

For the remainder of the PCV system, there is a line that comes off the turbo inlet elbow that splits off and runs to both valve covers. This line brings fresh air to the heads. You can see the split and where this line is on the turbo inlet in this picture.

http://www.main.experiencetherave.com/s ... P_6018.JPG

Pic of the passenger side going to the valve cover.

http://www.main.experiencetherave.com/s ... P_6015.JPG


The large port on the back of the engine is where all the crankcase vapors exit to be introduced back into the engine.

http://www.main.experiencetherave.com/s ... G_2550.JPG

The line coming off that vent tee's. One side goes into the PCV valve and the other end goes into a second port on the turbo inlet elbow. Under lower engine rpm (higher engine vacuum) the crankcase vapors are drawn into the engine through the PCV valve. Under higher engine rpm (lower engine vacuum) the crankcase vapors are drawn into the engine partly through the PCV valve and partly through the turbo inlet elbow. Under boost situations all the crankcase vapors are drawn into the engine through the turbo inlet elbow.

Just to clarify, the turbo inlet elbow has two separate ports on it. One to draw fresh air into the valve covers and one to vent crankcase vapors into the engine. Since these ports serve two different functions, they can not be combined.

Here is a scan from the factory manual which gives some more information on the PCV system.

http://www.main.experiencetherave.com/s ... _turbo.pdf

[James614]

One of the things I've always wondered is, how can the flow be properly directed under no vacuum conditions? With the PCV valve closed the fresh air draw and the evacuating air are within 1" of each other at the same point in the intake. There doesn't seem to be a good reason why the system wouldn't reverse flow under sustained boost.

I say this because the lines going to my head breathers still weep some oil into my intake. Less than the block breather, but still noticeable after a few months, and it's been that way since I bought my car stock. I never thought much about it in the past years but more recently after knowing how our PCV system flows it's been bugging me. If it flows like you say then it doesn't make sense for oil to make its way up that much tubing against opposing airflow and back into the intake.

[Legacy777]

I agree, there is probably "some" amount of backflow through the head vents. It's interesting the factory manual doesn't state that.

I've found that the setup I've got on my engine works pretty good to keep oil out of the intake. Just putting inline foam filter media in the head breather lines works very well to keep oil from going into the intake, and a catch can on the main vent line again does a pretty decent job. It's not 100%, but is pretty good.

[James614]

Yeah... I've got the catch can on the main breather and that's doing a decent job. When I get back I'll have to do something like the foam for the head breather.

[wtdash]

Thanks for the info....esp. about not combining the two. Pretty sure that others (me too) have done that.

That's why it takes two catch-cans to setup the system correctly?

And for the dumb question of the day.....do NA cars ever produce positive pressure - full load?

Td

[Legacy777]

You don't necessarily need two catch cans, and from my experience with the foam inserts, that seems to work very well. Since you don't have the volume or duration of exhaust gasses coming out of the valve covers that you do from main crank case vent, when the foam is placed in a vertical position it does a pretty good job of coalescing oil mist and allowing it to drain back down into the valve covers.


Regarding your question about do NA cars every produce positive pressure, the answer is no. (As long as they are function properly ) They can produce near atmospheric pressure, but no positive pressure.

[cj91legss]

So Josh, I'm wondering about this some as When you came to Seattle we had a small conversation about this. (which recently me and Stephen concluded that it is for sure a blown seal on my VF39)

I want to re do my PCV system to make sure it is functioning properly. BUT My ej20h heads only have 1 breather on each (unlike our ej22t heads.) The jdm motors do not have that vent between the alternator and power steering pump.

So what would happen if I block off that particular nipple coming off the block? I wanna say i read something on here when i was doing my engine build/head swap about that but don't recall where on here it was. Do you have a 20G pcv system available without having to search? If not i can easily search it tomorrow. Just curious.

[Legacy777]

I would think you have three options, you could either block off that center port on the block and not worry about it. The second option would be to drill and tap fittings into the valve covers so you can route that center port to separate ports than what is going into the intake. The third option is you could run the lines to the center port on the block and tee into the vent lines coming to the heads. However, I probably would NOT recommend this options for the reasons noted below that the ports going to the intake go through what appears to be some baffles.

From these pictures you can see that the ports that tie into the center vent just dump directly into the valve covers. While the ports that tie into the intake manifold go through what appears to be a set of baffles. I would assume that the ej20h valve covers have a similar baffling.

Driver's side valve cover:
http://www.main.experiencetherave.com/s ... P_4726.JPG

Passenger's side valve cover:
http://www.main.experiencetherave.com/s ... P_4728.JPG

Here's the PCV diagram for the overseas Liberty/Legacy. It's like the ej22t, minus the crossover piping/vent.
http://www.main.experiencetherave.com/s ... erseas.pdf

[cj91legss]

Awesome Josh, thanks alot.

Sent from my SGH-T989 using Tapatalk 2

[GTwagon]

can i use a wrx pcv system in a ej22t?
reason i ask is im finally in the process of replacing the short block in the turbo wagon.
in the process of removing the intake i broke a couple of plastic elbows and tee's.
im planning to use a tdo4 turbo and a 06+ tmic.
i know im not the only person that broke the plastic elbows and tee's.

[James614]

The WRX uses a completely different intake/PCV routing. Your best bet is to just use generic hose/pipe fittings and bulk transmission/oil lines (regular rubber vacuum/water hose will break down over time in contact with oil residue). It will likely be way cheaper and you can route the hoses differently if you want, or take the opportunity to install a catch-can.

[Legacy777]

This thread has info on where to get emissions hose for a pretty reasonable cost.

http://bbs.legacycentral.org/viewtopic.php?f=5&t=47500

[GTwagon]

tyvm for the info

[Legacy777]

You're welcome.

[GTwagon]

tyvm for the info

+1 on that

[turboboxer92]

i honestly cant get any pictures to come up on any of these links i believe they are dead

[Legacy777]

I was having issues with my ISP and DNS issues, so try again, they should be working now.