I have been thinking about a custom pcv system which would kind of be a hybrid of a few ideas I have found while reading around and Im having troubles understanding something about my pcv system. I understand that in most 1.8t during vacuum the valve on the tip is allowing air from the tip into the pcv system, the fresh air from the tip enters through the valve cover port which gets to the block through internal passages, and vacuum from the intake manifold is pulling air from the crankcase through the lower breather.
What I don't get is how this works on my system which has NO valve cover breather. Where is the air from the tip during vacuum going? If the i.m. is pulling air from the breather during vac and the tip pulling air from the breather during boost, then I guess the block has no chance to get fresh air...super sludge, audi fail. Well not a complete fail, they did just pull off a le mans 1,2,3

 

This isn't entirely accurate. The turbo inlet pipe is always under negative pressure. While technically speaking this is a "vacuum", it's not caused by the same forces as the vacuum you see on your boost gauge. The former is caused by the compressor wheel drawing air into the turbo. Because of this, air would never go from the TIP into the valve cover. Even if it wanted to, your "pancake" valve is there to prevent it from happening. The oil vapor under positive pressure in the valve cover and block is vented into the turbo inlet pipe to be run back through the intake tract and burned.

The vacuum reading on your boost gauge is created when the engine is running and consuming air, but the throttle is (even partly) closed. The engine consumes the air in the intake manifold faster than new air gets past the butterfly valve in the throttle body, creating a vacuum. This is why N/A cars also create vacuum.


The whole purpose of the PCV system is not to force fresh air INTO the block and valve cover, but to evacuate positive pressure. Hence the name: "Positive Crankcase Ventilation." While I agree that routing the oil vapor back into the turbo isn't the best way of doing it, this wasn't really the cause of the 1.8T's well known sludge woes. That was caused by bad oil change interval recommendations, under-sized oil filters, and a poorly designed oil pickup tube from the factory

What did you have in mind for your own PCV setup?

 

Damn, so this diagram is wrong?


I thought part of the function of the pcv system was to feed some fresh air into the crankcase to help flush out contaminants, this is based off of not just what I have been reading about the 1.8t, but just pcv systems in general.

What I am thinking of trying is using a dual can like thisStage 1 DC3 Product Page
There will be a T at the lower breather like the stock system, one of the hoses from the T will go to can#1, which will have its output connected to the intake manifold with a checkvalve after the can so it doesn't see boost, this will keep the vacuum on the block like the stock setup, but keep most of the oil from getting into the I.M.
The other hose from the T will go to can#2 which will have a filter on the output, this will allow the block to vent to atmos during boost like most other catch can installs I have seen, another check valve might have to be included there somewhere so when in vacuum unmetered air wont be allowed in.
The other part is where the fresh air comes in, I would change to a valve cover with a breather port, leave the prv on the tip and connect it straight to the valve cover, according to the diagram above the prv will send fresh air to the vc while not in boost. There will be a check valve in that hose so during boost the prv will not be sucking oil from the valve cover.

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I wouldn't say that diagram is WRONG, but I disagree with the part about fresh air coming into the valve cover from the turbo inlet pipe. That just doesn't make any sense. Even when you're not making boost, the turbo is still sucking air through the air filter, into the turbo inlet, through the blades of the compressor wheel and eventually into the motor. Air would never move from an area of low pressure in the turbo inlet to an area of relatively high pressure in the valve cover. Makes no sense and would defy physics.

As for your drawing, I would run one line from your block breather and one from your valve cover. You could either "Y" them together and then run one line to a catch can, or run two lines to a larger catch can. Then you can either vent that to atmosphere (provided you plug the TIP) or plumb the can back into the TIP. The catch can will fill up with condensed oil vapor and the cleaner air will go back into the intake.

I ran a vented catch can for a few months this spring and it was pretty stinky for a daily driver, FYI.

 

I see what your saying and there is quite a bit of debate on this flow of air, I think I will pull the hose off the prv and see if it is blowing or sucking air at idle. Would the valve cover really be at high pressure during vacuum with with vac pulling on the crankcase?

 

Nah the positive pressure created by blow by is very low relative to the vacuum/boost created by the engine/turbo. I can assure you, though, that your engine is not sucking fresh air into the valve cover or crank case out of the turbo inlet.

 

So is there fresh air getting to the stock system at all? Pcv systems normally have a fresh air tube from the airbox to either the block or valve cover, there must be a way to run a catch can and allow the flow of fresh air into the crankcase.

I definately don't care about the smell from a vented catch can, this daily driver is already very loud and bumpy, and has vibrations from the high density trans/motor mounts.

If I do away with the tip to vc connection, will the rest of the set up work with the dual catch can and T on the lower breather and keeping the vac connection from the intake manifold? I have seen this setup on alot of other cars, but not on a 1.8t.

 

you got this backwards. the stock pcv system only allows for crankcase gases to be recirculated from the crankcase to the intake. if you want to run a catch can, youll have to block the hole in the tip and run the crankcase breather hose to the catch can, or an open dump. but yes, if you did it this way you would need to have a filter on the can (unless you dont care about fresh air). ive seen heavily built 1.8t's which actually have multiple crankcase breather ports.