VIP GT-BBB

[Yowie]

But I simply couldn’t find any reference anywhere at all that the fresh air breather was anything but a fresh air supply to the crankcase, and that blow-by exiting this port into the inlet is a fault or failsafe mode, according to the design intent of stock pcv systems.

What have you found? Just a description as a "breather" and a non-exhaustive description that air enters through there?

The valve cover "breathers" probably draw in air under vacuum conditions (because the greater manifold vacuum is drawing crank gas through the PCV) but surely under boost conditions both the crank breather and valve cover breathers are expelling gas due to the ported vacuum from the pre-turbo pipe. EDIT - the simple diagram you've pasted above seems to confirm the "direction of gas depends" matter.

If my brand-new engine* collects fluid in the valve cover "breathers" catch can AND having a catch can in that loop is a common Subaru aftermarket thing, surely this brings an end to the hypothesis that gas only ever flows in one direction through the valve cover "breathers".

[* albeit with exaggerated E85 crank water vapour, some enlarged breather passages and two catch cans]


Agreed it would be interesting to get more of a handle on different routing, restrictions, extreme crank vacuum methods etc. For my car I'm happy with the factory loops with catch cans added (and some flow enlargement). My further experimentation will probably be limited to "better gas scrubbing" within the same basic architecture for an even cleaner intercooler.

[bigBADbenny]

If you can make a setup run, i am interested in looking at it as well. ATM i've got the 2 catch can setup and gives so many extra lines.

It should work as intended, especially if you keep the fresh air breather closer to the air filter and keep the smaller diameter sump return via the pcv three way close to the compressor, as Subaru intended, as per the appropriate Perrin or PSR V2 silicone inlet pipe.

You can do it the AVO etc way with the two returns closer to the air filter, but expect less vacuum on the sump breather.

[bigBADbenny]

Regarding Yowies comments:

A pcv system became a smog law and the specific arrangement was baked into the laws, for manufacturers to adopt.

Prior to that, draft tubes were used unless a vehicle was expected to wade eg military vehicles, thus the very first pcv systems were developed for wading..

So the fresh air heads breather and the flow of that breather from after the air filter to a port in the head or the valve cover, returning to the intake manifold via the crankcase/sump, is the actual intent of a pcv system.

As such you will not find any alternative arrangement for a four stroke petrol motor with a wet sump.
I did search for alternatives…


There’s a somewhat limited amount of online resources as many simply quote the basic Wikipedia article, and so far I haven’t even found much detail on n/a vs turbo setups.

I’m still looking, indeed!

I’ll dig up the diagram from the back pages of the my08 FSM which shows the breather as an inlet only.

Iirc the weak point of that setup is high G corners (mainly left handers?), where the heads might fill with oil, potentially flooding the inlet, regardless of blowby gas flow due to gravity.

Part of Subarus overly complex solution was surely the heads balance hoses which should have helped oil surge but didn’t completely.

As for using the same setup on your newly built engine…
Any meaningful residue in the breather can will be proof that the arrangement pulls less vacuum from the sump under boost than the stock arrangement, and especially if any oil surge in high G corners is experienced.

Again, the intent of the stock arrangement is to help oil return to the sump using both intake and inlet vacuum.

My interest is academic and as I mentioned, ultimately to inform a potential project that simply builds on the discovery stage of the theory.

I haven’t really found out much new info, the recent wave of bedtime reading just reinforced what I already suspected.

The key items being the pcv valve being considered a vacuum regulator with a choked flow orifice and a spring loaded one way valve (FI cars), other chokes in the system (similar sized but potentially staged) and a very basic Bernoulli tube/venturi arrangement to to keep the blowby flow going from the breather to the sump to the pcv inlet return under boost conditions...

Its just about job done other than the exhaustive enquiry into both other makes and models plus an analysis of the evolutionary aspect of pcv systems.

Additionally we can look at simplifying the system or potentially marking it more complex and even more redundant.

[bigBADbenny]

Ok there are some exceptions to all petrol motors have a pcv valve… that’s some antique engines and maybe bmw valvetronic, the latter doesn’t use a throttle plate.

I’m going to have to post links with various pcv system explainers and my comments regarding.

Especially as I’m struggling to solve the mystery of the n/a partially closing pcv valves as seen in H6 and other models.

[bigBADbenny]

https://www.underhoodservice.com/turboc ... cv-system/

Discusses turbo vs n/a pcv system differences, including the importance of the sump breather return to the inlet *in front* of the turbo compressor inducer.
The site doesn’t quote references but the concept is logically rooted in basic fluid dynamics.

[Yowie]

https://www.underhoodservice.com/turboc ... cv-system/

Discusses turbo vs n/a pcv system differences, including the importance of the sump breather return to the inlet *in front* of the turbo compressor inducer.
The site doesn’t quote references but the concept is logically rooted in basic fluid dynamics.

(emphasis added)

Do you mean "somewhere before the turbo" (which is all the article says*) or "as close to the inducer blades as possible" (which the article doesn't say)?

More generally, the article just seems to describe in very general terms the fact that turbocharged engines need the secondary method to scavenge crank gas. I don't think we can read any more specifics into it.

*"The area before the turbocharger is typically where the vapors from the crankcase are fed into the engine."

[bigBADbenny]

One might refer to the FSM workshop manual, which is explicit about the flow path. From page 5300 of the 2008 FSM:

EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES)
CRANKCASE EMISSION CONTROL SYSTEM
EC(H4DOTC)-6
3. Crankcase Emission Control System

: The positive crankcase ventilation (PCV) system prevents air pollution that will be caused by
blow-by gas being emitted from the crankcase.
The system consists of a sealed oil filler cap, rocker covers with fresh air inlet, connecting hoses, a
PCV valve and an air intake duct.

: In a part-throttle condition, the blow-by gas in the crankcase flows into the intake manifold through
the connecting hose of crankcase and PCV valve by the strong vacuum created in the intake manifold.
Under this condition, fresh air is introduced into the crankcase through the connecting hose of
the rocker cover.

: In a wide-open-throttle condition, a part of blow-by gas flows into the air intake duct through the
connecting hose and is drawn into the throttle chamber, because under this is condition, the intake
manifold vacuum is not strong enough to introduce through the PCV valve all blow-by gases that
increase in the amount with engine speed.

: The PCV hose is provided with a leak detection function.

Screen Shot 2022-11-21 at 13.10.28.jpg (79.14 KiB) Viewed 942 times

[Yowie]

What is/are the specific point/s you're seeking to make with the workshop manual extract?

[bigBADbenny]

Well this part is quite clear:
fresh air is introduced into the crankcase through the connecting hose of
the rocker cover.

: In a wide-open-throttle condition, a part of blow-by gas flows into the air intake duct through the
connecting hose and is drawn into the throttle chamber, because under this is condition,

So the FSM states how it should work, as opposed to how it shouldn’t work.

An aos/cc on the fresh air breather exists as a failsafe for oil surge or a blocked pcv valve.

If oil or sludge is in the aos/cc on the fresh air breather it’s very likely that that aos/cc system is compromised by way of setup or a blocked pcv and its operation should be verified by logging crankcase pressure under normal operating conditions: *edit* unless the car has experienced oil surge during high g cornering.


But as regards my line of enquiry:

The scope is broad, but I want to understand:

1: Stock pcv system operation.

2: The difference between n/a and turbo pcv systems.

3: Why Subaru added so much extra complexity over several generations to the stock turbo pcv system.

4: In what ways this system isn’t foolproof due to maintenance issues, fuel type and driving in extremis.

5: What are the common aftermarket solutions to these shortcomings and what are the ideal usage scenarios/applications.

6: How aftermarket solutions might be properly and improperly implemented.

7: Testing and verification of pcv systems.

8: How to add extra redundancy and/or performance in an upgraded pcv system.

[Yowie]

Well this part is quite clear:
fresh air is introduced into the crankcase through the connecting hose of
the rocker cover.

: In a wide-open-throttle condition, a part of blow-by gas flows into the air intake duct through the
connecting hose and is drawn into the throttle chamber, because under this is condition,

So the FSM states how it should work, as opposed to how it shouldn’t work.

An aos/cc on the fresh air breather exists as a failsafe for oil surge or a blocked pcv valve.

If oil or sludge is in the aos/cc on the fresh air breather it’s very likely that that aos/cc system is compromised by way of setup or a blocked pcv and its operation should be verified by logging crankcase pressure under normal operating conditions.

Fuller FSM extract pasted below for reference (footnotes & emphasis added):

": In a part-throttle condition, the blow-by gas in the crankcase flows into the intake manifold through
the connecting hose of crankcase and PCV valve by the strong vacuum created in the intake manifold.
Under this condition (1), fresh air is introduced into the crankcase through the connecting hose of
the rocker cover.

: In a wide-open-throttle condition, a part of (2) blow-by gas flows into the air intake duct through the
connecting hose and is drawn into the throttle chamber, because under this is condition, the intake
manifold vacuum is not strong enough to introduce through the PCV valve all blow-by gases that
increase in the amount with engine speed."


My footnote comments as follows:

(1) the FSM comment about fresh air entering the rocker covers is expressly limited to manifold vacuum conditions;

(2) under manifold boost conditions, if only "part of" the blow-by gas enters the pre-turbo pipe through the rear loop, what happens to the rest of the blow-by gas? The options seem to be:

(a) it exits the crank case via the rocker covers then also enters the pre-turbo pipe through the separate hose (as evidence by collected catch can contents on that loop);

(b) ???????


A fan of physics would surely recognise that, if there are two unblocked holes in a pressure vessel, pressure will exit out of both holes?

Common-sense, all cited documents and the lived experience (front loop catch cans collect fluid) do not support the alternative hypothesis that, under manifold boost & higher blow-by conditions, "pressurised oily gas in the crank case will only exit one of the two available holes unless something has gone drastically wrong".


Finally, if this alternative hypothesis thing is a trolling exercise to wind people up on the internet - well done, you got me hook line and sinker.

[bigBADbenny]

It ultimately does not matter to me who or what theory is correct, since any effects can be verified by monitoring or logging crankcase pressure, and by keeping track of the contents of each can.

What really matters to me is understanding the fundamentals so I can implement an upgrade on my car or advise others, with reasonable explanation of the considerations.

Its great having a differing point of view to mine, as the more I look into it, the clearer my position becomes.

If anyone can prove otherwise, that’s great, it’s only going to further my understanding, so thank you, again.

Its not a hill to die on, but a solution to what is a massive grey area in Subaru knowledge.

The question (pcv system unreliability) is even the subject of a class action stateside.

[bigBADbenny]

A fan of physics would surely recognise that, if there are two unblocked holes in a pressure vessel, pressure will exit out of both holes?

Could you please familiarise yourself with the basics of fluid dynamics, pressure versus flow, Venturis, Bernoulli’s principle etc?

Just scan the articles no need to dig that deep.

Eg: https://en.wikipedia.org/wiki/Venturi_effect

[Yowie]

Could you please familiarise yourself with the basics of fluid dynamics, pressure versus flow, Venturis, Bernoulli’s principle etc?

Just scan the articles no need to dig that deep.

Eg: https://en.wikipedia.org/wiki/Venturi_effect

It's not up to me to try to draw [specifics / "what Ben is trying to say"] out of incredibly general resources you might happen to link.

I've stated the case (with specific reasoning, specific physical evidence and your own written evidence from the Field Service Manual) why it's two-way traffic through the rocker cover breathers.

If you still think that's not how it works AND you want others to be persuaded to your view, the onus is on you to disprove it with specific reasoning and evidence.

"It's fluid dynamics, it's Bernoulli, it's Mabo, it's the vibe, do your own research" will not cut it.

I have done my best to assist with your admirable public quest for knowledge about Subaru PCV systems. I wish you the best for the rest of your learning journey.

[bigBADbenny]

But it’s your own statements that have absolutely no grounding in reality.

Here’s how the venturi effect works to move fresh air into the crankcase to the sump to return to the inlet in front of the turbo compressor, when the pcv valve is closed when the turbo makes boost.


To the left is the turbo compressor creating a vacuum, drawing against the air filter to the right.
The flow is right to left, instead of positive pressure, this system is always in slight vacuum, drawn either at the pcv valve with the throttle partially closed, intake manifold under vacuum or during boost with the vacuum generated in front of the compressor inducer.

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If both the pcv system to inlet returns are further away from the compressor impeller, and potentially both in the fat section of the inlet pipe, the differential pressure effect might be negated, probably under boost. One might establish the minimum can emptying interval, drive the vehicle off boost for the duration, inspect the can, then drive a shorter duration with lots of boost and reinspect for evidence in the can.

Anyway thanks for providing the initial feedback that lead me to ask: why is that particular aos/cc system not quite working as the owner expected?

I’ll attempt to install a functional dual can setup on my car to see if it helps the severe oil burning.
A functional aos/cc system can be a magic bullet for some afflicted engines, probably when the fault is the rings, as opposed to a valve stem seal.
I’ll see when I implement the mod.

[bigBADbenny]

As for real world proof, that’s going to require breaking out mechanical manometers (I have a set donated by HardwareBob), or buying an electronic manometer, buy some clear hose and fittings, a sealed bucket with my smoke generator in it, and attach a spare stock inlet to the inducer of a leaf blower.

If everything works as expected, any smoke n the sealed bucket would be drawn out by the return closest to the leaf blower inlet.

The test might even work with the blower fired into the inlet pipe and with the clear hoses dipped into water.

But why not go the whole hog, use an airbox, filter, sfb, inlet pipe all hooked up to a leaf blower inlet.