AOS/CC discussion about air oil separators and catch cans...

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Re: AOS/CC discussion about air oil separators and catch can

Postby bigBADbenny » Tue Aug 24, 2021 8:28 pm

So the first step is with a stock/ish car, use inlet pressure testing to verify there’s no leaks in the pcv system.
At some point you may need soap or bubbles, brake cleaner or unlit propane: see the inlet pressure testing diy post.

You’ll also want to replace, fix or more likely upgrade the following parts which are known failure points that will ultimately affect your fuel trims with negative results.
Stock tmic: upgrade or recrimp
Inlet pipe: silicone, eg psr v2, Perrin.
Throttlebody hose: essential, Kobe or AVO etc.
Pcv valve: check clean or replace.
Pcv hoses: oem is good for packaging, not forever durable… I reinforce the ends with glue heat shrink to protect the ends.
Upper and lower inlet manifold gaskets & orings.
Injector seat seals.

The stock system is ok for stockish boost, not because it holds any pressure, but because of stock blow by with a healthy engine.
As such you want check and especially clean out however much oil is present in your inlet tract over how many km.

You then repeat the test at a later date to see if it’s minimal, still a few drops of oil, or if you have an issue requiring mitigation.
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Re: AOS/CC discussion about air oil separators and catch can

Postby bigBADbenny » Tue Aug 24, 2021 9:29 pm

Even the new TPP inlets do eeeet!!!
I don’t even need to do the science…
viewtopic.php?f=10&t=36905&p=451725#p451725

Ok the apotheosis spoiler: 42 minutes in.
Discussing pcv inundation…
https://www.youtube.com/watch?v=HAvxjSdQsXI
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Re: AOS/CC discussion about air oil separators and catch can

Postby Yowie » Tue Aug 24, 2021 10:16 pm

Ok the apotheosis spoiler: 42 minutes in.
Discussing pcv inundation…
https://www.youtube.com/watch?v=HAvxjSdQsXI


My summary of the Flatirons video points from the 42 minute mark is as follows:
- Jon has a racecar with a pressure gauge connected to the crank;
- Jon's racecar has the PCV blocked-off, relying on ported vacuum only to evacuate crank & valve cover pressure;
- Jon incorrectly connected the clean hose of his catch can to the "near the bend" spigot rather than the "closer to compressor" spigot in the pre-turbo pipe;
- no issues on the street & shorter throttle applications
- ineffective to reduce crank case pressure at 5+ second wide-open-throttle applications on track.
- fixed when clean hose moved to the "closer to compressor" spigot in the pre-turbo pipe.

It's interesting, but it doesn't support an assertion that:

Using a Perrin or psr v2 silicone inlet (more affordable) should maintain the correct pressure differential in the inlet for the pcv returns, meaning less reliance on an aos/cc setup for a street driven car not running e85.


(adopting the plain meaning "in comparison to another aftermarket pre-turbo pipe" rather than "in comparison to some daft way of connecting or failing to connect breather hoses")

I will now stop hounding you for evidence and take it that you have nothing further to add and that the assertion will remain unsupported by evidence.
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Re: AOS/CC discussion about air oil separators and catch can

Postby bigBADbenny » Wed Aug 25, 2021 9:21 am

https://www.youtube.com/watch?v=HAvxjSdQsXI

At 3:40 minutes…

No the hounding is expected and welcome.
Let’s keep it out of unrelated posts tho. The fervour might scare innocent posters off!
You’re absolutely doing me a favour, in terms of solidifying the theory.

Even Flatirons haven’t gotten down to doing a manometer test, probably because they can verify various setups on track, with the help of major partners.

That and the penny dropped regarding the intent of the factory setup.

We need to fully understand what is a complex system in order to move forward with mods and upgrades.
Grandfathered mods get in the way of this.

Only a few more hours of study to go, I’ll get to all of it soon enough.
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Re: AOS/CC discussion about air oil separators and catch can

Postby bigBADbenny » Fri Aug 27, 2021 6:46 am

Here’s the crux of the matter:
Yowie wrote:
I also only have a single spot on the inlet to run a breather hose to, so they would need to be combined at some point before this anyway.


That makes a lot of sense - necessity through lack of pre-turbo pipe spigots overruling everything else.


I have also wondered whether the venturi effect would be greater if the inlet port was closer to the turbo.


I assume you are referencing that comment in the Flatirons video? A mate's project involved such considerations. Each of us separately spoke to different engineers who both said that the amount of vacuum in the pre-turbo pipe should be the same over its length (subject to disturbance caused by pipe irregularities).

As such, Flatirons' conclusion that there is more vacuum close to the compressor wheel and less vacuum further away doesn't seem to be supported by engineering theory. If Flatirons achieved better results moving the point-of-join closer, there may be other reasons for that.

If you only have the one pre-turbo pipe spigot it's all theoretical anyway.

Likewise, if both cans are draining fluid and your intact tract is cleaner than "without catch cans" then it must be working well enough for your application.



Did you speak to a structural engineer?

Whoever’s advice you’re taking, they have no understanding of basic fluid dynamics.

Take a look at the Wikipedia articles on the Bernoulli principle and the Venturi effect.

Lookup Delta P in scuba diving, see why divers place cages over low pressure orifices for safety.

Check out the difference between pressure gradient and pressure differential.

See the similarities between the subaru inlet tract and an actual venturi meter.

D61DC0B3-D8A6-48CD-AAAA-667B8D304790.png
D61DC0B3-D8A6-48CD-AAAA-667B8D304790.png (32.06 KiB) Viewed 299 times


Does this look familiar?

Testing? Absolutely, I’ve compared my crankcase pressure data to Subarus with three into one return aos cc.

Mine hovers around zero psi (corrected boost gauge) and trends slightly downwards by 0.3psi on boost. This is expected as its a working stock system.

A properly setup race 3 in one out hits around -1.5 psi on boost, again as expected, since there’s no fresh air port from the inlet to balance flow.
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Re: AOS/CC discussion about air oil separators and catch can

Postby bigBADbenny » Fri Aug 27, 2021 7:14 am

So with Bernoulli principal, the air accelerates where the pipe narrows.

Just like in the inlet pipe pre turbo.

So the avo style pipe is huge at the pcv return port: low pressure acting on that port.

Delta P, the pressure gradient in the inlet pipe:

Think of a down feather blowing around in a room with a single open window and a strong vaccum outside (created by wind or a storm etc).

The feather swirls around the room lazily till it gets in range of the window, then its suddenly caught by the vaccum, accelerates and is sucked out.

This is the same effect as you approach the turbo inducer in the inlet pipe.

Under boost, vacuum is greater at the turbo end.

The pcv return at the other end of the inlet pipe receives less vacuum by being further from the impeller, an by being in a larger section of the pipe.



Now I feel bad that this info might kill some business for companies that don’t offer the option of having the stock pcv return in the stock inlet position, however I want to clarify that for a race style 3in one out arrangement aos/cc, its fine, because there will be enough vacuum in the inlet for crankcase scavenging, regardless.

This setup is probably the arrangement on turbo subie engines that didn’t have the heads/crankcase balance pipes, to limit excessive crankcase vacuum.

We’ll get to the specifics by breaking down the evolution of the subaru pcv system over various generations.
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Re: AOS/CC discussion about air oil separators and catch can

Postby Yowie » Sat Aug 28, 2021 8:59 pm

https://www.youtube.com/watch?v=HAvxjSdQsXI

At 3:40 minutes…


3:40 until when? Somewhere between 3:40 and 42 minutes where you invited me to drop in last time? I've seen this video before. Who do you think "Yowie 08" is with the youtube comment 12 months ago?

I've pulled the pin at the 10 minute mark. I'm not going to re-watch a 56 minute video I've seen before on a vague tipoff that there might be evidence for your statement:

Using a Perrin or psr v2 silicone inlet (more affordable) should maintain the correct pressure differential in the inlet for the pcv returns, meaning less reliance on an aos/cc setup for a street driven car not running e85.



Turning now to the following statement:

So the avo style pipe is huge at the pcv return port: low pressure acting on that port.


Great. Please produce the evidence (anecdotal, testing, whatever) that engines with AVO pre-turbo pipes have high pressure sumps compared to engines with different pre-turbo pipes. Alternatively, admit that it is speculation. Speculation based on some science sure, but speculation.


Now I feel bad that this info might kill some business for companies that don’t offer the option of having the stock pcv return in the stock inlet position,


I'm sure they will be fine until some evidence comes to light.


...however I want to clarify that for a race style 3in one out arrangement aos/cc, its fine, because there will be enough vacuum in the inlet for crankcase scavenging, regardless.


I think you and I are starting to agree. I also think it doesn't matter exactly where "in the general neighborhood of stock position" the ported vacuum draws on the engine blow-by pressure - if that is more-or-less what you're saying.

Interesting stuff Re "straight-pipe = equal pressure" engineering fundamentals versus the convoluted path (elbows, narrow bits etc.) of a real pre-turbo pipe. There may very well be differences at different points on one pre-turbo pipe, or the spigot points on different pre-turbo pipes. It's all speculation until evidence comes to light that brand X pre-turbo pipes have high pressure sumps compared to engines with Brand Y pre-turbo pipes.


I genuinely enjoy a good speculation/debate about which design for an engine thing is materially better. The trouble is, we live in a world where testing is expensive and marketing is rampant. As a result car modification parts seem to fall into three categories:

(a) stuff that is proven to be beneficial that everyone just does (bigger dump pipes, bigger turbos etc.);

(b) stuff that is probably better by small percentages, popular with racing teams chasing the "one-percenters" but not good value for most road-car and track day enthusiasts; and

(c) stuff that is endlessly debated (divorced dump pipes vs bell-mouth dump pipes vs dump pipes with little MRT splitters welded into them) with no definitive evidence to hand to settle the matter.

[Years ago I saw some great commentary on golf equipment developments and marketing. There are "remarkable" changes (large titanium driver heads, carbon fibre shafts etc) and there are "unremarkable" changes ("this year's clubs hit 1.78321 metres further due to Racing Stripes (TM) Techology!").]

As consumers and indeed opinion-leaders on a car modification forum (particularly where new members seeking advice are concerned) I think we need to be clear in our own minds and in our communication:

(a) which mods are objectively, provably better;

(b) which are "better but probably not materially/measurably"; and

(c) which are "probably good but opinion-based and unproven"

I freely admit that a lot of tinkering I do with my car is in category (b) and/or (c). Especially if it is cheap to do and personally interesting. See the throttle body porting and airbox ribs removal. If you can find a post where I tell someone else those changes are essential I'll post you a packet of biscuits.



see why divers place cages over low pressure orifices for safety.


Good advice to cover one's orifice.
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Re: AOS/CC discussion about air oil separators and catch can

Postby bigBADbenny » Sun Aug 29, 2021 10:02 am

The point of this thread is to eventually get down to a clear set of recommendations for aos/cc setups for a variety of applications.

To get to this, I’m looking at the intent behind, and the evolution of, the stock subaru pcv system.

At the other end of the scale are race style 3 into 1 systems, and ultimately dry sump systems.

The middle ground is the most interesting grey area, where the aim is to improve the performance of the stock system as regards pcv oil control.


On the subject of testing the stock setup, I can do a temporary back to back test (eventually) of crankcase pressure with the fresh air heads ports open and closed.

I’d expect to see crankcase pressure decrease below stock , on boost with only the pcv return active.



I understand it’s frustrating and hypocritical, to offer advice in other peoples mod posts about this subject, and then refuse to debate the point there. From my point of view the subject is way to complex to discuss elsewhere, especially when we don’t even know the scope of said projects.


So your engineers comment refers to static pressure in the inlet, engine not running.
That’s absolutely correct. The moment you have airflow in the inlet, filter at one end and a vacuum inducer at the other, the tapering of the inlet and the location & diameter of the pcv returns come into play, with the fundamentals of fluid dynamics affecting the flow path, pressure gradient and pressure differential.




An avo style inlet with the single forward pcv system return port is going to work just fine, if the crankcase vent is still the primary link to inlet vacuum, presumably this would mean blocking off the heads breather return to inlet and keeping the heads/ccv balance pipes.

I need to go over the AVO LGT install PDF to refresh myself on the recommended hose routing, but as described above, this would help the stock system cope with added blow-by from a moderate increase in boost.

And this type of setup was probably the default prior to Subaru moving over to the gen4 style dual pcv return to inlet solution.

Yes I’ve seen many aos/cc setups over the years that were incorrectly routed.
They were either completely ineffective or made pcv & turbo oil control worse.
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Re: AOS/CC discussion about air oil separators and catch can

Postby bigBADbenny » Sun Sep 12, 2021 11:05 am

Well obviously there's more work to do aggregating all the recommendations, but here's some positive feedback from an enthusiast that brought his highly tuned Lib to one of the health check days, last year (2020?) in Collingwood.

I forget the exact details of his aos/cc setup, but I thought the schema was a bit "off" based on my emerging understanding of the general intent.
As usual I don't disparage the setup, just raise concerns, and offer potentially more efficient solutions...

aos cc Screen Shot 2021-09-12 at 08.50.09.jpg
aos cc Screen Shot 2021-09-12 at 08.50.09.jpg (20.66 KiB) Viewed 116 times


aos cc Screen Shot 2021-09-12 at 08.52.59.jpg
aos cc Screen Shot 2021-09-12 at 08.52.59.jpg (25.06 KiB) Viewed 116 times


I wanna say maybe the head balance pipes that tee to the crankcase vent were routed past a catch can, which wont do a thing for oil control.
Hopefully the owner will get back with some photos outlining the setup.
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Re: AOS/CC discussion about air oil separators and catch can

Postby bigBADbenny » Tue Sep 14, 2021 5:54 pm

So I’m thinking to test the stock my07 pcv system…

My car can return a clean LV as currently running, tuned for stock boost on a vf46, 3port & tbe.
It does burn oil, mostly on boost, blue smoke.

It has stock levels of blow by oil in the inlet, eg wipe out a smear every year or so.

The crankcase pressure test revealed a range of around +/- 0.25psi, and I gave the car at least medium beans during test, mostly 0-60kph.

Based on findings on goefit (guild of efi tuners fb), simply blocking the head breathers will convert the pcv system to a 3 into 2 return system, with vacuum being drawn on the sump breather by the intake via the pcv, and the inlet on boost.

This should result in more crankcase vacuum over the stock system, potentially less smoke from the exhaust.

The zero sum vacuum stock pcv system was clearly designed for a number of reasons, one of which I’d conject to be preventing crankcase vacuum from wiping the bores in conjunction with oem economy type oil.

My car is way beyond that, currently using less penrite 20w60, and clearly there’s room for improvement.

So to do the test, I’ll use adjustable clamps to gradually choke the hoses closed, and log some driving.

I need to get the Aem 30-4900 hooked up to rpm, hopefully a 5 minut job, currently it logs afr and boost only.

I’m really interested to see the result, hopefully not oil surge from the sump inundating the inlet and intake :P

That’s obviously where a catch can would be go, but I can’t see any harm starting gently gradually increasing the choke on the hoses and analysing the logs. Meaning I’m still at the data gathering stage.

Alternatively I can choke the flow partially or completely at the head breather return.
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