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Old 07-18-2018, 10:38 PM   #1
VinceS2
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Cool Oil Catch Cans - Installation 101 the Easy Neat Cheap Way; What You Need to Know

You may have read the FAQ at https://forums.nasioc.com/forums/sho...d.php?t=754710 and got confused. I know I did, and stuffed it up a beauty. Story here: https://forums.nasioc.com/forums/sho....php?t=2879197. Short story, I gathered all the hoses from crankcase and valve covers and ran them through the catch can and into the PCV at the manifold. Bad move. On boost PCV is shut and only place for the blow-by is up the turbo oil drain hose, and the now-not-draining oil goes into the turbo discharge and .... lotsa smoke! Whoops!

Understanding this error, then understanding the total system properly, I worked out a neat, cheap and effective way to do this mod. Here it is in pictures, for an '04 sTi daily driver, but likely directly applicable to a wide range of Subaru motors, and I will explain exactly what is going on below:






I am a BE Mech engineer, which doesn't make me invulnerable to errors or a knower of all things, but I've previously modified a lot of (non-Subaru) vehicles and it does mean I am not guessing about mechanical stuff I do understand. As you will have read in the FAQ, there are two reasons you want to do a catch can. First is to stop the buildup of oily deposits on the turbo blades and intercooler, as this reduces their efficiency. How much is a worthwhile question, but it is likely to be a small but noticeable percentage. The other reason is to reduce oil mist content going to the cylinders, as that lowers octane ratings, with potential to upset a tune and do damage. But more likely the quantum of the problem in reality is a potential smidge of lost performance, and who wants that!

My assessment, without having quantifiable facts, is that the coating of the intercooler is going to be the biggest issue here so I will fix that, and the octane reduction on-boost, but not be too fussed about the octane reduction issue off-boost since I think that is pointless. But easy to add a second can to also do that bit if you wish.

When you first look at the oil breather system for a Subaru engine, it is quite confusing. So let's separate the bits. The valve covers have two hoses connected to them. The big 3/4" one connects across to the other valve cover via two hoses and a steel pipe section. This looks important, but is irrelevant as it is just dealing with uneven blow-by / piston dynamics for reasons Subaru will have figured out are important. Just don't touch this one, it is good as is. The smaller 1/2" inch hose line is joined to the same 1/2" outlet on the other valve cover via a pipe that has a T connector, and that runs off to the front turbo inlet pipe fitting for the fumes to be sucked into the turbo / IC. An interesting (but irrelevant to catch cans) detail is that the big pipe connects to the top corner of the cover as it appears, but the smaller one has internal porting that makes the pick-up point in the geometrical center of the cover, but with leakage permitted near the top also. Didn't think too hard about why this is so, but a fascinating detail nevertheless.

At the back left corner of the engine, looking from above, is a dual outlet fitting which is the crankcase breather. It has two short hoses. One goes to the PCV on the inlet manifold and the other goes to the rear turbo inlet pipe fitting. The PCV sucks crankcase blow-by fumes into the manifold if off-boost. If on-boost, the PCV shuts and blow-by fumes go into the turbo inlet pipe. Also in the event that more blow-by fumes are generated than the PCV can take, they will go into the turbo inlet pipe anyway.

What I want to do is capture the valve cover fumes and the on-boost crankcase blowby fumes. I don't care about the off-boost crankcase fumes as I don't see why that would affect performance or any other thing I care about, let those suckers burn! What is depicted in the pics does, in effect, is to T the valve cover and blow-by fumes together and run them via the catch can into the turbo inlet pipe, hopefully just as clean air but certainly as much cleaner air, IF YOU GOT THE CATCH CAN BIT RIGHT. More on that later.

There is a SMALL compromise with this set-up. Remember we originally had a turbo inlet connection point for the crankcase fumes and another for the valve covers, separated in a way that meant one couldn't influence the other. That was no accident by Mrs Subaru, she knows her plumbing! Now what will happen is, in on-boost, there is a reduction in cross-sectional area of the connector so we will not be able to flow quite as much blow-by and thus crankcase pressures will be slightly higher, like a poofteenth or two. Also, in off boost, some of the valve cover fumes will be sucked directly into the PCV directly and not go via the oil catch can. When you consider how stuffed a motor can be before this breather system starts to become part of the problem, I don't think either of these 'concerns' are of any significance at all as far as any relevant outcome seen at the engine. But if it bothers you, do go add that second can in the PCV line!

Re the oil can, you only need a regular one inlet, one outlet plus a drain outlet and sight gauge type. My one is shown has two inlets and no sight gauge (because of how I originally, incorrectly, used it to gather all feeds), so I turned the second inlet into a sight gauge (again using the heat gun to stop the sight hose kinking when bent). All very important, but even more so is the damn thing needs to actually catch the entrained oil in the fumes! No cheapie will do that without modifications. Unfortunately, I forgot to take an internal pic of the kind of mods needed, so I'll try to describe them.

In mine, there is a little deflector plate on the inlet and there is a perforated baffle plate about 1cm below that. Seriously dunno how that was ever supposed to DO anything, but it looks cool and sells more tanks I guess. So I bent the deflector plate out of the way and opened up the ~11mm inlet hole with a 1/2" drill which was a tight fit for 1/2" copper pipe. I cut the end of about 6cm of copper pipe at about 45 deg, and used red loctite then pressed (tapped) it into the inlet hole so it was about 5mm clear of the bottom. Used the tin-snips on the baffle plate to clear the new pipe and refitted it, mostly to help stop the stainless steel wool pad from packing down over time. These s/s scourers, which one fitted nicely in mine, distributed both sides of the baffle plate, are the ducks' guts for this job. Not too course, not too fine, with decent exposed surface area and likely to pull mist out just fine.

That being said, and you do need to have something internally to actually extract the air, mine is just a toy and a much taller one would be better. I am concerned there won't be much oil in there before the extended inlet pipe 'goes under' and will be blubbing away in there. Don't know if this will matter and certainly should only make it more efficient collector, to a point anyway. Plus I can see something like this getting totally 'gunked out' as all the lovely carbons and carcinogens play out their merry dance in the royal rexie ballet room. So, just buy a bigger one, and extend the pipe 2/3 the way down to give some capacity without lost efficiency, something I will undoubtedly be doing in the future!

If there are any errors / omissions in this post, please kindly point them out and I will modify accordingly. I hope people can find this info and it is useful... enjoy.
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Old 01-07-2019, 12:50 PM   #2
blackerst
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Nice addition to the original post and quality pictures! I'd been pondering the plumbing of the breather hoses myself as a thought experiment but have yet to get in the guts of my engine bay and verify the hose routing. From the original post combined with your input the assumption I made was the following:

1) Crank case breather is not plumbed into the valve cover breather lines and is completely separate, hence the two turbo inlets.
1.1) On boost both crank case and valve covers have approximately equal vacuum from turbo inlet source (I understand vacuum is split between valve covers so overall vacuum is greater at crank case).

1.2) Off boost crank case receives greater vacuum from intake manifold while valve covers receive minor vacuum from turbo inlet due to throttle being nearly/completely closed. Also turbo inlet for crank case breather may be providing some air directly to intake manifold bypassing the throttle body, but idle air valve may offset this extra input or the air bypassing throttle body is negligible?

Proposed Solution: Similar to yours with a slight modification which covers both on boost/off boost oil catching:

1) T the crank case and combined valve cover plumbing together and run it to the catch can. This ensures all breathers run through the catch can first before entering the intake plumbing. Also yes, the crank case does not T immediatly with one output running to the PCV and the other outlet to the catch can, the crank case runs directly to the catch can first.

2) The catch can outlet is T'd with one outlet running to the turbo inlet and the other to the PCV.


Potential issues:
1) Reduced crank case ventilation during off boost as it doesn't run directly to the PCV and is combined with the valve cover plumbing reducing the overall cross sectional volume of the hoses running from the breather T to the catch can and beyond.

2) On boost vacuum overall is reduced due to combining all three breathers together.

Potential solution to issues: Increase catch can outlet diameter to PCV/turbo inlet T and match T size to new catch can outlet. Reduce hose size from catch can outlet T to PCV but maintain hose size from catch can outlet T to turbo inlet. The hose running from the catch can outlet T should then be T'd before reaching the turbo inlet, connecting both ends of the T to the two turbo inlet nipples that used to connect the valve cover breathers and crank case breather respectively.

This would increase the vacuum force (amount is negligible as any increase is beneficial at this point) during on boost and off boost scenarios.

Diagram for reference of final solution: https://photos.app.goo.gl/LZ1RT2APQ19yPNtc9

Thoughts? Suggestions?
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Old 01-07-2019, 06:01 PM   #3
VinceS2
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Wink Cute, but...

This would totally trash the scavenging capacity to the crankcase as the PCV would just suck air out of the inlet tube, being path of least resistance.

Also, being pedantic, the concept of more / less vacuum is an illusion; doesn't happen. The vacuum is the same. The actual vacuum at the end (crankcase / valve covers) could only POSSIBLY be different if there was a substantially different flow rate; ie dynamic effects. Which could only happen if there was substantially greater source of air into one area or the other and flow down oil drain paths didn't equalise it, with those tiddly 1/2" hoses being the only way out. Pedantic physics, but largely irrelevant!

I have personally made such leap-of-faith logic errors before and I so know how you got there; bin der dun dat ... Take the opening para for example!

Edit: Just to add, your solution is basically the exact situation I created in my opening para except you are adding a T in the line to the PCV to go to the turbo inlet, nominally to avoid the positive pressure situation. For me, this would have been the super-easy solution but I ruled it out as I wanted system integrity to be maintained. As well as deleting scavenging, this could also expose the turbo oil return to some increased pressure, but not enough to create the smoke curtain I managed - that needed the crankcase to see boost pressure through my own logic fail.

Last edited by VinceS2; 01-07-2019 at 06:15 PM.
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Old 01-07-2019, 09:55 PM   #4
blackerst
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Talking Thanks, I really like my design!

Quote:
This would totally trash the scavenging capacity to the crankcase as the PCV would just suck air out of the inlet tube, being path of least resistance.
I agree it allows air to run from the turbo inlet to the manifold off boost due to path of least resistance. I disagree that crankcase scavenging is worsened given the stock design crankcase breather is T'd with one end going to the turbo inlet and the other to the PCV already, so scavenging off boost is already an issue with stock if what you say is true.

A potential solution I'm also considering to alleviate the inlet to manifold flow during off boost is to put a PCV valve between the turbo inlet and the T to insure gases only flow to manifold PCV when off boost. Though given crankcase is already piped to the PCV and inlet I reasoned there wouldn't be a negative impact to retain the stock path except add in the valve cover breathers.
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Old 01-07-2019, 11:36 PM   #5
VinceS2
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Have a Nice Day? Aah well, once you're wedded to it, testosterone wins out every time...

Quote:
Originally Posted by blackerst View Post
I disagree that crankcase scavenging is worsened given the stock design crankcase breather is T'd with one end going to the turbo inlet and the other to the PCV already, so scavenging off boost is already an issue with stock if what you say is true.
I would say here is "check your base assumptions". You are talking about a designed in fault on a pretty basic system. I would be backing Subaru to have got it right, and the rest of us to not know exactly what the core logic here is. I have gone for a basic solution of just putting a can in the existing vent lines to solve the problem. Disrupting the system further to, at best, get a smidge of off-boost octane back, whilst adding obstacles to flow as originally designed, seems a pretty pointless pursuit to me. But that's me.

Whilst I would say I have laid out my logic fairly completely and am probably at the limit of adding value to debate around other prospective 'solutions', I would suggest a careful look at the crankcase breather fitting first before jumping to conclusions. There is a chance it is not the same size fitting from the larger crank inlet to the two hose outlets.

Regardless, the problem Subaru is trying to solve is blow-by, not suction. So maybe T-ing the PCV across is an answer as blowby will get through the can and into a place where it doesn't really matter which way it goes. Do let us know what you find out .
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Old 01-08-2019, 12:45 AM   #6
blackerst
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Thank you for the input and again thank you for your initial post detailing your solution! I'll definitely be taking a good long study of the breather pipping once I start pulling parts out of the engine bay.

Hope you have a great day!
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Old 02-26-2019, 09:53 PM   #7
Mozawrx
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Ty for the good read
Learned a few things from all of this
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