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Old 07-18-2018, 10:38 PM   #1
Scooby Newbie
Member#: 353338
Join Date: Apr 2013
Chapter/Region: International
Location: Newcastle Australia
2005 EVO 8 WRX + STi

Cool Oil Catch Cans - Installation 101 the Easy Neat Cheap Way; What You Need to Know

You may have read the FAQ at and got confused. I know I did, and stuffed it up a beauty. Story here: 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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