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Old 04-18-2005, 08:14 PM   #1
The Deliverator
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Default Catch Can FAQ

Catch Can FAQ
(with apologies to The Unabomber for 'borrowing' his format)

The point of a catch can is to minimize the amount of oil vapor that's fed into an engine's air intake tract.

If you've taken the intercooler off a car with even a few thousand miles on it, you'll likely have found the inside of it to be coated with a fine layer of oil. This oil coating is also typically found in the 'Y" pipe, the throttlebody, the cold side of the turbo, and the intercooler to throttlebody hose. This oil coating is very common and isn't usually indicative of a major issue.


Why would I want a catch can?


Catch cans are useful on many, many different kinds of cars- NA and Turbo Subaru's included.

There are two, maybe three specific reasons you might want a catch can:

1) You don't want to effectively lower the octane of the gas you're running- the oil vapor that doesn't condense on the walls of your intake tract ends up in the cylinder, to be burned away for emissions purposes. The effective octane of oil is significantly lower than that of gasoline; having it in the mix will effectively lower the octane of whatever gas you're running.
2) You've got an intercooler- by definition an intercooler is part of your intake tract, and It's generally agreed that an intercooler which is internally coated with oil won't be as effective as one that's clean.
3) You want something pretty under the hood.


Where does this oil come from?

On a WRX, this oil gets introduced into the intake tract via two hoses- the one coming from the PCV (Postive Crankcase Ventilation) valve and the valve cover vent lines.

Valve cover vent lines? There's more than one?
Yes, but they're T'd into that black tube which runs in front of your intercooler. The third part of the T is a hose running to your inlet pipe.


Who makes catch cans?

Everybody. Call your favorite vendor or hop over to their web site and see what they carry. Or build one yourself (see below).


Can't I just vent these lines to atmosphere?


You could indeed, but there are several reasons why you shouldn't.
1) You'll lose the benefit of having the inlet's vacuum help suck filthy air out of your crank case (thanks to Wylde Horses for this one)
2) It's bad for the environment
3) It'll make a mess of your engine compartment.
4) It'd let metered air out of the system


Which manufacturer is best?

This topic isn't really hotly debated because hardly anyone cares enough about a catch can to start a flame war over it. There are just too many factors to consider and they all do the same job. The main category breakdowns are:
a. Price
b. Looks
c. Composition
d. Features


What material is best?


Catch cans are commonly composed of aluminum, steel, and/or silicone. It's been reported that catch cans made from silicone cannot be used in certain configurations due to their occasionally collapsing when subject to vacuum.


What features might I want in a catch can?

Some catch cans include features whereby you can see how full the can is without having to remove the can from the car.

Others include little faucets so they can be emptied without having to remove them from the car.

Still others incoroprate breather filters. Essentially, these cans provide most of the downsides of venting the crank breather or PCV lines to atmosphere with the expense of having to buy extra gear to do it. If you'd like to avoid the consequences listed under "Can't I just vent these lines to atmosphere" you should probably avoid this sort of can.



Can I build a catch can myself?


You very likely can. Catch cans are relatively simple contraptions; not at all difficult to construct.


Why would I want to build my own catch cans?


Cost, mostly. Two separate cans are required for a complete installation, and using name-brand cans this can easily come to over $200. For around 1/4 of that, you can build a pair of catch cans yourself that do the job just as well.


Why would I want to buy a catch can?


Appearance and simplicity. Commercial catch cans are often very shiny and pretty. It's also rather easier to pull one out of a box and slap it in than it is to piece your own together. Vendor support might be another consideration.


What do I need to build my own catch can?

People 'roll their own' using everything from aluminum fuel bottles to sections of pvc pipe with end caps. Pretty much any sealable (airtight) container that can withstand underhood temperatures is a reasonable candidate for the body of a catch can.

You'll also need:
Various fittings -you want to end up with 3/8" male hose connectors coming out of your can(s).
3/8" ID fuel-safe hose
Some way to mount your can(s)



I want to keep oil mist out of my intake tract, but I'm too cheap to buy a catch can and too lazy to build one. Isn't there something else I can do?
Some people have had success sticking a small fuel filters between the inlet and the oil-introducing hoses. They don't hold much, though, and would likely require very frequent cleaning.


How should a catch can be installed?

There are a variety of ways to install a catch can. There's one right way, two sort-of right ways, and some would argue that at least one common installation method which is wrong.

Sort of right ways:
1) Disconnect the hose which runs between the PCV valve and the turbo inlet pipe. Run a line from the PCV to the catch can, and another line from the catch can to the turbo inlet hose.

2) Disconnect the hose running between the crank case breather lines and the turbo inlet pipe. Run new hose between the breather line and the catch can and another hose between the can and the inlet pipe.

Right way:
Use two catch cans. Configure one as discussed in option one and the other as described in option 2. For optimal effectiveness, this is the way to go.

Wrong way:
Disconnect the PCV<->Inlet line as well as the Crank Vent<->Inlet line. "T" the PCV and Crank vent lines together, and run the remaining end of the "T" to a single nipple on the inlet pipe. Cap the remaining nipple on the inlet. You'll be sending boost into places that definitely shouldn't ever see any. Bad idea.


Why isn't my catch can catching anything?


Apart from being installed incorrectly, the most common reason catch cans don't catch anything is that air flow through them is too smooth to cause the oil to condense while it passes through the can. Some sort of filtering media is often needed to help sort this out.


What should I use for filtering media?

People have used a lot of different things as filtering media. Foam lawnmower filters are a safe and popular choice.

If you decide to use media that's made of metal (steel wool, chain, etc) and have a metal catch can, be sure that they're made of the same stuff. They'll corrode if they're not, and you don't want the resulting debris being fed into your motor.

Why is my catch can full of water?

Condensation, most likely. It's almost certainly not something you need to worry about.

Blowby products (the filthy vapor in your crankcase) cool as they go through the catch can and associated tubing. As the temperature drops, water vapor condenses. This can cause a surprising amount of water to accumulate in the can. For example, I didn't check my can for about 5k miles over this last winter; poured out around half a liter of almost-entirely-water when I finally got around to it.


Where should I mount my catch can(s)?

The two most common places for mounting catch cans on a WRX are the passenger side strut tower and in front of the battery.


Are there any downsides to catch cans?

Having to remember to empty it out periodically is about the only major downside.

The only other potential downside is cost. Brand name catch cans typically cost $75 or more. Knock-offs can often be found on ebay for more reasonable prices- until you factor in their crazy shipping costs. Rolling your own (or buying used) goes a long way toward mitigating this downside.


Are there any more specific instructions out there about building a custom catch can?
Tons and tons. Search here and on Google as catch cans are a common mod on many different vehicles. Though the fitment isn't the same, the parts generally are.

Examples:
http://www.nissansilvia.co.nz/tech/g...hettocatch.htm
http://www.integra.orcon.net.nz/catchcan
http://www.srtforums.com/forums/showthread.php?t=97923
http://www.autospeed.com/cms/article.html?&A=1101&P=1
http://www.iwsti.com/forums/showthread.php?t=17214


Editor's Note

This post was created because questions about catch cans keep popping up. I came up with the text based on LOTS of searching, here and elsewhere, when I was building my own catch can setup. Upon reading this you should have an idea of whether or not a catch can is for you and whether you'd prefer to build one yourself or buy a commercial can.

The format of this FAQ (and much of the text in this "Editor's Note") was shamelessly stolen from the many excellent FAQ's the Unabomber's put together. If you like it, most of the credit should be his. If you hate it, it's all my fault.
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Last edited by stanh04wrx; 02-06-2006 at 09:29 PM.
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Old 04-18-2005, 08:39 PM   #2
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Uhhh. The PCV system takes fresh air from the airbox runs it into the valve covers where it is then drawn into the crankcase and up through a hose and into the throttlebody. Its the vacuum in the throttlebody that makes it all work.

If there is oil in your intercooler, its from the bearings in your turbo.

Edit: There's a check valve in the PCV hose so it won't work backwards under boost.

Last edited by OneOBS; 04-18-2005 at 08:52 PM.
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Old 04-18-2005, 09:12 PM   #3
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Repent and thou shalt be saved! The whole idea is to pressurize the crankcase and get blowby gasses out of it.

All modern PCV systems (pretty much everything made after like 1980) have a check valve.

Everybody without a catchcan gets oil coating in their intercooler, intake (downstream of the PCV), etc.

Read! http://www.autoshop101.com/forms/h63.pdf
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Old 04-18-2005, 09:33 PM   #4
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Quote:
Originally Posted by Al_Smokemcrack
Repent and thou shalt be saved! The whole idea is to pressurize the crankcase and get blowby gasses out of it.
LOL! I did that in a Saab once and blew out all the oil seals.
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Old 04-18-2005, 10:18 PM   #5
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saabs suck I had one. Anyway does anyone have some picture directions on how to install two cans? Just so we know for sure and there are no questions about it. Cause I'll buy the greddy cans those look hot. Or I'll just get a crawford 2.8 with sleeves. and a t67 yep.

Last edited by boosted20; 04-18-2005 at 10:26 PM.
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Old 04-19-2005, 01:34 AM   #6
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LOL! I got quoted in a nabisco FAQ, I feel so special - now I can tell people I'm not just a windbag/jackass.

As for the materials to make one, I have heard that you can buy certain air compressor parts that work really well for making a catchcan. I'm not sure what they are called or any sort of part numbers though. From those that I have seen, they look very similar to the aftermarket catchcans, they only differ in a few specific areas.
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Old 04-19-2005, 11:34 AM   #7
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Quote:
Originally Posted by Wylde Horses
As for the materials to make one, I have heard that you can buy certain air compressor parts that work really well for making a catchcan. I'm not sure what they are called or any sort of part numbers though. From those that I have seen, they look very similar to the aftermarket catchcans, they only differ in a few specific areas.
Home Made Oil Catch Can info here:
http://www.pbase.com/rsrock/oil_catch_can

It's for a Honda S2000, but the air compressor filter shown is the one I've seen other people use in home made setups before.
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Old 04-19-2005, 06:25 PM   #8
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Congrats on a nice write up.

I'd axe the catch can brand names through as you are missing some and adding every maker of catch cans would take forever and always change.

Also research and post links to homemade catch cans. Search here and google as catch cans apply to many vehicles. Though the fitment isn't the same, the parts generally are. Example

Check out the links on page one of this link for some good info.

Once this gets viewed, responded to, and edited, I'll sticky it for you!
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Old 04-19-2005, 07:32 PM   #9
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Quote:
Originally Posted by Unabomber
Congrats on a nice write up.

I'd axe the catch can brand names through as you are missing some and adding every maker of catch cans would take forever and always change.

Also research and post links to homemade catch cans. Search here and google as catch cans apply to many vehicles. Though the fitment isn't the same, the parts generally are. Example

Check out the links on page one of this link for some good info.

Once this gets viewed, responded to, and edited, I'll sticky it for you!
Thanks for the suggestions! Manufacturer list has been jettisoned and I've added links to homebrew cans. I've deliberately neglected to link to any homebrew units that use breather filters because I don't want to encourage people to go that route.

If it's ok with you, I'd also like to include a bit of the text from your post; "search here and google..." because that strikes me as a very succinct way of putting it.

Thanks again....



Cool, my own sticky.
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Old 04-19-2005, 10:10 PM   #10
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If you don't mind me suggesting it, I would include the setups with breather filters, along with a little description of how they work and what side effects there may be with them and let the reader make up their own mind. That way the FAQ is as complete as possible while not "overlooking" what some say is a viable alternative to dealing with the same problem. Or you could just do up a separate FAQ just on breathers... Just MHO.
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Old 04-19-2005, 10:24 PM   #11
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One Sort of big thing you did'nt mention was their application on n/a cars. They work there too and net the same results.

I would'nt want people to read it and think it does'nt apply to them because the're n/a.

Other than that, good job.
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Old 04-20-2005, 09:10 AM   #12
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Quote:
Originally Posted by Tim Sanderson
One Sort of big thing you did'nt mention was their application on n/a cars. They work there too and net the same results.

I would'nt want people to read it and think it does'nt apply to them because the're n/a.

Good point. I took Unabomber's tip and mentioned that they apply to many different cars, but now that you mention it, I think that should also be pointed out under "Why would I want one?". I put it in "service and maintenance" because it does apply to most cars, so I should account for that in the faq iteslf.

Thanks!
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Old 04-20-2005, 09:16 AM   #13
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Quote:
Originally Posted by Wylde Horses
If you don't mind me suggesting it, I would include the setups with breather filters, along with a little description of how they work and what side effects there may be with them and let the reader make up their own mind. That way the FAQ is as complete as possible while not "overlooking" what some say is a viable alternative to dealing with the same problem. Or you could just do up a separate FAQ just on breathers... Just MHO.
Interesting... You point out the perils of venting to atmosphere, but then suggest that I include links showing people how to do just that. I think the closest I'm willing to get to that is to include "What about catch cans with breather filters" and say "They're essentially as bas as venting straight to atmosphere". :-) Yep, I'll use that. Thanks for the suggestion.

I already tell them what the consequences can be, and if they're determined to ignore those consequences they can do their own google'ing for ways to go about it.
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Old 04-20-2005, 10:24 AM   #14
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Hehe, yeah, I'm all for letting people make informed decisions - even if they happen to be the ones I don't agree with. In this case, I only make the suggestion as a matter of completeness for the FAQ, so that all pertinent areas are covered. But I agree 100% that breathers are bad juju.
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Old 04-20-2005, 12:10 PM   #15
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Please feel free, if you want, to include links to my custom dual catch can setup. I have received many compliments and many want to duplicate it.

Points of note:
1) No Bling. No need to draw attention to catch cans.
2) PCV and Breather are separate systems.
3) High Quality.
4) Something relatively compact.
5) No stock hoses or body panels are harmed in this process. Everything is 100% fully reversible if you want to go back to stock.

Pictures:
http://www.bescaredracing.com/sti/catchcans/complete/

Diary of installation:
http://www.imprezawrxsti.com/forums/...ad.php?t=17214

t
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Old 04-20-2005, 10:40 PM   #16
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Quote:
Originally Posted by WolfPlayer
Please feel free, if you want, to include links to my custom dual catch can setup. I have received many compliments and many want to duplicate it.

Points of note:
1) No Bling. No need to draw attention to catch cans.
2) PCV and Breather are separate systems.
3) High Quality.
4) Something relatively compact.
5) No stock hoses or body panels are harmed in this process. Everything is 100% fully reversible if you want to go back to stock.

Pictures:
http://www.bescaredracing.com/sti/catchcans/complete/

Diary of installation:
http://www.imprezawrxsti.com/forums/...ad.php?t=17214

t
Thanks for posting that up! I believe when I'm ready to get catchcans for both my cars, I'll definitely be going with those! I like the look far better then most of the cans out there with the clear tube on the end.
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Old 04-21-2005, 01:02 AM   #17
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Quote:
Originally Posted by OneOBS
LOL! I did that in a Saab once and blew out all the oil seals.
That's not cool. What I mean by that, though, is pressurize it a little. Pressurized it a little and allow someplace for the gasses to go.
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Old 04-21-2005, 05:58 AM   #18
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Quote:
Originally Posted by WolfPlayer
Please feel free, if you want, to include links to my custom dual catch can setup. I have received many compliments and many want to duplicate it.

Points of note:
1) No Bling. No need to draw attention to catch cans.
2) PCV and Breather are separate systems.
3) High Quality.
4) Something relatively compact.
5) No stock hoses or body panels are harmed in this process. Everything is 100% fully reversible if you want to go back to stock.

Pictures:
http://www.bescaredracing.com/sti/catchcans/complete/

Diary of installation:
http://www.imprezawrxsti.com/forums/...ad.php?t=17214

t
That is definitely a super clean installation. If I were going to pay $200 for a pair of catch cans I'd buy those for sure. But I'm not, 'cause I'm way too cheap for that. Instead I think I'll fashion a similar mounting bracket, paint my cans black, and mount them in the same spot. :-)

Definitely gets a link in the FAQ. Thanks!
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Old 04-21-2005, 01:09 PM   #19
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The link that you posted in the main post to the catchcan install doesn't work. You need to actually visit the page and copy the URL from that page and then paste it in.

http://www.imprezawrxsti.com/forums/...ad.php?t=17214
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Old 04-24-2005, 08:46 AM   #20
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Quote:
Originally Posted by WolfPlayer
The link that you posted in the main post to the catchcan install doesn't work. You need to actually visit the page and copy the URL from that page and then paste it in.

http://www.imprezawrxsti.com/forums/...ad.php?t=17214
Fixed.
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Old 05-03-2005, 12:07 AM   #21
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Quote:
2) Disconnect the hose running between the crank case breather lines and the turbo inlet pipe. Run new hose between the breather line and the catch can and another hose between the can and the inlet pipe.
In this statement are you refering to the valve cover breather lines? You already covered the crank case (PCV) in the first one.

Last edited by TyranosaurusWRX; 05-03-2005 at 12:17 AM.
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Old 05-15-2005, 09:06 PM   #22
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t - just wanted to compliment this install one more time, really clean (took me a second to even see them in the pics!), and a nice howto writeup on IWS..

Quote:
Originally Posted by WolfPlayer
Please feel free, if you want, to include links to my custom dual catch can setup. I have received many compliments and many want to duplicate it.

Points of note:
1) No Bling. No need to draw attention to catch cans.
2) PCV and Breather are separate systems.
3) High Quality.
4) Something relatively compact.
5) No stock hoses or body panels are harmed in this process. Everything is 100% fully reversible if you want to go back to stock.

Pictures:
http://www.bescaredracing.com/sti/catchcans/complete/

Diary of installation:
http://www.imprezawrxsti.com/forums/...ad.php?t=17214

t
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Old 05-16-2005, 02:44 PM   #23
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Quote:
Originally Posted by jmolaver
t - just wanted to compliment this install one more time, really clean (took me a second to even see them in the pics!), and a nice howto writeup on IWS..
Thanks! I still have the cardboard sitting at home that I used to create the bracket. I was going to take the measurements from that and post it but decided that it would be best to actually remove the cans and measure the REAL bracket. Not sure when I'll get around to doing that ... so - maybe I will just remeasure the cardboad template.

t
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Old 06-21-2005, 08:12 AM   #24
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Accurate Machine Works waiting list...
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Old 07-10-2005, 11:11 AM   #25
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OK, I may be wrong....but:

1- this statement "Exhaust gasses cool as they go through the catch can and associated tubing. As the temperature drops, water vapor condenses. This can cause a surprising amount of water to accumulate in the can. " is wrong in this thread. No exhaust gases end up in the oil catch can.
What you state is correct (and is why you see water dripping out of exhaust tips) in another context, but not here.

2-You don't really want to PRESSURIZE an engine block (in the oil gallery), you really only want to pull a vacuum. this reduces some of the pumping of air underneath the pistons as well as removes combustion chamber gases that get by the piston rings.
Burning the oil mist and the gases reduces the emissions of the car. Leaky valve guides, leaky piston rings and slinging oil around are what produce the mess that the PCV and the valve cover breathers dump back into the intake system.

Add to above if you want, delete if you want.

BTW, I am a NA guy.
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