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PostSubject: Catch Can FAQ   Catch Can FAQ I_icon_minitime2010-11-29, 6:45 pm

Catch Can FAQ

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 turbo discharge 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 turbocharged vehicles.

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 StarQuest, this oil gets introduced into the intake tract via two hoses- the one coming from the PCV (Postive Crankcase Ventilation) valve and the vent hose from the stock air/oil separator when it gets clogged.

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.
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 intake manifold. Run a line from the PCV to the catch can, and another line from the catch can to the intake manifold.

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.

Where should I mount my catch can(s)?

The two most common places for mounting catch cans on a StarQuest are the passenger side strut tower and near the airbox.

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.


Editor's Note

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.

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