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#173 Oil Catch Can Breather

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Andre can you please shed some light on what would be a good rule of thumb to determine what size the breather should be. We sell catch cans at work and they have a dual input of an-10 and a single an-10 breather with a filter ontop. My co worker seems to think that if it has dual an-10 then it should also have dual an-10 breathers. Is he right in saying so?



I'm not sure if I could say there is a rule of thumb. The required breather size will depend on engine capacity, number of breathers, and how hard the engine is being worked (boost pressure for example). Generally I will use two -10 breathers on a 4 cylinder running moderate to high boost. It's not strictly essential to have the same cross sectional area for the breather out of the catch can as the lines running into it. We want large lines into the catch can to reduce the velocity of the blow by gasses which helps reduce the amount of oil that gets dragged along for the ride. Once the gasses have made it into the catch can this is less of a problem for the actual breather.

since its related to this webinar i thought i would ask here instead of starting a new topic.

would adding a Tee into a -10an oil drain (close to the block with the vent facing up) and running this to a catch can act as a crank case vent? i have been told no as the oil from the turbo is to aerated.

are there any down sides to a trunk mounted catch can? wouldn't the lines will with oil under the car and create a pressure or is this negligible? this is appealing to 4cyl with cramped east west engines as CAMS require a 3L catch can and there is little to know engine bay room.

does having the catch can vent routed to between the turbo and air filter create enough vacuum to warrant the length of hose and risk of oil in the intake? or is having a small air filter on the catch can effective enough?

I personally wouldn't tee off the oil drain from the turbo. You're always going to have a constant flow of oil out the drain which means your breather is more likely to pull oil vapour with it.

Mounting the catch can in the trunk is quite common in drag cars. this actually has the advantage of preventing any oil getting on the tyres if there is an engine failure. Here in NZ the requirements for a 2L turbo engine are a 2L catch can which is all but impossible to fit in a modern engine bay. It seems however that our officials know this and don't enforce the rule. Some measure of common sense seems to have crept in which is surprisingly rare.

I wouldn't plumb the breather back into the intake unless you're forced to. There's no upside and the downside is that you're definitely going to end up with some oil vapour in the intake system.

might look at moving the catch can to the boot.

currently the 3L can i have in the bay is an eye sore of a location.

was just worried with the lines being blocked with oil.


one last query, what happens when the lines running from the engine to the boot (trunk for the Americans) fill with oil?

i assume it will eventually push some of the oil into the catch can which isn't a great concern? i worry it will creates a pressure as it the oil has to be pushed up into the catch can, will this pressure be enough to worry the crankcase?

all of my concern centers around the lines under the car being lower than the catch can and the engine, but if its tried and trued i cant wait to move my catch can to the boot on my circuit car. engine bay is already over crowded.

thanks again.

I understand your concerns, however I can confirm that it's a non issue. You're going to be moving a sizeable volume of air through the breathers under high load and high rpm so this tends to keep the oil vapour moving through the breather hose.

sorry to bring up an older subject

A 4.0 6cyl running approx 30-35 psi and on E85, I'm going to keep the PCV system but through it's own baffled catch can, but was going to run 2 bigger lines to catch can/s, would 2 x -8 fittings and hose be sufficient or just go 2 x -10 size?

PCV check valve question. I have an ECOTEC 1.4L with a BW EFR 6258 turbo, running about 25 psi boost at the moment. A tuner recommended a vented oil cap with a -10 AN line to a catch can vented to atmosphere in addition to the original PCV from the head through a -10 AN line to a second catch can vented to atmosphere. With both being vented to atmosphere, would either require a check valve?

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