what are they for are they needed ?

 

they catch oil vapour from the heads and/or crank case breathers that recirclate air back into the intake system before the intercooler which can clog it up and reduce its efficency

 

so i take it our cars dont actually have one already - so are these essential ?

 

no they dont have em, i just replaced my intercooler (due to bent fins) and that had oil residue inside nothing major but not to sure how much it effects performance. I cant see them being essential or subaru would of designed them in the car however dont know if they become essential after a certain level of modding

 

the oil vapor wont help with the air going into the engine as well, and lowers performance a smig

 

oil vapour if it is laden with oil is low octane and can reduce the octane rating of your fuel leading to det.
Catch cans are more applicable in heavily modded cars running higher boost settings.
The test if you need one is if there is clear evidence of oil on either side of the turbo i.e inlet pipe/intercooler tract and that your oil usage is above normal. There are then two options 1. interupt the vent lines from crank and heads with the catch cans keeping the feed back into the intake pipe and maintaining the operation of the PCV valve. 2. remove the PCV valve and vent the catchcan to atmosphere, running the outlet well under the car, blanking all unused ports.
The latter although less green is what most tuners recommend because it completely removes the possibility of oil in the air intake.
It would be interesting to run a poll to find who installed catch cans and didn't catch any oil, I would be one of them.
Trev

 

cheers - so seeing as im not heavily modded its rather pointless me having one and it would just be engine bling for me

 

and more weight

 

Why is it that some peeps run the output from the catch can back to the sump rather than vent to atmosphere?

TX.

 

In most of those cases you'll get a vent (whether to atmosphere or back to the inlet tract) and a return to the sump, and a separator inside the can designed to pull the oil droplets out of the vapour.

The reason why you have the return to the sump is because that's the best place for the oil you've separated out to end up. Far better to have it back where it can do some good lubricating your engine than vent it out into the environment.

 

i see mentioned above the removal of the PCV valve,WHAT IS THIS and WHERE IS IT located as i have fitted a catch tank with both crank and header feeds going straight to it with a filter on my catch tank for the venting part.i have not removed any valve so can someone give more info on this please.

 

If you look down to the bottem of your crank vent pipe there is a T piece, the pipe running off to the right comes around into the back of the intake manifold and connects there into the Positive Crank Vent valve( pcv). In normal off boost driving the valve is open and allows vented vapour from the crank into the intake manifold back through the engine thus reducing pressure build up in the crank shaft. Under boost pressure the valve shuts tight to allow the then hotter/oilier vapour to be drawn out by the turbo into the air intake etc and again reducing pressure build up. The modification is to remove the PCV valve, blanking the port, removing the T piece and running the vent pipe straight onto the vent port. At this point I understand the catch cans should vent to atmosphere.
If the car is doing a lot of track work then blanking the PCV and venting to atmosphere via catchcan or breather tank is the way to go but if not then maintaining the OM system but installing catchcans, if needed, is a reasonable option.
Trev

 

Cheers for that trev,as its a daily driver and no track days done or planned,i`ll just leave it in place then,much appreciated for the info.