peter zippy reid

iTrader: (25)

Ok thats better yes he blanked both of them thanks for the help

veerinder9

iTrader: (1)

Why is blocking the pcv and fitting a can bad? I'm confused i thought this was a common mod to stop crap gtting into intake etc? Or am i missing something?

bludgod

iTrader: (3)

read more on catch cans here:
http://forums.nasioc.com/forums/showthread.php?t=754710

FMJ

iTrader: (2)

Copied and pasted this a few times and it's just my own thoughts on the matter:

On the classic you have the breather system as follows:

There is a pipe which is connected to the intake hose near the power steering pump. The pipe splits into two and goes to each head.

You then have the crank breather located just to the right of the turbo and to the left of the throttle on the block. This is divided into two. There is a small opening which leads back to the intake hose just before the turbo. There is a bigger opening which is connected to the throttle body via a one way valve (PCV Valve).

On boost and full throttle the PCV is closed. The pressure in the crank case exits the engine via the hose into the intake hose pre-turbo. It may also vent a bit through the heads but not much.

When you are off boost and there is vacuum in the intake manifold the PCV opens and the vacuum pulls dirty air from the crank case vent into the intake manifold to burn it off in combustion. This causes a vacuum in the crank case and clean air is sucked into it via the two head vents. As the head vent pipes and the two way crank vent can vent are all connected to system post MAF it is all read equally (no air escapes after the MAF has accounted for it).

If you VTA then you lose the metered air.

If you VTA and don't seal the PCV you will suck unmetered air in.

If you VTA you lose the vacum of the PCV which means new clean air is not introduced into the engine and the old acidic air will increase wear.

I have just been heavily researching this area myself and there is massive confusion about this system on the net. A lot of people think the rocker breathers are literally to let out pressure into the intake. Their real function is to let fresh air be drawn into the heads and down into the crank case. In the tech manual it confirms this.

Apparently the oil in your crank case gets quite acidic as it is used and these acids are bad for the engine (a reason you change your oil every six months even if you only did 500 miles). So ideally you want them properly flushed out via the positive ventilation system that Subaru designed. By just letting them breath then the dirty air to some degree will stay in there. Another reason the positive system is good is apparently it assists ring seal... not sure on that one but a few people seem to think this.

Now many have simply vented it all to atmosphere and probably noted no problems. But personally, now I understand how it works and the reasons behind it I am going to stick to a modified version of the original design that still works in the same way. I am going to run the crank breather through a catch can but keep it within the same routing of pipework.

I know its all a mess but it seems unlikely that Subaru would go to so much trouble making such a complex system just for the hell of it. For those who say "it's just for emissions we don't need it". I think this is part of the reason but if it were just for emissions the system could just be passive and they wouldn't have bothered with the PCV valve, the carefully designed t piece which gives priority to the PCV route rather then to the turbo intake and other bits.