I'd appreciate the help of you technical bods with my understanding of this topic. (2003 Spec-c ltd)

I think the factory system is what is known as Positive Crankcase Ventilation (PCV). This is where the vacuum in the intake system is used to draw air through the crankcase, clearing bypass gases back into the intake to be burnt off.

There are two rocker cover vents and a crankcase vent.

The crankcase vent has two pathways:
1. Under no-boost conditions the vacuum in the intake manifold opens the PCV valve and air/gas is sucked into the throttle body.
2. Under boost the PCV valve is closed and air/gas passes into the intake hose just before the turbo. This pathway is always open.

The fresh make up air is drawn in via the rocker cover vents. These connect together via the hard pipe attached to the intercooler and from there into the intake hose. I have often wondered why that hard pipe was made to give equal lengths to both vents, but I guess this is to even out the suction so both heads get equal ventilation.

Note that with this set up all the air is metered by the MAF sensor.

My car once had an oil catchtank that was installed in such a way that is would have been pretty useless as a catchtank, but even worse a possible engine killer:
From memory just the rocker cover vents went to the catchtank, which then vented to atmosphere.
The crankcase vent was still as factory. So unmetered fresh air would be drawn in through the catchtank, in the wrong direction, possibly causing a lean mix and doing nothing for removing heavy particles from the blow-by gases!

So, if I want to use a catchtank that vents to atmosphere I believe I would have to remove and plug off the PCV valve. The crankcase vent and rocker cover vents can then be run via a catchtank to atmosphere, BUT there would be no vacuum drawing air through the system.

Effectively this converts the factory Positive crankcase ventilation to Passive crankcase ventilation.
Is this a bad thing? How important is it suck the blow-by gases out of the system, or will they just clear naturally under any pressures created in the crankcase and rocker covers?

Any comments, catchtank recommendations, etc would be appreciated.

Manythanks, Andy

 

I think the rocker cover vents do the same as the crank case ventilation. They are 2 seperate systems. They both 'ventilate' and due to boost, 'dirty air/oil' is being sucked back into the intake.
Installing 2 catch tanks would be the best solution, so both systems stay separated.
Got a link from nasioc where some dude installed a dual setup, I'm going to do the same

http://forums.nasioc.com/forums/show....php?t=2443998

 

PCV is not needed & can be blancked off but the crank case breather must go into your catch can. I use an OBP can but if you run over standard boost then you must use the big port version. VTA for the can is fine I run a hose to the transmission tunnel for mine. Only one can is needed but it must have 3 ports & a vent.

 

Thanks for the useful replies:

To keep the first post short(ish) I didn't explain why I was asking. It's not for the normal reason of trying to keep crap out of the intake system, although that would be a nice silver lining. I want to minimise the connections into the intake pipe, as it's going to have to be readiliy removable for scrutineering of the turbo inlet size.

Therefore the dual tank set up shown by Highlander, and a good and very similar guide in the SN technical library don't acheive this. I have to vent to atmosphere, but was concerned that the lack of suction in the system will mean it's not effectively vented.

Richard's posts confirms that a 'passive' or 'open' system should be fine.

I run up to 1.8bar so will need a tank with larger connections.
Richard, this might be a stupid question but what's the 3rd inlet port for? Does this mean you ditch the OE connector pipe for the head vents and take them independently to the can?

Ta Andy

PS. I mentioned in the OE set up air is actually sucked down the head vents, as this is how the airflow is shown in the workshop manual. I assume thsi will be reversed in a vta set up, and blow-by gas will just be 'puffed' out of the vents?

 

One port per breather plus a separate one for the vent.

 

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. As you have rightly said in your first post 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 and other bits.

 

What about adding additional crankcase evacuation, via the downpipe.

 

This would be an ideal way of providing suction to pull crap out of the crank case without putting it back in your intake. Not sure on the effect it would have on a cat if you have one fitted or emissions for MOT time. Also you may need a one way valve to prevent it from putting exhaust in the crank case. Lastly it would need to be a properly designed venturi type fitting to get the suction.

You can get proper vacuum pumps that do this but they are expensive and probably excessive.

 

I use an ixiz set-up with the oil return to sump (constant oil return), as opposed to the rear breather/balance vent (stored oil runs back when pcv valve is open).
Still use the PCV set-up.
As you say, I used an extra hose, T into the vacuum line, a one-way valve and a slash cut tube welded into the downpipe, before the centre section flange, and you're away - Bernoulli's principle.

 

Trying to work that system out is hurting my pea-brain!
When the manifold is in vacuum and the PCV valve opens wouldn't the suction pull oily stuff from the drain back into the throttle body?

As I don't want any connections into the intake pipe, but from the above I think 'forced' ventilation is probably a good idea, I'm thinking a vacuum pump might be worth considering!

Remove PCV valve, and plug TB connection.
Take a hose from the crankcase vent to a single inlet catchtank
Fit vacuum pump on outlet of catchtank venting to atmosphere.
This gives constant suction on the cranckase.
Fit a small filter on the rocker cover vent, so no solids get sucked in, and this provides the make-up air so the crankcase is continuously ventilated with fresh air.

So where do you get a small 12v vacuum pump?

PS: might be an idea to know what mmHg suction the pump needs to pull. Fortunately I have an Apexi boost controller that will record max suction (min boost) over a run. So maybe I should take the car out tomorrow!

 

Warning! Ditching PCV could shorten your engine's life!

Having a general search for catchtanks I came across this article.
I know they are selling catchtanks but the 'history' of crankcase ventilation is interesting, and reinforces the fact that Positive Crankcase Ventilation is a bloody good idea:

http://oilcatchcan.com/

Looks like passive breathers could shorten your engine life quite dramatically if that is to be believed!

 

The can just keeps the oil out of the inlet & A VTA can with the blancked PCV keeps the maximum amount out of the inlet which is the best solution. High power high boost cars don't want oil in the inlet as it causes lean running & det as we'll as reducing the efficiency of the intercooler. Plenty of people have run this set up for many years at high power & boost with no issues.

 

Agreed, I initially was all for just venting all three holes to atmosphere.... I wanted to bin all the crap pipework. But on reading about it all I certainly won't be doing that now.

My intake is between -20 to -25 psi on over run max. So This would surely be the maximum amount of vacuum ever used to suck on the crank case. I would have thought the vacuum just prior to the turbo would be less? Then again if maximum boost is 18psi on my car would that not mean that at this point the vacuum just prior to the turbine in the intake would be -18psi vacuum? Or does it not equal out like that?

 

Oil would end up in exhaust. Also the pressure through exhaust would probably suck engine dry lol

 

All the catch tank is theoretically is a filter stopping crap oil residue going back into air intake. Thats what this diagram shows anyway. You leave pipework going to the same places but fit the catch tank inline. But leave pipe from pcv to throttle body

 

I don'st see a problem with oil in the exhaust. Better there then in the intake. It may mess up a cat if its fitted but all it would do to the metal exhaust is make is a bit oily inside... not a bad thing really. I have no idea about the suction levels. More then -20psi found in the intake manifold? I really have no idea.

With regards to the catch can. Yes as you say just fit it inline. But its venting to atmosphere that causes the problem or put all breathers into one can when the rocker ones are supposed to have fresh air.

 

That is all understood but positive crank case ventilation is also important. Ideally you want both. Both systems have their draw backs unless you combine them to allow PCV with a way of stopping oil in the inlet (in line catch can or separate vacuum source).

 

I understand what you are saying but lots of people use the same system as me to good effect, if it's not broken why try to fix it. You can still keep the PCV with my system & some people do, but many remove it with no issue. Some pipe the vent back in to the inlet but most don't as some oily gases will still go into the inlet which defeats the object. I fail to see how you can vent the can into the downpipe, as the exhaust creates pressure as does the breather system. The inlet manifold pulls vaccum so venting to the inlet or VTA are the only possibities unless you are going to rig up a vac pump to pull vented gasses out. On a high boost car a lot of pressure comes from the crankcase breather, even on tick over they constantly chuff pressure pressure out, so the pump would need to be a serious bit of kit.

 

I do not want to stop anybody doing what they are happy with or say they are wrong but just from what I have read it seems to be proven that having a vacuum pulling (not just venting naturally) crank case pressure is a good thing and can only help engine life and performance.

This page talks about vacuum systems on race engines and actually states that ideally about -6 to -8 psi is helpful to improve performance. So much so that people spend a lot of money on these systems for racing.

http://nutterracingengines.com/racin...uum_facts.html

As for pumps yes you would need a good pump. Little electric ones probably wouldn't be up to it. Hence this sort of thing: http://www.product-engr.com/vacuum_system.html

Yes very expensive I know!

As for the exhaust pressurising the crank case. No. You fit a one way valve which only allows air to be sucked into the exhaust not out. By using the correct venturi in the exhaust suction will be created. Think of the way a pressure washer picks up soap from a bottle or the way a shot blaster picks up sand. It shouldn't work against gravity like that but it does because there is a vacuum created as the air passes over it and this creates suction. This type of suction is what we are talking about.

All in all yes a lot of people have just vented to atmosphere. They may not encounter any problems at all. But its the sort of thing that you probably would not notice as it would be a long term issue that just increases wear which people would put down to wear and tear anyway and never look to lack of PCV as the cause. But the argument that lots of people do it and it's fine never holds with me. Lots of people never change the oil in their cars they just top it up. Those cars still run for years and years. But it's not a good thing and it will add up to an increase in wear.

Again people, do what you like. But I am sticking with the fact Subaru went to a lot of trouble designing this system. It is clearly not just for emissions as there are far more simple ways of solving that problem (all three breathers straight to the intake pipe with no PCV or extra piping). There are numerous resources which state that PCV is a good thing. I have found NO reliable sources that state that it should be binned other then people who did it and nothing bad happened to them. I'm sticking with the PCV system and just incorporating a catch can in the crank line to catch a bit of sludge. Yes some oily air will still go into the engine but if more has been going in for 14 years and 116'000miles I'm sure it doesn't really matter too much.

As with most mods its usually better to just leave it the way it was designed to work... but that's easier said then done when you like cars and moding

 

I dont know if you've ever spilt a drop of oil on exhaust but it smokes good n proper,so surely smoke from burning oil would come out back of exhaust. Why the need to vent to atmosphere if using a catch can?

 

I'm not criticising your ambition to develop a better way of doing something, I just don't see it working & think you'd be best using what is tried & tested. Iv never heard of an Impreza running a Venturi down pipe, only an up pipe like I use to help with spool. To develop this will cost a lot & if it doesn't work properly then it's a waste of time & money, are you planning to sell such a system if it works & is there a market for it? Good luck with it & if it works post your results to prove me wrong. Take a look at Andy Forrest's web site, there is some good info on there both for & against PCV & Andy knows a thing or two about Imprezas. Premature engine failure can't really be proved either way when modified Imprezas are running way beyond the power level intended by Subaru.

 

Er, hello.
Works fine. 45 degree end cut tube, mounted at 45 degrees. Tip of the tube to be central in the downpipe.
Above an outer curve is a good mounting point, as the gases are travelling fastest, at that point.

 

Why the big effort to keep the pcv, the time the engine is working its hardest & the most breathing cycle begins it shuts itself off, on newage engines it then recycles from the block breather to the cam covers & cam covers back into the inlet, which produces all the oil carryover in the intercooler & inlet tracks.
The pcv is fine for off boost conditions ie town driving etc to keep emissons under control, but any performance upgrades it really needs removing & the tried & tested catchcan needs fitting properly.

 

If it works then great, but is this a real system or theory? Some pictures would be helpfull if you are actually using this system & what power & boost do you run? I understand a one way valve may stop exhaust pressure entering the breather system but if exhaust pressure is greater than the breather system it won't vent & would have the same outcome as a blocked vent, or are you saying a Venturi solves this problem?

 

What about a daily driver with performance upgrades? Would you try it without the pcv?

 

I run 460BHP at 1.7 bar no PCV daily driver no issues now thanks to Motors S GT.

 

Wow. He sounds like he knows what he's doing.

 

Elaborate

 

I hate breather systems! still confused what to do with mine, im running a hyperflow catch can vented to inlet but im getting some oil in the intake pipework(dont want any)
Was going to remove pcv and vent to atmosphere this weekend but now im confused what to do.

 

There's nothing new about crankcase evacuation. Might be unusual on a turbo motor. Might also be unusual to retain a pcv.
Tick-over - pcv only in operation.
Part throttle - pcv and evacuation in operation
Full throttle - evacuation only in operation.
The exhaust gases, flowing past the tip of the draft tube, create a pressure differential.
The check valve stops pressurisation of the crankcase.