Why does the PCV setup connect to the inlet?

[dnb]

There is a breather to allow air IN to the crank case on the passenger side rocker.
The plenum is connected to the flame trap vent on the driver's side rocker, and also to the inlet pre-butterfly via a T piece.

The engine will suck the corrosive vapours from the crank case when it's pulling a vacuum. When the engine is under load then the manifold pressure is close to atmospheric pressure, so not much sucking of the crank case vapours (it's a thin tube, so there's not a huge flow of air)

So the question is: Why does this system need a connection to the inlet? All it's going to do is reduce the amount of suck the crank case "sees" under vacuum. Does the crank case not like being evacuated with such force?

[spend]

I'm not quite sure what your exact question is, does http://en.wikipedia.org/wiki/PCV_valve answer it anyway.

I was always convinced there was a restriction under the little foam breather, as well as 'T-piece' - there is definitely FA except scrunched up wire in the flame trap.

Dave

[trackcar]

Are you asking why there's also a larger bore tube upstream of the throtle plate? Thats to get rid of the much larger blowby gasses (espeically on high miler land rovers etc) which wouldnt be able to pass through the small restrictor into the vacuum side of the PCV tubing route.?

[dnb]

Yes Joo. I was planning to run it to a catch tank and vent it to atmosphere.

[GreenV8S]

dnb said:

Yes Joo. I was planning to run it to a catch tank and vent it to atmosphere.

If you're after any sort of performance that should be one of the first changes IMO. Oil and crank case gases in the intake dilute the fresh air and lower the octane of the charge. It's good for the environment for you to use your engine as an incinerator, but does cost power.

[dnb]

Exactly Pete - it costs engines (even less eco friendly ) on the Impreza due to detonation induced by the octane reduction.

The engine bay is VERY limited for space these days (v8 enhancement plenums fill it all up ) so I want the minimum of plumbing for simplicity:

The car does a lot of short runs, so getting rid of the corrosive gasses in the crank case is a must. I plan to use the plenum connection to suck the crank case gasses through a catch tank with built in separator and not bother with the connection to the inlet at all. Where is the flaw in my plan?

[spend]

Why not use a 'purge valve' type arrangement to operate when you don't need the performance. Presumably then you would be able to deploy a smaller catch tank and still use plenum vacuum to give a good 'clear the air' sucking out of the crankcase?

Dave

[trackcar]

Why not use the exhaust to scavenge the gases, use a one-way valve though so you dont get a backfire going back into the crankcase.

[2Munkys]

GreenV8S said:

dnb said:

Yes Joo. I was planning to run it to a catch tank and vent it to atmosphere.

If you're after any sort of performance that should be one of the first changes IMO. Oil and crank case gases in the intake dilute the fresh air and lower the octane of the charge. It's good for the environment for you to use your engine as an incinerator, but does cost power.

Hmm, that's interesting; it's something I'd not considered, nor has the idea as a performance gain been suggested to me. How significant is this contamination to the charge? Has anyone carried out some manner of experimentation on a rolling road? Any thoughts on a percentage of power to be gained through this route?

[dnb]

spend said:

Why not use a 'purge valve' type arrangement to operate when you don't need the performance. Presumably then you would be able to deploy a smaller catch tank and still use plenum vacuum to give a good 'clear the air' sucking out of the crankcase?

Dave

That is nearly what I was suggesting. There's no way of getting a NA engine into +ve boost, so the "purge valve" effect tends to happen automatically - less suck as the engine comes on load.

I don't want to have the bypass pipe going to the inlet (but will be keeping the connection to the plenum) because I can't fit it in easily...

With the catch tank in place, there should be no problem with oil getting everywhere it shouldn't...

[GreenV8S]

The 'positive' part of the ventilation is provided by the small connection downstream of the throttle. This applies manifold depression to the crank case via a restrictor. The restrictor is required because the crankcase seals are unlikely to cope with full manifold depression.

When the engine is on load the manifold depression is reduced and the breather gas flow rate is increased so the restricted breather becomes inadequate. Hence the unrestricted breather is connected upstream of the throttle. When the engine is on load there's more flow in the intake which means it can help pull the gases out.

The whole setup is a cheap simple way of doing the same thing that a PCV check valve does.

If you are going to replace it you could replace the restrictor with a PCV check valve and do away with the unrestricted upstream connection, but I don't see any point in doing that since it would still end up with all the breather gases being fed back into the engine intake. That's ultimately what causes the performance loss.

There are performance benefits from replacing it with a system that vents to atmosphere. A vent to atmosphere breather system should always go via a catch tank to ensure that you don't ever spray oil out since that can be dangerous.

[spend]

What I meant was a purge valve on the big pipe, I always assumed this big pipe was to give a big suck to attempt to make sure none of the earlier pipework got blocked..

Cleaning the trap & pipework out is part my service schedule (but rarely done I suspect). I am always surprised at the sticky mess in there - even when you still have nice clean engine internals. I suppose thats why I am kind of dubious about catch tanks without a 'vacuuming' to keep everything clear, unless you are prepared to constantly check & clean- but that maybe my extended oil life tight@rse syndrome

Dave