Breather filter.

[Guest chiefvinso]

If you are producing loads of blow-by, then it could affect your fuel / air mixture. There's lots of nasty stuff in blow-by gases, and not all of it is nice. At a rough calc I reckon you can expect WOT blow-by production to be in the region of 100 to 200 litres per minute worst case on a big single car on the overrun. However, the fat end of this will go through the part throttle breather into plenum.

 

This might sound like a lot, but the car will be trying to suck approx 10000 litres of air per minute, so you are talking about 1 to 2% of the total flow being blow-by.

Thanks, so I would assume from this that its best left standard on and going back through the airbox on a standard/unmodded vehicle.

[Digsy]

Thanks, so I would assume from this that its best left standard on and going back through the airbox on a standard/unmodded vehicle.

Oh, hell yes

[JohnA]

The crankcase should only really have a small fraction of a psi positive pressure, and that's the maximum allowed. That's what I've discovered in a few turbocharger manuals that were extensive enough to go into such depth.

 

I found this on the low side, so I made a few experiments myself, measuring crank pressure directly.

Also using a transparent oil return tube and watching the oil return flow with and without having the crank ventilation stuck on the compressor intake. The difference was substantial, even on idle low-load operation.

I redid the experiment after I had changed the piston rings (less blowby) and the difference was smaller.

 

So the older the engine (the more the blowby) the more you need the intake vacuum to reduce the crank pressure.

 

A catch-tank should only really be an intermediate stage and then hook back to the intake, NOT the atmosphere as it's usually done.

[mikeyb10supra]

The crankcase should only really have a small fraction of a psi positive pressure, and that's the maximum allowed. That's what I've discovered in a few turbocharger manuals that were extensive enough to go into such depth.

 

I found this on the low side, so I made a few experiments myself, measuring crank pressure directly.

Also using a transparent oil return tube and watching the oil return flow with and without having the crank ventilation stuck on the compressor intake. The difference was substantial, even on idle low-load operation.

I redid the experiment after I had changed the piston rings (less blowby) and the difference was smaller.

 

So the older the engine (the more the blowby) the more you need the intake vacuum to reduce the crank pressure.

 

A catch-tank should only really be an intermediate stage and then hook back to the intake, NOT the atmosphere as it's usually done.

 

 

Dont take this the wrong way im not meaning to sound funny at all. but dont you get a tad bored of doing all your tests and that???

[JohnA]

No mate, I find it soothing and interesting. I use the car as a testbed, a base for experimentation.

If you saw my garage you'd understand, he he...

[Alex]

Personally I like to drive mine....

[Digsy]

The crankcase should only really have a small fraction of a psi positive pressure, and that's the maximum allowed. That's what I've discovered in a few turbocharger manuals that were extensive enough to go into such depth.

If you turbo oil return line is a decent diameter, then the turbo bearing housing should be at pretty much crankcase pressure. Even so, the oil feed will be at, what - 1 to 2 bar above atmospheric, even if you take it from the cylinder head?

 

You would really have to go some to presurise your crankcase to a degree where is seriously impedes oil flow, and your front seal would spit itself out long before then, anyway.

 

I doubt that crankcase pressurisation is much of a factor in getting the oil out of the turbo.

[JohnA]

The oil feed is pressurised, yes.

The return isn't. If oil doesn't flow by gravity, then it piles up the bearing and gets pushed out of the seals.

 

That doesn't bring permanent damage immediately ofcourse, but it doesn't help either.

[Digsy]

The oil feed is pressurised, yes.

The return isn't. If oil doesn't flow by gravity, then it piles up the bearing and gets pushed out of the seals.

It will if you have a considerable pressure on the drain side, but as I said, in real terms you will get good flow aslong as there is a decent pressure drop across the bearing housing. I reckon 2 bar against whatever you are likely to see in a crankcase will still be a good drop.

 

Unless the turbo seals are shot of course.

[JohnA]

I've never tried to measure oil pressure at the drain side, so I can't comment on this.

 

But when a turbocharger manufacturer specifies a small fraction of a psi as the maximum allowed crankcase pressure, I listen very carefully.

It's a free country after all, we don't all have to agree on everything - that would make it a very boring world, wouldn't it?

 

What the hell, let's be brave, vent to the atmosphere it is, lol...

[Digsy]

No mention of it in the tech notes on the Garret (Allied Signal . Honeywell) website.

 

Just for clarification, I don't advocate venting a breather system to atmopshere.

[JohnA]

That's why it's a 'gotcha'

 

Holset recommend 0.12psi acceptable, 0.2psi max. This really shows the need for vacuum to suck out the crank gases, doesn't it?

 

Thanks to forums like this one, this sort of information can be disseminated, and now you know

[JohnK]

So far all the talk has been about the breather on the left side of the engine (as you look in the bay) What about the breather on the right hand side that goes from the rockerbox cover to the plenum (sp?) Surely this is under pressure when in positive boost forcing air into the cover?

[JohnA]

...Surely this is under pressure when in positive boost forcing air into the cover?

 

You'd better hope it isn't mate.

There is a one-way valve fitted there, to avoid this very condition.

 

Check out a recent thread on the subject

[JohnK]

Where is the valve? It's not in the hose itself.

[Digsy]

Where is the valve? It's not in the hose itself.

Its an integral pasrt of the stub pipe in the cam cover. You have to take the hose off to see it.

[Digsy]

That's why it's a 'gotcha'

 

Holset recommend 0.12psi acceptable, 0.2psi max. This really shows the need for vacuum to suck out the crank gases, doesn't it?

 

Thanks to forums like this one, this sort of information can be disseminated, and now you know

Well, I have to say that I can't see how the physics work around this at all. The way I see it you can look at the lubrication circuit on a turbo in two ways:

 

1) There is a steady flow through the turbo with a high presure on one side, a huge pressure drop accross the bearing and then a residual oil pressure on the other side. However the oil exiting the turbo centre section is so whipped up that it is falling out under gravity (hence the pipe must be a large diameter and pointing downwards), but there will be flow through the bearing at all times, unless the crankcase pressure equals the oil supply pressure, which will never really happen.

 

2) The pressure drop accross the bearing is so high that you can consider the exit to be completely decoupled from the feed - i.e. as soon as the oil exits the bearing it is at crankcase pressure. Even in this case it would not matter what the crankcase pressure was because again the oil will fall down the drain pipe under gravity regardless of the pressure in the pipe. In fact even if the pressure in the crankcase and the oil drain pipe was almost the same as that in the turbo oil supply line, even though there would be hardly any flow, any oil that leaked through into the drain would still fall out under gravity alone.

 

But we both seem to have experience which contradicts the other, so I think we are going to have to aggree to disaggree on this one.

[JohnA]

.... Even in this case it would not matter what the crankcase pressure was because again the oil will fall down the drain pipe under gravity regardless of the pressure in the pipe. ...

I'm not sure you've got this from the right angle.

We want some pressure difference across the oil return pipe, right after the bearing and inside the sump, right?

 

There is not much oil pressure left after the bearing, so the oil needs to flow pulled by gravity.

Now imagine someone sealing the dipstick with an aircompressor fitting and pressurising it with 10bar or air. Would the oil return ever work? Not in a million years.

What if we pressurise it by 1 bar? Still nothing

It comes out that even 1 psi of pressure impedes the flow, that's all.

 

Venting an old and tired crankcase (full of blowby) to the atmosphere doesn't gurantee low enough pressure in the sump, intake vacuum is more effective, simple as that.

[Digsy]

There is not much oil pressure left after the bearing, so the oil needs to flow pulled by gravity.

Absolutely. Agreed.

 

Now imagine someone sealing the dipstick with an aircompressor fitting and pressurising it with 10bar or air. Would the oil return ever work? Not in a million years.

At 10 bar, no, it wouldn't work, because the pressure is way higher than the oil feed pressure.

 

What if we pressurise it by 1 bar? Still nothing

It comes out that even 1 psi of pressure impedes the flow, that's all.

OK, a pressure of 1psi will support an oil column of about 0.9m in height if the oil forms an effective self-seal in the drain pipe, but for this to happen the oil will have to be solid and in a relatively small diameter tube. I think, however, that with the foamy conditon of the oil coming out of the turbo and the large diameter of the drain pipe, the seal woud be poor enough that pressure would quickly equalize between the crankcase and the bearing housing and let the oil fall out. Once the pressure has equalised, then no matter how high it is, you will get a flow of oil under gravity. I suppose you might get some temporary airlocks and "glugging", which won't be good for overall flow but as the total pressure difference across the bearing housing is probably in the region of 30psi in favour of the oil flow, I doubt that the bearing lubrication will be seriously affected.

 

Venting an old and tired crankcase (full of blowby) to the atmosphere doesn't gurantee low enough pressure in the sump, intake vacuum is more effective, simple as that.

Yes. As I keep saying, I don't and never have said that venting a crankcase to atmosphere is a good thing.

[JohnA]

...I think, however, that with the foamy conditon of the oil coming out of the turbo .

I've heard this 'foamy' theory before, can't remember where.

I'm not sure where it originated, maybe one of Corky Bell's books or something similar.

I only treat these as extra resources, there are inaccuracies and mistakes in every single book I've read, some just typos others copy/paste mistakes (yes, I've seen those too, lol...)

 

I haven't seen this foam happen in real life, in fact it is sometimes difficult to tell if the oil moves at all within the tube because of the LACK of a single bubble to trace.

Maybe at high boost/load the foaming theory is more likely, I couldn't comment on that because I couldn't replicate this condition with myself underneath looking at the transparent hose (that may not have withstood those oil temps anyway)

[Andy-No-Knee]

The heater/aircon intake area is just below the windscreen, in a big wire mesh, and is funnily enough half covered by the bonnet. So fumes can and will and do get into the cabin.

[Alex]

The heater/aircon intake area is just below the windscreen, in a big wire mesh, and is funnily enough half covered by the bonnet. So fumes can and will and do get into the cabin.

 

It's a high pressure area on top of the bonnet in front of the screen...meaning the air is pushed into the cabin air feed from above. Also the undertray is pulling the air down and out from the engine bay, away from the cabin air feed.

 

So I still think the "issue" isn't infact an issue at all.

[Guest Usmann A]

What if your motor pulls too much Vacuum at idle, and starts to suck in the oil into the plenum, quite a bit .....