964-C2

And inside the turbo mount, is it just an open 3-way connection between the turbo oil exit / turbo oil return to sump hose / AOS connection ??

(I am sure this has been discussed earlier, I just could not find any info when I tried searching.. Sorry).

lee101315

That is the vent for the turbo drain. What do you have in mind? Are you planning on bypassing it or making sure the hose seals properly?

TonyG

The air oil separator (AOS), separates the blow by into air and oil.

The air gets recirculated into the intake.

The oil gets drained back into the drain pan through the pipe from the side of the AOS, to the motor mount, back to the oil pan.

TonyG

964-C2

Are you sure about this??
Does oil really go from the AOS to the turbo mount, then to the sump?

TonyG

Yes.

There is a hose from the side of the AOS which runs down to the side of the motor mount, where the plenum is. It's the same plenum that the turbo drains into.

TonyG

Thom

No.

The line between the turbo mount and the AOS is to create a vacuum past the turbo CHRA so that oil flows through it. This is needed when the oil return line to the sump ends below the oil level in the sump.

TonyG

That's incorrect.

The turbo doesn't need vacuum to drain. It needs gravity which it has (just like 911's which only use gravity to drain their turbos into their little sumps).

The reason the oil drain is below the oil level in the pan is so that crank case pressure doesn't affect it's ability do drain. If it were above the oil level, crank case pressure would pressurize the drain and it wouldn't drain.

As another example, I always plumb in an aftermarket AOS/Catch Can, which drains to below the oil level in the pan so that I don't have to deal with emptying it at the track. Works perfectly without anything creating a vacuum (and keeps the intake tract bone dry btw)

TonyG

Thom

If you remove this line, the oil won't drop down through the turbo because the air below would have nowhere to go, and it has nowhere to go because the return line is below the oil level in the sump.

There is always vacuum in this line, unless the top of the AOS is not connected to the turbo inlet.

TonyG

Oil is heavier than air. It will fall through any air in the line, and will sit in the oil return line at the exact level of the oil in the pan.

There is no pressure in the line because it's below the oil level.

You don't need a vacuum in a drain line for it to drain. In fact, look at every other turbo application out there . None have a vacuum line connected to the turbo oil drain. All use gravity to drain.

And like I said... we also take an aftermarket AOS/catch cans, and plumb them down to the bottom of the oil pan below the oil level. They all drain great.

TonyG

Thom

Precisely, and this why oil fumes are "vented" from the turbo mount to the AOS through this line, but not the other way around.

TonyG

Oil fumes?

Really?

What happens to the oil from my AOS/catch can? It too drains to the bottom of the oil pan.

And the 911... which gravity feed to a sump (with oil in it), then must rise above a certain level to get to its scavenge pump).

And.... in case you weren't aware... a lot of the 951 high end race engines don't even have the factory AOS, and yet their turbos dump straight down into the same place... with the same motor mount.

TonyG

Thom

Oil fume or whatever you call the air/oil mixture that takes shape below the turbo or anywhere else in the crankcase, the point being that the crankcase should not be open to atmospheric pressure when the engine is running.

As you said the turbo oil return line acts as separate "oil sump" for the turbo, but since it's separate from the crankcase, the purpose of the line between the mount and the AOS is to balance the pressure/vacuum produced by the pistons moving in the crankcase.

As the AOS is routed back to the compressor inlet, there is always going to be vacuum in the line linking the AOS to the turbo mount line when a 951 engine is running correctly, and it makes no sense to say that "The oil gets drained back into the drain pan through the pipe from the side of the AOS, to the motor mount, back to the oil pan.", because there is already a hole at the bottom of the AOS that does just that. You can read the threads about turbine seals leaking at idle, which happen to stop leaking when just adding vacuum into the AOS.

Btw I don't question race engines, since they are not mostly designed for long term reliability.

TonyG

Nobody said anything about the crankcase being open to atmosphere.

I never said the any oil line acts like a oil sump.

And no... the purpose of that line is not to balance the pressure by the pistons moving up and down.

No. Only when the engine is under boost, will the line from the the top of the AOS have a vacuum on it. And the quantity of vacuum is never very much because it's created by a simple venturi layout (a pipe 90 degrees to the air going into the turbo).

When the engine is not on boost, the the intake charge completely bypasses the turbo. And with the turbo not spooled, no venturi effect is created on the line to the AOS which is directly in front of the turbo. So there is no vacuum created.

Thus it's incorrect to say that there is always vacuum present on the AOS.

Further, the quantity of crankcase blowby will far exceed the small vacuum created by the venturi of the hose being in front of the turbo. By far.

The top of the AOS to the intake tract is what would normally be the PCV setup. Except that with the config of the turbo layout you don't need the 1-way PCV valve.



Go cut apart a AOS

Most of what's written on the internet is written by people that shouldn't even be working on their cars.

With respect to turbo seals leaking:

Not all turbos seal the same for various reasons. Some far worse than others.

But if the turbo seals are functioning properly, you don't need vacuum to get the turbo to seal. Even saying that vacuum could physically cause a turbo to seal is silly. At best, vacuum under the turbo would cause a leaky turbo not to leak oil into the intake track and would to some extent lessen the resulting smoke at idle.

In that case... get the turbo built properly and the problem goes away.


TonyG

Thom

Well, since you sound like someone who wants to know better than the factory engineers, I will leave you with your certainties.

Perhaps you'd need to look at a stock engine with the original unmodified breather system to see how it works.