odonnell

I'm trying to figure out a good approach for the vent line that goes to the turbo mount/drain. In short, I have no choice but to use an NA AOS, since my block is not machined for the different sized ports of the real 951 AOS.

Summary: https://rennlist.com/forums/944-turb...nstorming.html

I've been going over my options for integrating that vent line into the AOS but in order to respect the original entry angle (which has to do with the vortex of the vapor inside the AOS), I'm basically limited to plastic welding a nipple on.

So I was thinking about going to a catch can instead, and leaving the AOS the way it is...one less thing to worry about failing due to a mod gone wrong. My concern is that if there's a relatively high amount of oil actually being sucked out of the drain, that would quickly fill up my catch can. With the AOS obviously it just drains back to the sump.

Anyone have a rough idea of how much oil actually would be sucked out? Or if having a return line from the catch can to the block is a viable option?

V2Rocket

I don't think any oil actually goes through there other than the odd bit of mist.

I think it's more of a pressure equalizer from the crankcase to aid draining the turbo oil to below sump level.

odonnell

Someone tell me if any of this is flawed logic.

The vacuum hose going to the AOS plugs into the intake plenum boot, which is before the turbo. Therefore the pressure in that hose can be no higher than atmospheric. Even under boost, the air demand from the turbo probably creates lower pressure than atmosphere inside that plenum. It's probably close to atmosphere though.

Now I'm picturing how the AOS is managing the crankcase pressure. There's a tendency for pressurization, especially under boost, so the AOS is the relief - at worst, it's atmospheric.

I've heard that not having the vent from the turbo drain to the AOS causes smoking under boost. I'm wondering if perhaps the crankcase pressure, which is acting on the surface area of the oil in the pan, is pushing or pulling the oil column in the drain tube up or down. Under high boost, you would expect the crankcase pressure to be higher than normal...you have more blowby and the AOS has more work to do to relieve that. Maybe the oil is actually being forced UP the drain tube. All it would take is a fraction of a millimeter of the oil level in the sump to be depressed by slightly-higher-than-atmosphere pressure and it could easily be forced up the tube a few centimeters. The oil pressure coming out of the turbo can't be that high at all to resist this.

What if the turbo mount scavenge tube is actually there to suck up some of that oil before it gets to the turbo?

V2Rocket

My understanding is that there is a chamber inside the engine mount (this is hearsay - I have not cut up a turbo mount) to which the AOS hose is connected.

The oil draining from the turbo CHRA goes into this chamber via gravity. Crankcase pressure is introduced via the AOS hose. The oil which drains into the pan enters below sump level, draining via gravity only. So having crank pressure on the "top" and "bottom" end of the drain cancels out so only gravity is making the oil flow down, rather than fighting "pressurized" oil coming back up.

odonnell

See I thought about that, but the area over which the pressure applied is varied greatly. The oil in the drain tube is maybe a square inch or so, where the area of the oil in pan is hundreds of times more. So just like how you can use a hydraulic jack to raise a car using only hand strength (Pascal's principle, force = pressure x area), the pressure inside the mount would have to be a multiple of the pressure in the crankcase to achieve balance.

V2Rocket

I think of it like an FPR...manifold pressure on the outlet side of the injector fights against the fuel pressure in the rail, unless you give it a manifold signal on the "other side" of the fuel pressure to cancel out.

Fuel injector nozzle is tiny compared to even the 3mm vacuum hose carrying the MAP signal, and the FPR diaphragm is pretty honkin big in comparison to both.

odonnell

I did a little research and it seems like a common thing on other cars with the drain below the oil level. The logic seems that because the negative crankcase pressure can't make it up the drain tube (since it's full time full of oil), an auxiliary vacuum source has to be added to help draw the oil through the turbo. Makes sense now. And to my original post, it looks like this is operable.