Brendan,

With a turbo mounted below the sump level, you need a pump to return the oil from the turbo to the sump. Some use mechanical pumps driven off of a pulley while others use electric pumps. The oil pressure on the 928 is very high compared to alot of cars. The pressure needs to be reduced in some cases to between 40 - 55 psi. That can be achieved by a restrictor at the oil inlet on the turbo or better yet, by a bypass hose because you want to reduce the pressure but keep the volume up as high as you can comfortably scavenge. The restrictor can be made interchangeable to make the trial and error part easier. Drill inside your inlet fitting and tap it for 1/8" pipe thread. Then take some 1/8" socket head pipe plugs and drill a tiny hole in the center. Now this can be installed into the fitting and you can swap it out for a larger or smaller size as needed. ( I believe .080" is a good starting point. But I'll have to look at my notes ) For the bypass hose you would put the restrictor in the inlet to the hose and the hose would tee into the inlet and outlet of the turbo oil ports. This allows some oil to bypass the turbo altogether and lowers the pressure of that oil. Some turbo's are more susceptable to bypassing oil internally causing the turbo to blow smoke. There are lots of ways to eliminate this. One thing you can do is to install a small catch can at the oil outlet of the turbo and scavenge from there.

Mine has an electric scavenge pump hooked directly to the oil outlet and I don't have a problem with smoking. Once, I tried to tee into the oil return line and ran the other leg up to the oil catch/fill can ( 16V fill location ) to help get rid of any oil pushed up there. I immediatly started having problems with my turbo smoking, even at idle. I guess the extra hose running to the catch/fill can eliminated the pumps ability to scavenge the oil properly from the turbo. ( Kind of like when you get a hole in the side of your drinking straw and can't get much of the drink to come up into your mouth.)

 

I was thinking of doing this on a track car for fun. Do you have any pictures of your sump and scavenge pump?

Wouldn't a restrictor increase the pressure through that part of the line?

 

The restrictor cannot increase system pressure. The oil supply will have a higher pressure upstream of the restrictor but downstream, at the turbo, it will be much lower. The bypass is a better option all around.

I don't have any pics here. The car is currently at the paint shop and I lost my hard drive a couple of months ago and all my pics along with it. I'll see if I can't get you a pic soon. I will tell you that it's not much to look at. Just an oil supply line from the engine block ( oil cooler port ) to the turbo, then out of the turbo to the electric scavenge pump then back to the sump. Nothing fancy or difficult.

I will reroute it eventually as I plan to install an oil cooler.

 

If you could find them, the trick would be to get air foil bearings...no oiling necessary and faster spool times to boot.

 

Or superconductors and magnetic bearings

Do these really even exist in an automotive turbo?

 

Yes, actually there are a couple companies who have built turbos using air foil bearings, but they are not for the general market. Foil bearings have been used in large turbo machinery (read: jet engines) for some time, so obviously the science is sound, it's just finding them in the right size that would be the tricky bit. Check out Mohawk innovative technology, or just go here:

http://www.miti.cc/newsletters/06_oi...plications.pdf

That was in '99 and those guys were only doing it as a proof-of-concept for building the next generation of jet engine, but I believe toyota has at least done some looking into it. The reason why it isn't prevalent is that the bearings are too expensive to use on the mass market, when they aren't sizing the turbos properly for racing enthusiasts anyway, and the oiling system is well-understood. However for a one-off type of thing, especially a rear-mount turbo, where EGT is much lower, you need all the help you can get. I'll look into it once I get other problems sorted, but there are several companies which make custom air foil bearings, so I'll grab a turbo out of a junkyard and pull it apart and see what it needs and if it can be done, and if so, minimum units/price etc. First priority though is getting the car to stop burning oil.

 

I plan on the restrictor/ pump out of sump method. Fabio is right on the money.