| your question. It has everything to do with the design of the turbo and how the pressures in the system work. When the car is under boost, the turbine seal is subjected to positive pressure from the "outside" of the lubrication system. This prevents oil from moving from the inside of the cartridge through the seal and into the exhaust tract where it will get burned. When you let out of the gas after boosting, the engine is typically at higher RPM and the oil pressure in the engine is higher as well. This pumps more oil at higher pressure into the cartridge, but this isn't the main contributor to the leaking. What happens in the exhaust without cats is lower backpressure on the turbine seal, which contributes to oil passing by the seal and into the exhaust tract to get burned. When you have cats in the system, they produce a degree of backpressure on the seal and this prevents oil from getting past the seal. Here is a picture of the rear turbine shaft seal bore:
 Here is a picture of the seal itself:
 As you can see, the rear seal is just like a piston ring. This seal fits into the bore of the housing and the ends of the ring are supposed to close up. If the seat bore is a little too big, the ends of the seal wont be within spec and allow some oil to leak. In addition to this, the seal also moves in and out, axially to the shaft and bore. This is due to the fact that the entire rotating assembly has thrust loads on it, of which the thrust bearings are there to support. There is always a small amount of axial shaft play on any turbocharger, even with BB turbos. This seal is designed to move in and out of the bore as the thrust loads change on the turbo while in operation. As the turbo moves from 0psi up to its maximum boost, the thrust loads change on the rotating group and the shaft will move from back to front as loads change. Because this seal has to be able to move as well as seal the oil inside of the cartridge, they call it a "dynamic seal". The operating conditions of this type of seal require that everything about it as well as its bore seat AND shaft seat must be perfect in order for it to work properly. Which brings me back around to my original statement - variations in the rebuilding processes used on these turbos is what accounts for the variation in success in using them without cats, and the nature of the components within the turbocharger are what account for the fact that they will smoke without backpressure (on a turbo that hasn't been built to proper specifications). I guess I am saying that if your Sport500 smokes without DP or TP, it is one of these semi-defective turbos. That conclusion is supported by the fact that not all Sport500s will smoke when you install DP or TP.
[ ashspecz.com ]
[ agpowers@bellsouth.net ] Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the tailor.
Albert Einstein
|
I am uploading my turbo rebuild images to help answer - 
