ok everyone need some help, 1st question why does a turbo have to spin at 120,000 rpm why? and second if my friend kept gettin oil in his exaust side of his turbo replacing it and nothing being worng wiht it what is goin on with his turbo please help qucikly!!!

 

Well, a turbo doesn't HAVE to spin at that high a speed. However, if you want it to work, it should. The faster a turbo spins, the greater pressure (boost) it is able to create within the plenum. If a turbo is sized too large for a particular situation (ie, not enough flow to spin the exhaust turbine fast enough to build pressure) the air will leak back around the intake turbine and create a situation called "surging" (often lumped into the "Turbo Lag" catergory). Smaller turbos spin up quickly, but can not force as much air, because the do not have the surface area. Larger turbos move greater volume, and can support greater pressures, but require more flow to spin up to max. Both are limited mainly be the laws of thermodynamics (HOT HOT HOT!).

As for the second question, I'm not sure what you mean. Are you saying oil was leaking from the exhaust side, so he replaced his turbo, and the oil is still leaking? Or are you saying that, after swapping the turbo, oil is still coming out of the exhaust pipe, and you believe it to be coming from the turbo?