If the wheel has signs of FOD or Foreign object damage, IE: chipped or bent blades, the turbo needs to be rebuilt by a reputable turbo repair shop. (Limit, PTE, Forced Inductions) Simply changing the compressor wheel will not fix the hidden problems that might have already occurred. The rotating assembly can typically be dynamically balanced, or in most cases, each part would be balanced from the manufacturer. Whenever something happens to either the compressor wheel or the turbine wheel, (something hits the wheel and causes a blade to become chipped or broken) the rotating assembly becomes un-balanced. Over a period of time, this un-balanced condition will cause wear on the journal bearings, the thrust bearing and the oil seals. If not addressed soon enough, the rotating assembly will get worse and worse, eventually leading to the blade fins coming into contact with either the compressor housing, or the turbine housing. This is what is referred to as "rubbing". It's all down hill from that point. If the compressor wheel continues to rub on the compressor housing, blade and comp cover material will make it's way into the motor and eventually into the oil system and straight to the bearings of the motor. On an intercooled car, this material can get stuck in the intercooler, depending on the type of core the IC has, (flushing of the intercooler is highly recommended at that point), but on a non intercooled car, this material will go straight into the motor, circulate throughout the oil system, possibly causing damage to piston rings, and eventually bearings. This type of oil contamination can get stuck into your factory oil cooler located in the factory radiator. The factory oil coolers can not be thoroughly flushed out, they need to be replaced or totally eliminated if you've had a motor fail or bearings fail, and in some cases a severely blown head gasket. This FOD in the oil cooler will eventually leach back into the motor over time.
Sorry to be so long worded. I just see people make these mistakes all the time and they wonder why their brand new turbo keeps failing or their fresh rebuilt motor takes a dump after only 2k miles of driving.
HTH
Patrick
