The Wastegate valve can be "internal" or "external". For internal Wastegates, the valve itself is integrated into the turbine housing and is opened by a turbo-mounted boost-referenced actuator.
An external Wastegate is a self-contained valve and actuator unit that is completely separate from the turbocharger.
In either case, the actuator is calibrated (or set electronically with an electronic boost controller) by internal spring pressure to begin opening the Wastegate valve at a predetermined boost level.
When this boost level is reached, the valve will open and begin to bypass exhaust gas, preventing boost from increasing.

BOV and a Bypass Valves are mounted on the intake pipe after the turbo but before the throttle body. A BOV's purpose is to prevent compressor surge. When the throttle valve is closed, the vacuum generated in the intake manifold acts on the actuator to open the valve, venting boost pressure in order to keep the compressor out of surge. Bypass valves are also referred to as compressor bypass valves, anti-surge valves, or recirculating valves. The bypass valve serves the same function as a BOV, but recirculates the vented air back to the compressor inlet, rather than to the atmosphere as with a BOV.

A properly assembled and balanced turbo requires no specific break-in procedure. However, for new installations a close inspection is recommended to insure proper installation and function. Common problems are generally associated with leaks (oil, water, inlet or exhaust).

Shaft play is caused by the bearings in the center section of the turbo wearing out over time. When a bearing is worn, shaft play, a side to side wiggling motion of the shaft occurs. This in turn causes the shaft to scrape against the inside of the turbo and often produces a high-pitched whine or whizzing noise. This is a potentially serious condition that can lead to internal damage or complete failure of the turbine wheel or the turbo itself.

My turbo is sounding like a sewing machine's whistle a distinct cyclic noise cause by unstable compressor operating conditions known as compressor surge. This aerodynamic instability is the most noticeable during a rapid lift of the throttle, following operation at full boost.

I want to make x horsepower so select a turbocharger to achieve desired performance. Performance includes boost response, peak power and total area under the power curve. Further decision factors will include the intended application. The best turbo kit dictated by how well it meets your needs. Kits that bolt on without any modification are best if you don't have fabrication capabilities. Less refined kits can be cost effective if you access to fabrication capabilities.

Oil requirements depend on the turbo's bearing system type. There are two types of bearing systems; traditional journal bearing; and ball bearing. The journal bearing system in a turbo functions very similarly to the rod or crank bearings in an engine. These bearings require enough oil pressure to keep the components separated by a hydrodynamic film. If the oil pressure is too low, the metal components will come in contact causing premature wear and ultimately failure. If the oil pressure is too high, leakage may occur from the turbocharger seals. An oil restrictor is generally not needed for a journal-bearing turbocharger except for those applications with oil-pressure-induced seal leakage. Remember to address all other potential causes of leakage first (e.g., inadequate/improper oil drain out of the turbocharger, excessive crankcase pressure, turbocharger past its useful service life, etc.) and use a restrictor as a last resort. Restrictor size will always depend on how much oil pressure your engine is generating-there is no single restrictor size suited for all engines.

Ball-bearing turbochargers can benefit from the addition of an oil restrictor, as most engines deliver more pressure than a ball bearing turbo requires. The benefit is seen in improved boost response due to less windage of oil in the bearing. In addition, lower oil flow further reduces the risk of oil leakage compared to journal-bearing turbochargers. Oil pressure entering a ball-bearing turbocharger needs to be between 40 psi and 45 psi at the maximum engine operating speed. For many common passenger vehicle engines, this generally translates into a restrictor with a minimum of 0.040" diameter orifice upstream of the oil inlet on the turbocharger center section. Again, it is imperative that the restrictor be sized according to the oil pressure characteristics of the engine to which the turbo is attached. Always verify that the appropriate oil pressure is reaching the turbo.

The use of an oil restrictor can (but not always) help ensure that you have the proper oil flow/pressure entering the turbocharger, as well as extract the maximum performance.