Turbo Charger
Turbo system discussion invariably begins with the heart of the system, the turbocharger itself. Conventional naturally aspirated engines use the movement of pistons or a rotor to vary the volume of a combustion space. When the volume varies, the engine achieves aspiration (the act of breathing), with the pistons/rotor performing the duty of pulling air into the engine, compressing it for combustion, and then expelling waste gases. A turbocharger is a type of engine, except combustion is handled externally; in this case, combustion is happening inside the engine. Combustion waste gases spin an impeller (pictured) much like a river spins a waterwheel. Here, the housing is made of a material suitable for the handling of hot exhaust gas, in this case, cast iron. Gas flows into the impeller housing (or "turbine housing"[/i]) through the flange at the 6 o'clock position in this photo. The gases flow around the interior of the snail-shaped turbine housing and exit out the center. Typically, the center exit is bolted to a conventional exhaust system. The more exhaust gas the engine creates, the faster the turbine can spin, and thus the more work it can do.

A shaft conjoins the "hot" turbine side of the turbo to the "cold" compressor side. (No gears or mystery mechanisms inside the turbo ?it's just a shaft bathed in engine oil.) While the turbine only moves exhaust gas, the compressor only moves fresh air. It spins in the opposite direction from the turbine when viewed face-on, and the flow of air is also opposite, with intake in the center and output on the side, much like a hair dryer. Imagine placing your hand over the throttle body of a running engine ?normally, you'd feel vacuum ("suction"[/i]). With a properly selected turbocharger in place, this vacuum is replaced with pressure ?lots of pressure, as there is enough leftover energy in exhaust gas to spin the turbo at speeds that can apply 30 PSI and more of air pressure to the intake valves, known as "turbo boost." With positive intake pressure, not only is the engine breathing easier, it is actually getting force-fed.
article from
http://overboost.com/story.asp?id=1048to be continued...