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TURBOCHARGERS
A turbocharger is a means of harnessing the energy in the exhaust gases of an engine, which would otherwise be wasted. The turbocharger is basically an exhaust driven air pump. The turbo is used to increase the volume of intake air that an engine sees. Usually an engine has to "suck" for air. This is why you can measure"vacuum" on an engine. A turbocharger has the unique ability of pressurizing the intake charge to push the volumetric efficiency of an engine over 100%. This means that if you have a 1.3 litre engine, if it were normally aspirated, and able to draw air unrestricted to obtain 100% volumetric efficiency that it would draw in 1.3 litres of air per revolution. Ambient air pressure is 14.7 psi. If the turbocharger were to run at 14.7 psi, it in effect would be doubling the amount of air (O2 being the important igredient) being fed into the engine. Power is in direct relation to the amount of air that moves through an engine, and therefore power is nearly doubled as well.
Turbochargers (or pumps) only build pressure when they are moving more fluid than the downstream side can handle. Pressure in this case can be thought of as a restriction to flow. An engine will normally accelerate by itself if it has free access to fuel and air. The throttle on an engine restricts the amount of air that an engine can have at any given time in order to maintain a certain RPM. A turbocharger builds boost, even under open throttle because it is supplying more air than the engine can use at that time. This is usually because the car is under load (moving the vehicle), and the engine cannot accelerate as fast as it can under no load conditions.
Think of the case where your vehicle is sitting parked, in neutral. If you rev the engine, you are lucky to build one psi of boost. This is because the engine can accelerate almost as fast as the air is supplied to it, and therefore no pressure builds in the intake manifold. If your engine would accelerate as quickly loaded, as it does unloaded, I'll bet you would see a lot less boost pressure than you normally do. The heat generated under loaded conditions also plays a factor as hotter exhaust contains more energy. Viscosuty of the hotter air also comes into play, but the effects may be negligible. Less viscous air has less mass to move the turbine, but also flows more easily and quickly. Time is another factor in building boost. The engine is usually not in the lower gears long enough for the turbo to build up full flow and pressure. That is why the most boost pressure is usually seen in gears 3,4 and 5.
Turbochargers, like other pumps have a pump curve. This is a plot of efficiency vs. speed. There will be an optimum speed range for the turbo to run at. Beyond that speed, flow drops off, the pressure ceases to increase, and more heat is generated. None of that is good for a turbocharged car. That is why turbocharger selection is so important. If the turbo peaks too early, the car runs out of breath at higher revs. If the turbo peaks too late, the car feels like a dog until the turbo comes into it's optimal operating range.