Adiabatic Efficiency

A 100% adiabatic efficiency means that there is no gain or loss of heat during compression. Most turbochargers will have a 65-75% adiabatic efficiency. Some narrow range turbo's can get higher, these types of turbo's work well in engines that operate over a narrow rpm range. In general the wide range turbo's don't have as good peak efficiency, but have better average efficiency and work better on engine that operate over a wide rpm range.

Pressure Ratio

This is the inlet pressure compared to the outlet pressure of the turbocharger's compressor. For single stage turbo's, the inlet pressure will usually be atmospheric (14.7 psi) and the outlet will be atmospheric + boost pressure. For staged turbo's the inlet pressure will be the outlet pressure of the turbo before it + atmospheric, and the outlet will be inlet pressure + additional boost from that turbo.

Density Ratio

Turbochargers compress the air to make it more dense, this is what allows more oxygen in the engine and give the potential to make more power. The density of the inlet air compared to the density of the outlet air is the density ratio.

Turbine

The side of the turbocharger that converts the energy of the exhaust into mechanical energy to turn the compressor.

Compressor

The side of the turbocharger that compresses the incoming air charge and directs it to your engine.

Cartridge

This is the center section of your turbocharger. It houses the bearings for your turbocharger, they have oil passages to lubricate the bearings and some have water jackets for water cooling.

Intercooler

When intake air is compressed by a turbocharger it is also heated, even more so than when supercharging due to the turbo being heated by the exhaust. Hot intake air is not good for power and will increase the chance of detonation. An intercooler reduces the intake temperature by pushing the air through a heat exchanger (much like a small radiator) that absorbs some of the heat out of the charge. With less heat, you'll need less boost pressure to get the desired power and decrease the chance of detonation. Anything that reduces the intake temperature is a big plus in a supercharged engine.

Turbo Lag

A turbocharger uses a centrifugal compressor, which needs rpm to make boost, and it is driven off the exhaust pressure, so it cannot make instant boost. It is especially hard to make boost at low rpm. The turbo takes time to accelerate before full boost comes in, it is this delay that is known as turbo lag. To limit lag, it is important to make the rotating parts of the turbocharger as light as possible. Larger turbo's for high boost applications will also have more lag that smaller turbo's, due to the increase in centrifugal mass. Impeller design, and the whole engine combo also have a large effect on the amount of lag. Turbo lag is often confused with the term boost threshold, but they are not the same thing, lag is nothing more the the delay from when the throttle is opened to the time noticeable boost is achieved.

Turbo Boost

Usually measured in pounds per square inch, it is the pressure the turbocharger makes in the intake manifold. One of the ways to increase airflow through a passage is to increase the pressure differential across the passage. By boosting the intake manifold pressure, airflow into the engine will increase, making more power potential. Boost is also measured in Bar. One Bar equals 14.7 psi.

Boost Threshold

Unlike turbo lag, which is the delay of boost, boost threshold is the lowest possible rpm at which there can be noticeable boost. A low boost threshold is important when accelerating from very low rpm, but at higher rpm, lag is the delay that you feel when you go from light to hard throttle settings.

Wastegate

The wastegate is a valve that allows the exhaust gasses to bypass the turbine. The waste gate relies on boost pressure to open it. Spliced into the wastegate pressure feed there must be some form of pressure bleed. By bleeding pressure to the wastegate, it is possible to control the amount of boost by reducing the pressure at the wastegate.

Turbo Cool Down

A turbocharger is cooled by engine oil, and in many cases, engine coolant as well. Turbo's get very hot when making boost, when you shut the engine down the oil and coolant stop flowing. If you shut the engine down when the turbo is hot, the oil can burn and build up in the unit (known as "coking") and eventually cause it to leak oil (this is the most common turbocharger problem). It is a good idea to let the engine idle for at least 2 minutes after any time you ran under boost. This will cool the turbo down and help prevent coking