Turbocharger Glossary:
A/R: A formula used to determine the size of the compressor and turbine housing configuration. The letters stand for Area divided by Radius, where Area is the cross-sectional area of the scroll (the finned wheel), and Radius is the shortest distance from the centerline of A to the centerline of the turbine shaft. The basic rule of thumb about A/R: A turbo with a smaller A/R value will spool up quicker and be more responsive, though it won't produce as much peak power as one with a larger A/R value. The turbo with the larger A/R value has the greater capacity to produce power, but will spool up slower and exhibit turbo lag.
Bar: A means of expressing air pressure, like psi. A measurement of 1.0 bar translates to 14.7 psi, which is equivalent to normal atmospheric pressure at sea level.
Bearings: The shaft between the turbine and compressor wheel rides on bearings inside the turbo's center housing. In a standard turbo those bearings are made of metal, and a thin film of oil separates the shaft from the bearings. These bearings do wear, but they can be rebuilt if damaged. Some turbos use ball bearings in the center housing instead of metal bearings, which reduces friction and speeds up the turbo's response time. Ball-bearing turbos are pricey to rebuild, though. If you damage a ball-bearing turbo, you may have to sell your sister to rebuild it.
Blow-Off Valve: A device used to release excess pressure in the intake system between the compressor wheel and the throttle plates, to prevent compressor surge. Also known as a Pop-Off Valve.
Boost: A measure of the air charge sent from the compressor to the combustion chamber, typically indicated in either pounds per square inch (psi) or bar.
Compressor: The finned wheel on the intake side of the turbocharger that is driven by the exhaust-side turbine. It sucks fresh air through the center inlet, compresses it, and sends it to the combustion chambers. (Sometimes by way of an intercooler.)
Compressor Surge: A condition caused by compressed intake air caught between the compressor wheel and closed throttle plates. When you get off the throttle after a burst of acceleration, the plates close but the compressor wheel may still be sending compressed air into the system. If the compressed intake charge cannot flow through the throttle plates, it will flow back to the compressor, which could result in bearing damage.
Downpipe: The pipe that carries exhaust gas away from the turbine side of the turbocharger and into the exhaust system. A large-diameter, free-flowing downpipe eases backpressure that could impede the exhaust flow at the turbine and slow it down.
Housings: The snail-shaped enclosures inside of which the turbine and compressor wheels spin. Like A/R, housing shape and size have a tremendous effect on the turbocharger's output, as well as its ability to dissipate heat. A turbo with a larger housing can produce higher boost, and therefore higher horsepower, but is generally slower to spool. A turbo with a smaller housing spools quicker but cannot make high levels of boost.