Journal bearings: Hollow brass sleeves suspended in oil that allow the turbo shaft to spin freely inside the centersection.
Mass flow rate: A representation of how much air (based on density) is being output by the turbo for use by the engine.
Oil restrictor: A device that reduces the amount of oil delivered to a ball-bearing turbo because it requires less oil pressure than a stock journal-bearing unit.
Oil supply/return lines: Routes engine oil to and from the turbo to lubricate the bearings in the centersection.
Pressure ratio: The absolute compressor outlet divided by the absolute inlet pressure and represented on the left side of a compressor map.
Shaft: The piece that travels through the centersection and connects the turbine wheel to the compressor wheel.
Shaft play: Condition in a worn turbo where the shaft is allowed to move (other than spin). Signs of play indicate the turbo is ready to fail by allowing the fan blades to contact the housings or other problems. An abnormal whine or scraping sound can be a sign that a turbo is suffering from shaft play.
Spool: Another term for turbo boost. A turbo is spooled up when it is creating boost in the intake manifold.
Supercharger: Unlike a turbocharger, it is powered by a belt on the front of the engine that spins a compressor to create boost. It's less efficient than a turbocharger and is not used in light-duty diesel vehicles.
Surge: When boost pressure builds up to the point that it causes the compressor wheel to stall. This can be prevented with the use of bypass valves. Also known as bark.
Surge line: The left boundary of a compressor map that represents operating conditions where a surge is likely.
Trim: The ratio of the inducer area divided by the exducer area.
Turbine wheel: A wheel that is spun by exhaust gases that pass through the fins and into the housing before dumping into the exhaust pipe.
Turbo lag: The time it takes for a turbo to spool up after the throttle has been increased.
Turbo speed lines: Lines on a compressor map that represent the rotational speed of the compressor wheel.
Turbo timer: An electronic device that keeps an engine running before shutdown to ensure EGT is low enough to prevent oil from cooking in the centersection.
Twin turbos: A system using two turbos mounted in parallel.
Variable-geometry turbo (VGT): A turbo that uses variable vanes or a sliding nozzle to alter the volume inside the exhaust housing to maximize turbo speed at low engine rpm. Also known as variable-turbine geometry (VTG).
Variable nozzle: A sliding nozzle in the exhaust side of the turbo that can reduce the volume around the turbine fan blades to increase turbo rpm at low engine loads.
Variable vanes: Adjustable blades that route exhaust gases directly into the turbine wheel at low engine rpm to increase spool on tap at low speeds.
Wastegate: Bypass that diverts excess exhaust gases away from the turbine once a preset boost level is reached in the compressor side of the turbo. It can be built in to the exhaust-turbine housing (internal) or can be separate from the turbo housing (external).
Water-cooled turbo: A turbo assembly that incorporates channels in the centersection to circulate engine coolant to keep the bearing assembly and related parts cool and prevent heat-soak from the exhaust turbine and housing.