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Just to clarify a couple of technical terms that get banded about, so when people are talking away you know full well what they are on about!
A/R - A/R describes a geometric property of all compressor and turbine housings, it relates to the inlet/exit area of the housing to the radius upon which the fluid tangentially exits/enters the housing. Increasing compressor A/R optimises the performance for low boost applications. Changing turbine A/R has many effects. By going to a larger turbine A/R, the turbo comes up on boost at a higher engine speed, the flow capacity of the turbine is increased and less flow is wastegated, there is less engine back pressure, and the engine volumetric effeciency is increased resulting in more overall power.
Cartridge (CHRA) - Centre housing rotating assembly, includes a complete turbocharger minus the compressor and turbine housing, and actuator.
Clipped Turbine Wheels - When an angle is machined on the turbine wheel exducer (outlet side), the wheel is said to be 'clipped' or 'cut back'. Clipping causes a minor increase in the wheel's flow capability, however it dramatically lowers the turbo efficiency. This reduction causes the turbo to come up on boost at a later engine speed (increased turbo lag).
Free-float - A free floating turbo charger has no wastegate device, so can not control its own boost levels, therefore an external wastegate must be installed.
GT - The GT designation refers to Garrett's state of the art turbocharger line. All GT turbo's use modern compressor and turbine aerodynamics which represent huge efficiency improvements over the old T2, T3, T3/T4, T04 products. The net result is increased durability, higher boost levels, and more engine power over the old product line.
Trim - Trim is an area ratio used to describe both turbine and compressor wheels. Trim is calculated using the inducer and exducer diameters. As trim is increased, the wheel can support more air/gas flow.
Wastegate - A wastegated turbo charger includes an integral device to limit turbo boost. This consist of a pneumatic actuator connected to a valve assembly inside the turbine housing. By connecting the pneumatic actuator to boost pressure the turbo is able to limit its maximum boost output. The net result is increased durability, quicker time to boost, and adjustability of boost.
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