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Variable geometry turbochargers (VGTs) are a family of turbochargers, usually designed to allow the effective A/R ratio of the turbo to be altered as conditions change. This is done because optimum A/R at low engine speeds is very different from that at high engine speeds. If the A/R ratio is too large, the turbo will fail to create boost at low speeds; if the A/R ratio is too small, the turbo will choke the engine at high speeds, leading to large exhaust manifold pressures, high pumping losses and ultimately lower power output. By altering the geometry of the turbine housing as the engine accelerates, the turbo's A/R ratio can be maintained at its optimum. Because of this, VGTs have a minimal amount of lag, have a low boost threshold and are very efficient at higher engine speeds. In many configurations, VGTs do not even require a wastegate, however this depends on whether the fully open position is sufficiently open to allow boost to be controlled to the desired level at all times. Some VGT implementations have been known to over-boost if a wastegate is not fitted.

The most common implementation is a set of several aerodynamically-shaped vanes in the turbine housing near the turbine inlet. As these vanes move, the area between the tips of them change, thereby leading to a variable A/R ratio. Usually, the vanes are controlled by a membrane actuator identical to the one on a wastegate, although electric servo actuated vanes are becoming more common.

The first production car to use these turbos was the limited-production 1989 Shelby CSX-VNT, equipped with a 2.2L petrol engine. The Shelby CSX-VNT utilised a turbo from Garrett, called the VNT-25 because it uses the same compressor and shaft as the more common Garrett T-25. This type of turbine is called a Variable Nozzle Turbine (VNT). Turbocharger manufacturer Aerocharger uses the term 'Variable Area Turbine Nozzle' (VATN) to describe this type of turbine nozzle. Other common terms include Variable Turbine Geometry (VTG), Variable Geometry Turbo (VGT) and Variable Vane Turbine (VVT).

The 2006 Porsche 911 Turbo has a twin turbocharged 3.6-litre flat six, and the turbos used are BorgWarner's Variable Turbine Geometry (VTGs). This is significant because although VGTs have been used on advanced turbo diesel engines for a few years and on the Shelby CSX-VNT, this is the first time the technology has been implemented on a high production petrol car (only 500 Shelby CSX-VNTs were ever produced). Exhaust temperatures in petrol cars are much higher than in diesel cars and this normally has adverse effects on the delicate, moveable vanes of the turbo. BorgWarner engineers however have managed to combat this problem with the new 911 Turbo.

• How does Variable Turbine Geometry Work?
• Variable Turbine Geometry (VTG) explanation with pictures