Variable Turbine Geometry (VTG)

Variable Turbine Geometry technology is mostly used in turbo diesel engines, but there is no evidence that it could not benefit petrol engine. It is said that (don't ask me why): turbine makes best use of exhaust gas flow if the latter hit the blades at right angle under low speed, and at narrow angle under high speed. Variable Turbine Geometry mechanism therefore varies the direction of the exhaust nozzle according to speed, thus improve the acceleration of turbine.
Another Variable Turbine Geometry alters the cross-sectional area through which the exhaust gas flows, thus controls the amount of boost pressure. This is implemented by adjusting the position of guide vanes inside the turbocharger. At lower engine speeds, they restrict the flow and therefore increase boost pressure; at higher engine speeds they open wide and reduce the exhaust back-pressure.

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Golf mkII ´91 GT Turbo Diesel Intercooler
VW Golf Diesel site