There is a drawback with wastegate turbo. A small unit spins up quickly given good low speed response, but won’t handle high volumes of exhaust and air under full engine load. V large turbo gives high load performance, but poor response at low speeds. So the best compromise of size is fitted to the engine.
Variable nozzle turbo (TD4, TDV6, and TDV8) overcome this compromise by varying the angle of the inlet nozzle that direct exhaust gas on to the turbine. At low engine speed low exhaust flow, the nozzle close in, increasing the gas speed over the turbine blades, speeding the turbine and producing a quick high boost pressure that provides low speed torque. At high engine speeds load (high exhaust flow) the nozzle open out, maintaining sufficient boost for high torque while preventing over boost. This reduces exhaust back pressure, so less engine power is used pumping exhaust out.
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