For the sake of clarity, the twin-turbo installations in the six cylinder 3.0i and 3.0d engines differ as follows.
In the 3.0i Petrol the turbos are identical, light pressure units mounted in parallel, meaning that one turbo is fed independently from 3 cylinders and the other is fed, also via a separate exhaust manifold, by the other three cylinders. Neither cylinder has any influence on the level of boost created by the other. It is a charmingly simple approach with devastating results.
In the 3.0d Diesel powered engines, the turbo's are mounted sequentially off a single exhaust manifold fed by all six cylinders. The arrangement is complex and requires some deft electronic control. At low rpm, the engine only produces enough volumetric flow to speed the turbine of the low pressure turbo. This turbo, as it spools up, supplies pressurised air to the engine which then produces greater volumetric flow. This causes the larger turbo to spool up and deliver the desired amount of boost pressure at higher rpm. The interaction between the two turbos is electronically controlled. This enables this 3.0d engine to push out a superlative 200 kW for its displacement and enables it to rev to an unheard-of-for-a-diesel 5000 rpm.
Here is a cut-away image of the sequential turbo installation.
