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The 2.5 V6 VAG diesel engine is not generally a useful prime mover when compared with the 4 cyl 1.9 PD for several reasons:
It's an old tech engine with a rotary pump. Injection pressures are low and the specific fuel consumption very high. Compare the CO2/tax ratings for an auto version of a Passat/Superb when fitted with with each engine. Then compare the performance differences - there's not a lot in it.
Slush box autos are a useful test bed (possibly their only use when mated to a diesel) for comparative engine performance figures because they are so very lossy and therefore make the engine work hard. Under such conditions differences in specific fuel consumption and CO2 emissions are thrown into sharp relief. Due to the high torques achieved generally by forced induction diesel engines, torque convertor autos are useless, lossy and unreliable and don't make for a good partnership with this type of engine. Hence the DSG.
Belt changes on the V6 need to be expertly accomplished in order to maintain performance and a measure of reliability. It's a stupid setup with the water pump bearings being very highly stressed (driven by the cambelt at the bottom of the Vee) and the VP44 pump being separately driven and requiring careful re-timing. Maintaining these engines properly results in costs which are quite out of scale with their utility and which nullify any diesel savings.
The jewel in the crown of the V6 is the Bosch VP44 rotary pump. As a radial piston pump, (made allowable when CAV's patent on this configuration expired) it's reasonable, but the disaster is the incorporation of a fair chunk of (flimsy) electronics inside the pump. Utter madness. The reliability record of this pump is dreadful in all applications regardless of engine maker - and repairs are expensive. There is also the not insignificant matter of getting at it and reassembly with new belts.
Not my favourite engine as you may by now have gathered. The 1.9PD (130 PS) trounces it. Pity about the later PDs.
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