Ever since it was launched in 2009, Porsche’s Cayenne Diesel has been – how shall we say this – a bit of a an embarrassment to a marque which prides itself on performance.

While BMW offers the X5/X6 M50d with a stump-pulling 376bhp and 546lb-ft (740Nm) of torque, Porsche were almost apologetic with only 242bhp and 406lb-ft (550Nm) in the Cayenne Diesel.

The Cayenne’s 4.2-litre V8 diesel engine delivers 376bhp and 627lb-ft (850Nm) of torque.

Mercedes have the ML 350 BlueTec with 254bhp and 457lb-ft (620Nm), but in the battle for SUV supremacy the folks in Munich had the class well and truly owned.

So what did Porsche do? Well if you can’t beat them, why not cheat?

Whereas BMW and Mercedes took the most efficient route with 3-litre six-cylinder powerplants, Porsche has opted for a bi-turbo 4.2-litre eight-cylinder engine to imbue the Cayenne S Diesel with 376bhp and 627lb-ft (850Nm). Take that BMW and to hell with the fuel economy..

As you’d expect, Porsche’s press release is littered with phrases such as ‘efficiency’ and ‘intelligent performance’, but the simple fact of the matter is an average of 34.0 mpg and 218 g/km of CO2 emissions puts the new Cayenne S Diesel well behind its competitors, despite its auto start/stop system and 1,200 km touring range.

For the record, Mercedes’ ML 350 BlueTec achieves 39.2 mpg and 189 g/km of CO2 emissions, while BMW’s M50d offers 36.7 mpg and 204 g/km.

You won’t mistake this for a V6 Cayenne Diesel – once it’s on the move.

Volkswagen offers the same 4.2-litre displacement in its V8 TDI Touareg, but with only 335bhp and 590lb-ft (800Nm) of torque and 239 g/km of CO2 emissions. Porsche’s 3-litre petrol Cayenne S Hybrid, despite almost 375 bhp and 428lb-ft (580Nm) offers only slightly better fuel efficiency with 34.4mpg and 193 g/km of CO2 emissions.

Performance-wise, the Cayenne S will comfortably annihilate most sports cars with zero to 62mph acceleration in 5.7 seconds and a top speed is 158 mph. Quick, but the X6 M50d is even quicker, with zero to 62mph covered in 5.3 seconds (5.4 seconds for the X5 M50d) and a top speed of 155 mph.

Porsche describe the driving experience of the new Cayenne S Diesel as “..emotional like a sports car in its dynamic performance and sound”, with a smooth resonant V8 howl quite unlike the sometimes clattery V6 in the Cayenne Diesel.

To handle its gargantuan torque Porsche has enhanced the Cayenne’s PTM active all-wheel drive, while offering the optional Porsche Torque Vectoring Plus (PTV Plus) to further improve handling and precision.

Apart from its rapid acceleration, you can distinguish the Cayenne S Diesel from it’s lesser siblings by larger silver brake calipers, black slats in the front apron which make the air intakes appear larger and the car wider, and of course its distinctive engine sound.

The Cayenne S Diesel will be available from January 2013 and cost 77,684 euros in Germany, just 2,142 euros more than the petrol V8 Cayenne S.

UK pricing is yet to be confirmed, but is likely to be around £58,500 (depending on final specification), which is £2,000 cheaper than BMW’s X5 M50d. Volkswagen’s similarly powered Touareg Altitude V8 TDI is available at £59,935, while the Cayenne S Hybrid can be yours from £61,074.