Now that diesel engines seem headed for an inexorable decline, even though technology and hybrid solutions have made them cleaner and more efficient than ever, it seems almost impossible to imagine units larger than 2.0 or 3.0 litres.....
However, in the early 2000s, escalating performance and increasing customer demands pushed leading premium manufacturers to push the boundaries and design diesel engines as 'big' as the most powerful petrol engines.
It was precisely in those years that the three main German rivals, Audi, BMW and Mercedes-Benz, threw down the gauntlet with three 3.3 to 4.0-litre V8 diesels initially only for flagships and later also for SUVs.
The first to appear, in late 1999 and early 2000, was the Bavarian entry, which debuted in the 7 Series E38 with the 740d model. This engine was the first exponent of the M67D series, dubbed the M67D39 as its effective displacement was 3,901 cm3, and was produced by joining two four-cylinder blocks from the 1,951 cm M473, while maintaining a bore and stroke of 84 x 88 mm.
BMW M67D 3.9
Power output of the first version was 238 PS at 4,000 rpm, with torque of 560 Nm at 2,000 rpm, but in 2000 power output was increased to 245 PS and maximum torque was reduced to 1,750 rpm. The top speed of the flagship diesel remained the same as before, 242 km/h (150 mph), but the 0-100 km/h (62 mph) dropped from 8.4 to exactly 8 seconds.
The engine remained largely unchanged for the next generation of BMW's 7 Series, the E65, which arrived in 2002, with performance still increasing: power rose to 258 PS at 4,000 rpm and torque to 600 Nm at 1,900 rpm, and remained so until 2005, when the 4.4-litre variant was introduced, reaching and exceeding 300 PS and changing the model name to 745d.
Audi AKF 3.3
The Volkswagen Group's response came in the early 2000s with the Audi A8 D2 series, with an aluminium-block V8 petrol derivative. The first, called AKF, had a displacement of 'only' 3.3 litres (3,328 cm3, with a bore and stroke of 78.3 × 86.4 mm), but an impressive output of 224 PS, making it proportionally first in the power-to-displacement ratio, with more than 67 PS/litre.
The layout was four-valve with twin overhead camshafts as on the competitors, but it was not yet equipped with common rail, but with simple direct injection with a compression ratio of 18.5:1. In return, it was equipped with two variable geometry turbochargers. Maximum output of 224 PS was achieved at 4,000 rpm, torque was 480 Nm between 1,800 and 3,000 rpm, and performance was close to that of the BMW: 242 km/h, with a 0-100 km/h time of 8.2 seconds.
In 2003, the A8 D3 series saw the arrival of the 4.0-litre ASE evolution, 3,937 cm3 to be exact, and came with chain and common rail timing, while the compression ratio was lowered to 17.5:1. With 275 PS and an impressive 650 Nm of torque, it was the most powerful turbodiesel on the market at the time of its launch, and pushed that A8 to a top speed of 250 km/h (155 mph), with 0-100 km/h in 6.7 seconds.
Mercedes OM628 4.0
Mercedes-Benz went a little slower, but got it right. Announced in late 1999, its V8 diesel arrived only a year after the S-Class W220 with the S 400 CDI. It was an all-aluminium engine with a 75° engine bed angle and balancer counter-shaft in the centre of the V, a square physiognomy (86 x 86 mm), an effective displacement of 3,996 cm3 twin-turbo with variable geometry, 18.5:1 compression ratio and common rail injection.
It debuted with an output of 250 PS at 4,000 rpm and 560 Nm of torque between 1,700 and 2,600 rpm. The saloon, which was only fitted to the standard wheelbase version and not the long-wheelbase version, claimed a top speed of 250 km/h with acceleration from 0-100 km/h in 7.8 seconds, but with a claimed fuel consumption of less than 10 litres per 100 km (28.2 mpg-UK). From 2001 to 2005 it was also fitted in the ML-Class and G-Class SUVs. In 2003, output was increased to 260 PS.
In mid-2005, the OM628 gave way to its evolution, the OM429, which retained its size and displacement but with improved fuel injection and turbocharging. Power output in the 2005 to 2010 model years ranged from 306 to 320 PS at 3,600 rpm, with torque of 700 Nm between 2,000 and 2,600 rpm, and 730 Nm at 2,200 rpm. In addition to the S and ML, it was also fitted in the GL and the E-Class, called 420 CDI and then 450 CDI, maintaining a displacement of 4.0 litres.
Volkswagen V10 TDI
And the others?
The first to pick up the gauntlet was Volkswagen, which in 2003 introduced a 5.0-litre V10 TDI with 313 PS and 750 Nm, later increased to 350 PS and 850 Nm in the special R50 version of the 2007 Touareg.
Land Rover arrived a little later with the V8 Lion, produced by Ford, which controlled the British marque, as an evolution of the TDV6 family produced jointly with PSA. It was a 272 PS 3.6-litre that found its way into the Range Rover and Range Rover Sport, later replaced by a newly designed, 'Ford DNA' 4.4 with 313 PS.
Finally, let's not forget the folly of Audi, which, to celebrate the twin-turbo V12 used in endurance racing in the victorious Audi R10 TDIs from 2006 to 2008, released a variant of the Q7 in 2007 with a 6.0-litre V12 TDI with 500 PS and no less than 1,000 Nm of torque.