Audi A4 Diesel vs Audi A8 Diesel

Boasting the sporty three-litre V6 TDi engine, which powered the A6 to a "Best Luxury Car over $57,000" gong in Australia's Best Car awards last year, the A4, priced at $86,700, delivers as "the most-powerful six-cylinder TDi in the B-segment".

The C220CDi Mercedes-Benz diesel offers 110kW and 430Nm, while the newly introduced BMW 320d produces 115kW and 330Nm, although both are four-cylinder offerings.

The new Audi V6 powerplant is a common-rail direct injection unit that uses piezo injectors (voltage-activated injectors that are very quick and precise) and runs 1600bar of pressure in the fuel delivery system.

Multiple fuel injections per cycle and two intercoolers for the single turbocharger are among the other engine bay highlights.

With 171kW of power and 450Nm of torque on tap, six-speed tiptronic transmission and speed-sensitive power steering, the A4 seamlessly (and frugally) cruises but has the capacity to reach 100km/h in 7.5 seconds.

Similarly, the $210,000 supercar A8, with its all-new 4.2-litre, eight-cylinder twin-turbo engine, lightweight Audi space frame and quattro all-wheel drive, also turns it on.

The turbodiesel V8 comes from the same family as the new V6 in the A4, using the same high-pressure common-rail direct-injection principles, and boasts a weight of 255kg, down by 15kg over its smaller predecessor.

Like the A4, the engine is of low-weight, compact design, with Audi claiming a new benchmark in acoustics.

The V8 has two turbochargers, with variable turbine geometry, assisting in reducing lag and generating higher torque at lower engine speeds, as well as two intercoolers.

With 240kW and 650Nm the high-performance limousine accelerates from 0 to 100 in just 5.9sec.

The new powerplant is so quiet it's easy to forget you are driving a diesel.

The A8's 4.2 TDi engine's twin-pipe exhaust system is also equipped with a diesel particulate filter — made to last the lifetime of the vehicle without additives or servicing — which further decreases the engine's emissions, which in greenhouse terms are already low.

And while Australian motorists do not have the privilege of discounted diesel prices, rebated registrations and the reduced taxation offered to their European counterparts, the models' fuel economy is incentive enough.

Averaging 8.4 litres per 100km under Australian Standards testing, the A4 offers around 750km per tank; with just 9.7 litres per 100km fuel consumption, the A8 pushes close to 1000km.

Both models feature electric front seats complete with electric lumbar support and leather upholstery.

Positioned behind the sports leather steering wheels, driving from Hobart to Launceston was a pleasure.

Travelling vast highways, curly mountain roads and even navigating unsealed stretches in winter conditions with ease, both the A4 and A8 lived up to the expectation that the sporty-yet-safe luxury models would be smooth operators.