Although an eight-cylinder engine would be even more compact, to make it into the five-litre class it would need large, heavy pistons and connecting rods, so its ability to rev freely would be impaired as a result. It is no coincidence that many competitors have generally concentrated simply on high torque with their large-capacity V8 engines, rather than attempting to squeeze sporty performance out of them too.

The 450 bhp V10 in the Audi S8 belongs to the next generation of Audi V-engines, all of which have a 90-degree included angle and a spacing of 90 millimetres between cylinder centres.

Its two banks of cylinders are offset by 18.5 mm. Key areas of the Audi version have been reengineered compared with the engine in the Lamborghini Gallardo. The bore has been increased from 82.5 to 84.5 mm. Its stroke is 92.8 mm, and its displacement 5,204 cc.

The crankcase of the Audi ten-cylinder engine is produced by low-pressure die-casting, from a hypereutectic aluminium alloy. This technology renders separate cylinder liners superfluous; the liners are instead honed directly from the material by exposing the hard silicon crystals. A so-called bedplate design - an intermediate frame - gives the crankcase extremely high torsional rigidity and improves its vibrational behaviour. The cast-in grey cast iron bearing bridges reduce the thermal expansion of the aluminium casing; they simultaneously keep the amount of play at the main crankshaft bearings within tight tolerances.

The connecting rods are made from forged steel, and the pistons from an aluminium alloy. At the rated engine speed, each of them covers an average distance of 21.7 metres per second; oil-jet cooling prevents temperature peaks from occurring. With its crankpin offset of 18 degrees, the V10 fires at the ideal spacing of 72 degrees crankshaft angle. A balancing shaft located within the "vee" eliminates the free inertial forces of the first degree and also contributes towards the engine's notable refinement.

All four camshafts of the ten-cylinder engine can be adjusted continuously by 42 degrees crankshaft angle via hydraulic camshaft adjusters, depending on the load and engine speed, to modify the valve opening overlap. They thus optimise filling of the combustion chambers and improve engine response.

The camshafts - complete with the balancing shaft, the oil and water pump and the auxiliaries - are driven by maintenance-free chains running on the reverse side of the engine. They actuate the 40 valves via roller cam followers with hydraulic valve-play compensation.

The diameter of the valves on the intake side is 32.5 mm, and 28.0 mm on the exhaust valves. The exhaust valves are sodium-filled, for a cooling effect.

The V10 in the new Audi S8 uses the petrol direct injection principle known as FSI; this permits a high compression ratio of 12.5:1 and a correspondingly effective combustion process based on a homogeneous lambda value of 1.

FSI technology has impressively demonstrated its dynamic potential in motor sport - Audi R8 racing car equipped with it participated in the Le Mans 24 Hours five times for Audi, emerging as winner on four of those occasions. In the Audi S8, the FSI system is managed by a highly advanced Bosch Motronic bearing the designation MED 9.1, operating with ten individual pencil coils and two control units according to the master/slave principle.

The common rail injection system delivers the fuel directly to the combustion chambers in precisely metered amounts, at a pressure of up to 100 bar. The variable intake manifold incorporates electronically controlled tumble flaps that induce a swirling movement in the air drawn in at low engine speeds and load, making the internal combustion process more efficient as a result. The two-stage layout of the magnesium intake manifold serves the same purpose. Depending on load and engine speed, at around 4,000 rpm an electronically controlled valve switches from the long intake paths - measuring 675 mm - to the short paths 307 mm long. From that point on, the emphasis is on output rather than torque.

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