Not all V10s are created equal
by Franklin Stoudameyer
The 10-cylinder engine long since used in the Lamborghini Gallardo has been integrated with Audi's race-proven technology and seated in the all-new S8 to provide another milestone in Audi's history of revolutionary automotive engineering. Using the Gallardo V10 as a base, Audi reengineered key differences to suit the specific needs of the S8. Audi's alterations to the engine allow for engine specifications to be tailored to the S8's design, form and function. Audi's V10 version has been restructured to give its 946-pound heavier, four-door sedan a chance to become synonymous with sporting performance and compete, to a degree, with its lighter, two-door Gallardo cousin.
The V10 engine utilizes two banks of cylinders that are offset by 18.5mm, twin five-cylinder banks arranged at a 90-degree included angle between the deck surface and intake runners, with a spacing of 90 millimeters between cylinder centers. Located within the center of the cylinder bank is a balance shaft that rotates at the same speed, but in the opposite direction, as the crankshaft. This opposite spin cancels out and effectively eliminates engine vibrations and contributes to the engine's refined sound. Vibration under load is smoothed by the uniform ignition spacing of 72 degrees between the cylinders, which is achieved by offsetting the crankpins 18 degrees.
Audi's overhaul of the V10 engine included increasing the engine bore by 2mm, from 82.5 to 84.5. The increased displacement from 4,961cc (5.0 liters) to 5,204cc (5.2-liters) subsequently necessitated redesigning the crankcase. Audi uses technology carried over from its motorsport program: low-pressure die-casting from a hypereutectic aluminum alloy to form an intermediate frame reinforcing the compact crankcase and providing high torsional rigidity.
Audi eliminated the standard fuel injection management and upgraded to its trademark FSI direct fuel injection borrowed from the Le Mans-winning R8. This change required Audi engineers to update the engine management system from the standard Lamborghini LIE management program to the highly advanced Bosch Motronic MED 9.1. This in turn allowed for an increased compression ratio, 11:1 to 12.5:1, and for a more accurate air-to-fuel ratio based on a lambda value of 1. The common rail injection system delivers the fuel directly to the combustion chambers in precisely metered amounts, at a pressure of up to 1,470 psi.
Although the S8's V10 puts out less maximum horsepower, 450 bhp at 7000 rpm compared to the Gallardo's 513 bhp at 8000 rpm, it makes up for the loss in torque. Realizing that higher torque achieved at a lower engine speeds was needed to give the S8 a dynamic, lively engine feel and increased power delivery, Audi implemented Variable Turbine Geometry (VTG). At low rpm, the speed of airflow is increased by directing the air through a longer path with limited capacity, but a shorter and more capable path opens when the load increases so that a greater amount of air can enter the combustion chamber. This allows the engine to deliver 90% of its maximum torque from as low as 2300 rpm. The Gallardo puts out a maximum 376 lb-ft of torque at 4500 rpm in comparison to the S8's 398 lb-ft at 3500 rpm.