Drivetrain: highly flexible, eight-speed tiptronic
The powerful 4.0 TFSI works together with an extremely convenient and fast-shifting automatic transmission. The tiptronic’s eight speeds are tightly spaced, while its high gear spread of 7.1:1 between the shortest and longest gears reduces fuel consumption. When starting off, the S8 accelerates dynamically, while on highways it rolls along with low, consumption-reducing revs.
The eight-speed tiptronic impresses with a high level of efficiency. One of the key elements to this efficiency is an efficient oil pump that reduces friction losses. A hydraulic impulse storage unit enables the start-stop function. For the restart, its oil volume is pressed into the system by spring pressure – meaning that the tiptronic is ready to start in no time.
The dynamic shift program (DSP) that controls the eight-speed tiptronic features the automatic operating modes D and S. It uses high gears at low revs whenever possible, but downshifts quickly and flexibly when needed. The DSP has access to the route data from the standard MMI navigation plus. The computer uses this data to incorporate upcoming curves into its gearshift strategy – allowing it to avoid unnecessary gear changes on meandering roads, for example.
If the driver wants to intervene, he or she can use the paddles behind the steering wheel or the lever on the center tunnel. It resembles the thrust lever of a yacht, and the integrated gear indicator is lit internally in red. The lever communicates with the transmission purely electronically. Gear changes are triggered by a brief tap of the lever, after which it automatically returns to its center position.
As in nearly every Audi model with a longitudinal engine, the eight-speed tiptronic is installed with a special layout: The differential is located in front of the torque converter.
The front axle is then moved far forwards – the advantages are a long wheelbase, a short front overhang and a finely balanced weight distribution. The rear axle differential in the quattro powertrain and the battery in the trunk also factor into this balance.
The heart of the quattro permanent all-wheel drive system in the S8 is a self-locking center differential. The purely mechanical planetary gear normally sends 60 percent of the torque to the rear axle differential and 40 percent to the front. If one wheel slips, the differential transfers most of the power to the axle with the better traction. Up to 80 percent can be directed to the rear wheels and up to 70 percent to the front wheels.
Standard: the sport differential
The standard sport differential is a perfect complement to the quattro drive. The high-end rear differential distributes the power in continuously variable proportions between the rear wheels. It does this using two superposition gears, each of which comprise two sun gears and an internal gear and are engaged by means of electrohydraulic multi-plate clutches. They rotate 10 percent faster than the drive shaft.
When the clutch closes, it steplessly forces the higher speed of the superposition gear onto the wheel. The additional torque is drawn off of the opposite wheel via the differential. In this way nearly all of the torque can be directed to one wheel. The maximum difference between the wheels is 1,800 Nm (1,327.61 lb-ft).
In fast curves, the sport differential literally pushes the sedan into the radius so that the S8 corners as if on rails. The system nips any tendency toward oversteer or understeer in the bud. Thanks to its short reaction time – less than 100 milliseconds – it works even faster than the ESP stabilization system. It is just as effective while coasting as it is under load. Control of the sport differential is integrated into the standard Audi drive select system.
Complementing the sport differential on the front axle is torque vectoring. This software solution monitors whether the front inside wheel in a curve is about to slip. In this case, it pulses the brakes very slightly so that the outside wheel can transfer more power to the road. The system improves handling, safety and traction.
The equipment, data and prices specified in this document refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.