A new forced-induction engine in the schema of the S4 is a big deal, but even bigger is a fundamental change to Audi’s quattro all-wheel drive system. Yes, the S4 still uses the tried and true 40:60 split Torsen center differential, but Audi introduces a new facet of quattro with the pairing its latest sport differential at the rear.
This evolved setup utilizes a superposition gear on both the left and right sides – connected via a multi-disc clutch that can apportion almost all of the power by an electro-hydraulic actuator to the right or left rear wheel. Action is controlled by computer able to act in under 100 milliseconds.
The main function of the sport differential is to send power to the outer rear wheel in a turn to help stabilize and even throttle-steer the car. Where ESP helps control the car via braking by utilizing sensors for yaw, speed, steering angle, throttle, etc., the sport differential does the same with throttle in an effort to neutralize the effects of understeer. The rear sport differential is optimized to work even when the car is coasting and even when the clutch is engaged. Think of it as the antithesis of ESP.
Cars like the test vehicles we encountered in Mallorca paired the sport differential with the Audi Drive Select (ADS) system. In addition to its three-tiered (Comfort, Auto, Dynamic) settings for steering weight and ratio, throttle, dynamic suspension and shift points in the case of an automatic car, the Sport Differential also gets three levels of tune when paired to ADS.