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Since model year 2004, the S4 sedan and S4 Avant are both equipped with a new, two-stage braking system. For normal deceleration, the servo ratio is 5.5:1, meaning the amount of braking pressure being applied by the driver is multiplied by a factor of 5.5. When rapid deceleration is called for and the driver rapidly engages the braking mechanism, the servo ratio more than doubles to a remarkable 12:1 for extraordinarily enhanced stopping power.