V8 Differential Story (long)

[cwhelmke@ccgate.hac.com]

     From the 91 V8 Quattro Engineering Factbook
     
     Automatic Trans "Intelligent" Electronic Locking Center Differential:
     
        A computer controlled electronic multi-disc clutch couples and 
     uncouples during driving to provide smooth modulation of torque 
     between front and rear axles. This center differential locking clutch 
     is activated when a given difference in wheel rotational speed occurs. 
        Whenever excess wheel slip at one or more wheels is detected, the 
     center differential locks up. Torque is divided 50/50 when lockup 
     condition does not exist. With lockup power (torque) can be divided 
     0/100 to 100/0
        Input information for calculating the degree of slip is transmitted 
     to the control system by the four wheel-speed sensors that also serve 
     the ABS. The control unit works so quickly that impending wheel spin 
     is recognized in less than twenty thousandths of a second. The 
     automatic locking provides maximum traction at all times - and 
     consequently maximum acceleration on all road surfaces.
        This applies particularly in cornering: the car's tendency to 
     moderately understeer due to its chassis tuning remains unaffected.
     This is a major advantage of Quattro over the other systems: it is 
     more predictable regardless of circumstances, because the basic 
     driveline reaction characteristics remain the same.
        In the extreme, the drive torque can be completely redistributed by 
     the locking function to the front or rear axle. When decelerating or 
     braking, the center differential lock is immediately disengaged, so 
     that ABS control is not adversely affected in any way. The locking 
     system is also deactivated when the transmission control unit 
     recognizes that the vehicle is coasting or when a constant cruising 
     speed of 62mph is attained.
     
     Five-Speed Manual Transmission with Torsen Center Differential:
     
        For those who prefer the sportiness of a manual transmission, the 
     V8 Quattro is also available with a five-speed providing quicker 
     acceleration (0-60 7.1 sec vs. 8.7sec) and a slightly higher top 
     speed, (152mph vs. 146mph) the five-speed also differs in the method 
     of power transfer within the Quattro system: a Torsen differential is 
     used in place of the electronic center differential found in the 
     automatic.
     
     Torsen Rear Differential
     
        For good traction on roads that have less grip on one side - for 
     example a snow covered road that has been cleared on only one side - a 
     second differential lock is desirable.
        This is achieved in the V8 Quattro with a Torsen self-locking 
     differential (conventional 80,90,100, and 200 Quattros, use a Torsen 
     center differential) that divides torque at the rear axle. The locking 
     effect of the gear system integrated into the Torsen differential can 
     instantaneously distribute between 20 and 80 percent of the available 
     torque as needed between the two rear wheels. Torque transfer occurs 
     automatically, according to wheel grip; it is not activated by the 
     driver and is effective up to maximum speed. Since the locking action 
     of the Torsen differential only occurs when the engine is under load, 
     the ABS function remains in effect over the entire speed range. The 
     advantages of optimum power distribution can be utilized even when 
     cornering at high speeds.
     
     -Chris
     90 V8Q
     Raytheon Optical Systems
     Lexington, MA