Automatic transmission is ZF 4HP22 EH with four forward gears:
- Straight ratio third gear
- Torque converter locked on third or fourth gear
- Selectable programs for economy, sport and 3-2-1
- Computer controlled hydraulic valves
Transmission control unit is integrated to engine control unit. With computer control change points can be optimized for better fuel economy and smoother operation. In sport mode third is highest gear engaged and torque converter is locked at just below 100 km/h. This gives good acceleration in highway speeds. And with locking torque converter fuel consumption is improved over predecessor. Transmission can be upgraded for better reliability and higher power output, this is not cheap though. In standard form it is said to hold 300hp on wheels for extended time.
The same basic ZF 4HP22 automatic transmissions was also used for several other BMWs and by other manufactures. Despite popularity it is not trouble-free, there are a lot of reports on erratic behavior and failures. How much of this due to lack of maintenance and what is for design shortcomings is not straight forward. One of common problems is failure of clutch pack when car is rev’ved in stand still (above 2000 rpm). Many transmissions gave up on emission testing in United States. There was even a technical bulletin issued by BMW how not break the transmission.
Rear differential is featured with typ 210 case, 25% limited slip and 2.93 long ratio. It is largest type for eighties production BMWs, motorsport cars included. Differential is very strong and well constructed. It is tested to hold over 800 whp for unspecific period of time, your mileage may vary. Stock 745i turbo power output is around 200hp on wheels. Limited slip versions are most desirable ones, not every 745i turbo came with one from factory. Cruising on fourth gear at 100 km/h translates to around 1700 engine revolutions per minute.
Same large differential was used on several BMW chassis (E23, E24, E28) at the time. Differentials between cars should be easily interchangeable. For improving acceleration there are many different gear ratios over stock to choose from. Either ratio 3.25 or 3.46 should give improvement. It must be noted though engine revolutions rise across the range and decrease comfort. With three-inch stainless steel exhaust this could be something to think of. Also shorter ratio decreases top speed and may effect to attained boost levels.