When you remap a BMW's engine, the ECU is tuned to produce significantly more power and torque. But the ZF8HP automatic gearbox still has its factory torque limits and shift calibration – written for a completely standard engine.
The result is a mismatch. The engine is producing more than the gearbox is programmed to handle, which means torque is being cut at the gearbox level to stay within factory limits. You're not getting everything the engine tune has to offer.
On top of that, the gearbox's shift logic – its speed, its gear hold behaviour, its response under load – is still calibrated for a stock engine. It doesn't know you've tuned it. The driving experience doesn't match the power.
Combining engine and gearbox calibration in the same session fixes both problems. Torque limits are raised to match the actual engine output, and the gearbox behaviour is recalibrated to complement the tune – creating a seamless, cohesive drivetrain.