Datalogging is the process of recording real-time engine parameters while driving. For custom calibration work, datalogs provide the calibration engineer with crucial feedback about how the engine is performing under your specific conditions — your fuel, your altitude, your driving style.
Key parameters to log include: requested vs actual boost pressure, ignition timing and knock correction, air-fuel ratio (lambda), intake air temperature, coolant temperature, oil temperature, throttle position, and engine load. The specific parameter list depends on your ECU platform and the calibration tool being used.
For BMW applications, common datalogging tools include MHD Flasher, xHP Flasher (for TCU), and professional tools like ISTA/D. Your calibration engineer will specify which parameters to log and the driving conditions to replicate.