akirk wrote:Those refer to fitted black box recorders...
does anyone know whether the car's built in computer maintains a permanent record of data - or does it only push it out in real time?
Alasdair
It's not permanent - they just don't have the memory capacity to store all data for all time. But they do have buffers which store some parameters for a period of time. How much depends on the memory and that varies from manufacturer to manufacturer.
These days many do have enough to reconstruct information about how the car is driven (for example you can identify harsh acceleration from fuel trim data), how long such data is kept varies from minutes to a few hours. Some high spec cars have track modes than include capture and storage of a lot of data (which is made available to the driver).
Aftermarket scan tools that read both OBDII and manufacturer specific data are widely available now so getting at the data is not difficult, and the police can and do extract it when investigating suspected offences where the effort to do so is warranted (eg fatalities).