Turbo timer.

[Niall]

Actually that's a point, phase 3 cars would need the transponder present for the engine to run.

Is the transponder only used for startup, or is it required permanently?

transponder is only needed for start up. If you could fool the bsi into not turning off till the timer runs out, it wouldnt lock the ECU but then your central locking wouldnt work till the timer ran out. Best option would be to do away with your immobiliser but personally i wouldnt be happy with that