to make sure the turbo is stopped rotating. when i shut the ignition off, the oil supply to the turbo is cut off.

 

it will never stop spinning. the reason you let the car run a bit before you turn it off its to let the turbo cool off. this is so the hot oil wont cook on the red hot turbo. let it cool off for 10-15 seconds or just take the last 30 seconds of yoru drive easy. or buy a turbo timer. it will keep your car running for 30 seconds after you remove the key to let the turbo cool off and you nto have to wait

 

cool. where can i get a turbo timer for my 2007 a4?

 

they are all the same. just get a greddy or apexi timer and your good.