Couple of questions regarding the cool down period after hard driving;

Firstly what is the rationale behind allowing the car to idle before shutting off, specifically why with any turbo engine? I know that it's best with any car to have a cool off whilst the engine is running to get the fluids cooled but surely this is best done at low speed so as to get some decent airflow rather than relying on the cooling fan(s).

Secondly how long should this period be - would 30 seconds suffice as that's how long the slow crawl over the speed bumps to my house from the main road is.

 

Depends how hard you've been driving..... If the oil stops circulating while the turbo is still very hot, then you get sludge buildup in your oil passages. There's also the issue of thermal shock when materials cool down rapidly. This can break seals between componants and even warp or crack casings. The worst thing that you could do is to thrash the car then immediately pressure wash the engine bay.....
Depending on my oil temperature, I'll drive the car gently for 2-5 minutes after a thrash to let it return to normal temps. Either that or sit with the car idling (or just use the turbo timer) if I'm already at my destination. 30 seconds doesn't seem quite enough to me.

 

It is far more effective to drive off boost for the last 2 miles or so, than to sit and idle with no cool air entering the engine bay.

 

It depends on the turbo temperature. Coming off the track it takes my car 10-15 mins to get the oil temp back to normal. This isn't really the main issue though. You don't need to idle the car for 2mins every time you go to the shops. If you've just pulled off the motorway from 90mph then I normally wait a good few mins, if you've been driving through the town for a few mins to get home then I just be gental with it and make stopping the engine the last thing I do before I get out.

There is no hard and fast answer it comes down to your own sense of how hot things are.

 

When a turbo has been used hard it gets so hot, that if oil were to be stationary in the turbo the bearings would burn the oil, leaving carbon deposits behind - killing the bearings. So the rationale behind it all is to let the circulating oil (and water) cool the turbo down to a temp that once turned off, the turbo isn't hot enough to burn the oil.

 

Cheers for the informative replies - wish it was just easier to stay off boost, especially with that roundabout just down the road