This is straight off Banks web site. I will ad that on the Turbo drag cars we run the engine is shut down quite often at the end of a full pass when going through the finish line and the turbos will be red hot and glowing.
You have to let a turbo-diesel idle for two minutes before you shut it off.
This is a current myth that has a basis of fact stemming from many years ago. It also has a kernel of truth regarding today's turbocharged gasoline engines that operate at higher peak exhaust temperatures than turbo-diesels. In the early days of turbochargers, the turbo shaft was supported by a babbitt bearing that could seize, or even melt, if the engine was shut off immediately after sustained boost conditions where the turbocharger would "heat soak". A two minute cool down at idle allowed the turbocharger to dissipate any remaining spinning inertia, and the oil circulation cooled the bearing and prevented oil "coking" in the bearing area. Turbochargers haven't used babbitt bearings for over 30 years, and today's oils resist coking. Synthetic oils won't coke, period. With a turbocharged gas engine, it's still good insurance to let the engine idle for 30 seconds to a minute to allow the turbo or turbos to dissipate any inertia and to cool the bearing area to prevent oil coking, especially if the engine has been worked hard just prior to shut-down. Of course, using quality synthetic oil eliminates this potential coking problem.
Today's turbo-diesels are a different story. There is really no reason to "cool down" a turbo-diesel these days, but you won't hurt anything by doing it either. You can still find people who swear you have to do it, but the myth is fading. Maybe they just like to sit and listen to the radio.