NefMoto

Good day.

There seems to be a lot of argument about running a turbo motor without a recirc and the damage it causes.  There's little solid information available and I thought this would be the best place to discuss it.

While I agree that running no valve allows for stalling when you let off the throttle, the turbine is under no load, so the stalling does not harm the bearing.  While the blade is loosing and gaining momentum very quickly which I can appreciate will reduce the turbine's life cycle, I probably won't see it.

Is that correct?

I want to hear engineering principles, not about how big Davie down the pub says its bad.

What loading is placed on the shaft and bearings during load, and then off load. 

This comes from the horses mouth https://www.turbobygarrett.com/turbobygarrett/compressor_surge

little information.. well  :)

when you run WOT and lift off throttle, compressed air moves backward and hit the wheel. if you take an assembled shaft, ull see that compressor wheel is much lighter than turbine. so when the load applied to the turbine wheel its ok, but when you loading the lighter comp wheel youve got an extremely torsional force to the shaft because turbine wheel much heavier. and its inertia too. but its not end of the story. there is another force that makes things really bad - inertia of exhaust gases, which load turbine wheel for a little time even after TB is closed.
your comp. wheel stopped by the comp. air but turbine wheel spins by own inertia and some force of exhause gases. your shaft is fucked from 2 sides. aaand youre hear that surge sound for some seconds. thats because it doesnt stop immidiately. and all this time your shaft is fucked. and bearings too because all that forces applies at different moments and angles to the shaft.

not good idea to run without dv  :-\