Turbocharger surge speed...

[Jim Zurlo]

>As I understand it, it is basically the fact that the turbo is trying to
>force more air into the engine than the engine can consume at that given
>moment based on rpm and volumetric efficency.  When this happens the air is
>forced backwards through the compressor, which causes shaft whip, increased
>exhaust backpressure and will try to spin the compressor nut off if it is
>right hand thread.  In severe cases it can cause the compressor wheel to tag
>the housing, in mild cases it shortens the life of the thrust bearing.

Severe turbo surge on a highly boosted engine (3:1 Pressure Ratio) can also
cause brown underwear.  

Where the throttle is located will also effect the tendency to surge.  If the
throttle is located downstream of the compressor then slamming the throttle
shut is likely to cause surge.  If the throttle is located upstream of the
compressor then surge is less likely.

Jim