Originally Posted by
GARETH T
ok lets talk about "same as when you shut the throttle and have no dump valve"
you close the throttle when the turbo is above its boost threshold,
there is a rapid reduction of torque being generated by the turbine.
the compressor desnt have the energy it needs to keep spinning, so its slows down rapidly, but there is still a volume or pressureised air in the intercooler/pipework, this tries forcing it way past the slowing compressor wheel
The compressor is to large for the specified engine. What stevie says is correct. Compressor is not able to generate the small amount of air at the given (wanted) pressure ratio and is forced into surge (stall), hence being to large.
There is no problem with turbine torque, as when over the surge line the compressor is actually overspooling.
Think about right when you are closing throttle you get "turbo chatter". That is surge because the air which has been moved by the compressor dont get swallowed by the engine; this is damaging for your turbocharger as it causes bending moment on your turbocharger shaft and wear out your bearings.
Stu on here described surge pretty well only days ago.