Surging?

[Ruan]

Surging is simply where the turbo is pushing out a chronic amount of air at a low shaft speed, therefore not much intertia behind the wheel, therefore it becomes easy to cause the flow to become unstable (that's the fluttering noise you'll hear) then when it gets really bad the turbo will start stalling... Imagine a 'stall' in aircraft, simply the aerofoil stops producing lift because the airflow over the wing is too high for the requested angle of attack... The turbo stops moving air and slows rapidly, causing massive stresses on the shaft... It's the same phonemenon as when a petrol lifts the throttle and the turbo barks away as it rapidly slows down, but the difference is that you're also on the throttle, there is drive to the turbine wheel, so its trying to rapidly bend the compressor off the shaft and press the compressor wheel back and forth... Hence we say it rapes the turbo...

Ways to solve it, reduce the boost at lower RPMs, make the head flow better at low rpms, but often its a combo of inlet pressure causing high exhaust pressure, causing high inlet pressure etc etc... I.e. the turbine side can't flow enough as the vanes are set too closed, the head can't flow enough at that rpm, nor can the engine consume that much at that rpm and the compressor is spinning to slow to increase the pressure, so it stalls as that's the easiest option...

This is why I say you have to understand that with VNTs boost is easy, you need to look into how to control it well...