[Twoforme2]

It can happen When you suddenly drop throttle. The pressure to the engine intake is higher on the compressor side than the turbine can support, so that pressure has to go somewhere and backs up through the compressor and, in essence tries or will turn the wheel backwards (ever blow an air compressor into a fan?)

This is a bad !

Turbo Surge can be controlled by enlarging the turbine wheel (The side that the engine exhaust controls) while maintaining the same compressor wheel (the one that makes the boost and goes to the engine intake) size. This is because it then takes more force on the compressor side to overcome the force on the turbine side, thus less likely to cause the turbo to surge.

Does that make sense?
I am still new to this stuff also, but have done a fair amount of reading to better understand these silly things.

 

[Twoforme2]

They are indeed two different things. "Barking" a turbo is from cutting the fuel supply off instantly, and air is still trying to move. Since the fuel supply is cut off, that air HAS to find a place to escape, and the easiest place for the air to go, is back through the charger.

"Surgeing" is more or less starving the charger. At certain rpm's and fuel, the charger will almost start "huffing"

Two TOTALLY different things

OK, you made me go look, because I thought one (barking) was a symptom of the other (surge)

According to Garrett.
Surge is the left hand boundary of the compressor map. Operation to the left of this line represents a region of flow instability. This region is characterized by mild flutter to wildly fluctuating boost and “barking” from the compressor. Continued operation within this region can lead to premature turbo failure due to heavy thrust loading.
Surge is most commonly experienced when one of two situations exist. The first and most damaging is surge under load. It can be an indication that your compressor is too large. Surge is also commonly experienced when the throttle is quickly closed after boosting. This occurs because mass flow is drastically reduced as the throttle is closed, but the turbo is still spinning and generating boost. This immediately drives the operating point to the far left of the compressor map, right into surge.



Surge will decay once the turbo speed finally slows enough to reduce the boost and move the operating point back into the stable region. This situation is commonly addressed by using a Blow-Off Valves (BOV) or bypass valve. A BOV functions to vent intake pressure to atmosphere so that the mass flow ramps down smoothly, keeping the compressor out of surge. In the case of a recirculating bypass valve, the airflow is recirculated back to the compressor inlet.