[Boosted98gsx]

Comp surge is too much air out of your compressor for your engine to digest or your blow off valve to release. Basically, your turbo makes too much air, and so instead of exiting the system through the combustion chamber, it reverts through the compressor housing.

This is bad because it causes your compressor wheel to spin backwards, which spins your turbine backwards, and puts backpressure on your engine, normally in pulses, which can make your car jerk, and hurt your drivetrain.

Too much load per given rpm is the main cause. Examples, going up a hill in a RPM too low, having your A/C on in an RPM too low, a turbo that spools very easily under load (small turbine / housing).

Also, a blow off valve that doesn't release enough airflow can cause surge (or BOV "flutter").

It can be avoided by increasing your turbine size or turbine housing A/R ratio, getting a larger BOV, or a less efficient turbo.