The compressor map also has an important landmark, the surge line. The area of compressor surge is a line bordering the islands on the far left side of the map. Surge is when the air pressure after the compressor is higher than the compressor can generate. This causes the airflow in the compressor wheel to back up and stall. This, in turn, causes the pressure to drop, allowing flow to resume until it stalls again. In severe surge this can become a violent oscillation that destroys the thrust bearing of the turbo and can even cause mechanical failure of the wheel. Surge can feel like anything from a slight fall off in power to a violent jerking when driving. Surge also makes a thrumming or chirping noise that can be distinctly heard over the engine. You'll often hear compressor surge during shifts, if the compressor bypass valve (or blow-off valve) is too small. Obviously surging any time other than during a shift is not good stuff, so any compressor match should avoid crossing over or dwelling in the surge area of the map especially after the wastegate opens.