Compressor surge is a phenomenon where the compressor cannot increase the pressure of the air it is pushing and results in the reversal flow of air through the compressor.
The fluttering noise is the sound of the compressor “chopping” through the air rather than pushing the air. The most common time which compressor surge occurs is during gear changes on a manual transmission car. Under acceleration, the turbocharger is flowing air and the engine is ingesting the air which means the air that the turbocharger is pushing is going somewhere at a certain pressure. When a gear change occurs, the throttle which allows air to flow into the engine is closed.
This results in a large pressure spike as the turbocharger is still trying to flow air due to the inertia of the compressor and turbine but there is nowhere for the air to go as the throttle is closed. The pressure within the intercooler piping continues to increase until the compressor reaches its pressure limit. When it reaches this limit, it cannot flow the air anymore and the built up air pressure inside the intercooler and pipe begins to flow backwards through the compressor which is trying to flow air forwards.
The result of this reversal flow of air is the immediate deceleration of the turbocharger and a high load on the bearings which support the compressor/turbine shaft. At low turbo speeds and low pressures, the deceleration of the turbo and the load on the bearings is low, i.e. a small amount of fluttering at low engine speeds and throttle movements is negligible.
At high turbo speeds and high pressures, compressor surge during a gear change can be damaging to the bearings of the turbocharger as the deceleration rate of the turbocharger is high and the reversal airflow through the compressor is high.
This deceleration will also reduce the boost response of the turbocharger when the throttle is reopened as the engine will need to work harder to increase the turbo speed back up to operating RPM.
A BOV is designed to maximize boost response and eliminate the problems associated with compressor surge. During a gear change the BOV will open up due to vacuum and boost pressure. This will vent the excess pressure build up and allow the turbocharger to continue to flow air.
This will also reduce the deceleration of the turbocharger as the turbo does not need to work against a closed throttle. Flow is another important specification of a BOV. If the BOV cannot flow enough air, the pressure increase in the intercooler piping when the throttle is closed can still be high enough to cause compressor surge.