In order to understand how an 'anti-surge' compressor housing works (which is technically a 'ported shroud'), you need understand what surge is, in the first place.
Surge: The 'Surge Line' is the leftmost boundary of a compressor map. This is where flow instability occurs - This is where the compressor wheel is spinning at a high enough speed to force more air than the engine can injest. On initial thought, one might think this would just give you more pressure in the intake - and this is true, as long as you are on the compressor map. Basically, the surge line is the MINIMUM airflow needed to keep the pressure from backing up, and causing reverse flow pressure waves - these are INCREDIBLY damaging to the turbocharger!
Follow that thought a little further - when are you constantly seeing high flow of air, but little entering the intake? That's right, when you get off the throttle from boosting. This surging is what a BOV protects the turbo from. You also see this, if you have attempt to spin a compressor wheel such that it is forcing more air into the intake than the engine can take - or attempting to get more boost per rpm of the compressor wheel, than it is designed for.
That said, the ported shroud allows the surge line to be moved further left - this is because the compressor housing allows some of the air that starts to get backed up in the intake, to flow back out the housing.
Here's a pic from Garrett:
Surge: The 'Surge Line' is the leftmost boundary of a compressor map. This is where flow instability occurs - This is where the compressor wheel is spinning at a high enough speed to force more air than the engine can injest. On initial thought, one might think this would just give you more pressure in the intake - and this is true, as long as you are on the compressor map. Basically, the surge line is the MINIMUM airflow needed to keep the pressure from backing up, and causing reverse flow pressure waves - these are INCREDIBLY damaging to the turbocharger!
Follow that thought a little further - when are you constantly seeing high flow of air, but little entering the intake? That's right, when you get off the throttle from boosting. This surging is what a BOV protects the turbo from. You also see this, if you have attempt to spin a compressor wheel such that it is forcing more air into the intake than the engine can take - or attempting to get more boost per rpm of the compressor wheel, than it is designed for.
That said, the ported shroud allows the surge line to be moved further left - this is because the compressor housing allows some of the air that starts to get backed up in the intake, to flow back out the housing.
Here's a pic from Garrett:
