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There is a lot of misinformation from the earliest posts so I recommend whoever is reading to disregard the early posts. Only the later posts (2008) have more relevance and is correct.
As mentioned earlier by someone, the car is a speed density system that utilizes a MAP sensor. The MAP sensor read vacuum/pressure in part with the o2 sensor to calculate fueling needs. Since a blow off valve/diverter valve/bypass valve/pop off valve (or whatever fancy name you want to call essentially what is the same thing) will not negatively hurt performance as the car will detect the change in pressure from the valve releasing and compensate correctly.
In a MAF sensor system, the MAF actually "reads" the airflow and that "metered" air is used to calculate fueling needs. When the blow off valve releases pressure, there is no way the car knows that metered air is released and it still adds fuel as if that metered air is still there, causing a rich condition. Now normally, that air is dumped back into the system AFTER the MAF sensor so that the metered air is still in the system, just recirculated to the front of the track.
Now, there is a performance aspect of the blow off valve and that is why it is replaced. It's not for the sound. The sound is just an after effect that many seem to like. The OEM diverter valve/blow off valve etc... is designed to operate on a OEM car. The spring used inside the diverter valve is rated around what maximum boost should be on a car. Since aftermarket tuned cars typically run almost double the boost of what a stock car runs, this may become a problem as the diverter valve may open slightly under boost when it's not suppose to, bleeding off boost pressure. Running an aftermarket blow off valve will prevent this as the spring will be stronger and capable of keeping the valve closed.
Another thing, aftermarket blow off valves also react faster, giving a better throttle response. The after effect is the high pitch screech. OEM valves typically react (open/close) slower as they do not want that high pitch.
Now, if you don't want the high pitch sound, just have it rerouted back into the intake like OEM does and that will greatly reduce the sound as it is muffled in the intake tract.
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