Its not that you run lean, in fact you'll run rich when it dumps. Since the MAF has already measured the amount of air that has passed it and the sends the correct amount of fuel to hit the target A/F ratio. Dump the air to the atmosphere (instead of recirculating it back into the turbo inlet) and the car goes pig rich (since theres no air...just fuel). Sometimes it causes stalling, more than likely you shoot fireballs. If you must have the sound, there are aftermarket BPV's out there than can hold increased pressures. Combine it with a CAI and you'll get all the sound you'll ever want or need. FWIW: the stock one is good up to any amount you'd be running on the stock turbo.