On the 4AGZE ( the GTZ motor ) the throttle body is before the supercharge so when you are boosting and lift off the throttle there is nothing for the boost to build up against so there is no need for a blow off valve.

For the die hard rice boy who does not care about there being any point in having a blow off valve, he just wants it for the sound. Sorry this can not be done because all the boost that is being created from the supercharge goes straight into the motor.

If you were wandering why blow off valves work on turbo car this is because the throttle body is after the turbo. When the throttle is closed the boost will build up against the throttle, it can then be blown off.