I have a 89 Supra automatic, I've just raised the boost, when I let off the gas, I hear a flutter sound. Do they come with factory BOV or is it a compressor surge? Thanks.

Take off the stock BPV, if you can blow through it, replace it with a Bosch unit.

They come with a bypass valve. Depending on the filter setup, you might hear the stock unit. A blow off valve vents to atmoshpere, yours is put back in the intake.