I always heard those blow off valves wher used on higher end turbo's to make the rpm's drop quicker when you let off the gas. Like a strong after market turbo without one will hold the boost pressure for a little while after you let off the gas wich keeps the rpm's from dropping down right away, wich makes it take a lil longer before you can shift to the next gear. Thats what I always thought anyway. I should probable research this a lil more.