The basic reason for a BOV (blow off valve) is to relieve a buildup of pressure in the intake tract when the throttle blades are closed suddenly. Many turbo cars, including the 86/87 intercooled GNs, have "blow through" throttle bodies. This means that the throttle body is placed in the intake stream after the turbo. In these setups, if you were to close the throttle blades suddenly, the turbo is still spinning very fast, and cramming air into the intake tract, but it has no place to go (the throttle blade is closed), so it backs up, and essentially puts the brake on the spinning turbo blades. This can be harmful to a turbo, if it happens a lot, and can decrease its useable life-span. On manual-tranny cars with a turbo, its almost a neccesity, as this happens every time you shift under boost (when you let your foot of the gas to shift). There is some debate as to whether it is neccesary in an automatic (like the 86/87 GNs), as this only happens rarely, if you were to be running at full boost, and then suddenly let of the gas. If you want, theres a ton of threads on this topic in the general tech section. On stock hotair 84/85 GNs, its is absolutely unnecesary, as our throttle bodies are immediately before the turbo, so if we close the throttle blade suddenly, it has no ill effect on the turbo. If however, you are running a modified hotair (like cool 84) that has had the throttle body re-located to be downstream of the turbo (as is often done when adding an intercooler to our cars) then youre in the same boat as the 86/87 GNs where there is some benefit of running one. So basically, if you have a hotair with the throttle body in the stock location, there is absolutely no reason whatsoever for getting a BOV, theres not even a place to install it on our cars. If you have moved your throttle body to be after the turbo, then it may or may not eb worth the trouble (read the threads on that topic to make up your mind for yourself, theres a few diff opinions ont eh subject). I hope this helped clear things up. Oh, and a standard disclaimer: i am by no means on expert on this stuff, and there are MANY who know more than me. If something i said here is incorrect, please feel free to correct me, thats the only way I learn
