I have a '91 940 SE that i have made some changes to, big exhaust, turbo timer, boost gauge, K&N filter. I also cut the bottom of the air box off to get alot more air into the engine. My question is, when I back off the gas I hear a loud swoosh of air (I think) from the engine compartment. It is a bit like having Darth Vader breathing into a Mr. Microphone. Sure it sounds cool but what is it and is it bad?? Does my car have a factory "blow off valve" if I do, is this sound from that??

I am not familiar with the volvo turbo system yet (soon will be I hope), but that woosh sound is the "pressure ridding" system!

When you let off the throttle at any amount of boost, the valve goes nuts because the pressure is high. The throttle valve is closed, so on that side of the system, your closing off an open whole, causing the pressure on that side to all of a sudden be able to easily increase, since your turbo is spinning still from momentum and still quite fast moving exhaust.

There is no harm done to the engine, because on the other side of the throttle valve, in your intake manifold, and then on to the cylinders, there is a much lesser pressure, maybe even a vacuum.

That is the problem with pressure cut of switches on the "before" side of the throttle valve, because they cut off when you let your heavy foot off the gas.

Its just a matter of your valve "catching up" with the complete system pressure.