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Hi Mark,
If you put a mighty vac tool on the decel valve's small vacuum line connection and put a hose on the larger of the two hose connections then pump a vacuum into the decel valve while blowing into the large hose you'll notice that the decel valve starts to open around 19" of vacuum and is fully open and passing air around 21" vacuum.
The average 930 motor with it's very inefficiant low 7:1 compression only makes about 15" of intake manifold vacuum at idle so the decel valve stays closed at idle. Even real slow mild decelerations don't open it.
The average normally aspirated car with 9:1 to 10:1 compression is much more efficiant and idles around 20-23" of manifold vacuum when in good tune because each power stroke is making more power with the higher compression than a motor that has only 7:1 static compression.
I have mine vented to atmosphere and I like it that way.
Like Keith said it makes life for the turbo's thrust bearing easier and probably helps keep it spinning fast between shifts by acting like a second small blow off valve.
I like the little fooosh it makes as it blows out air pressure when you let off the throttle between high speed full boost shifts too.
I put a small valve I had sitting around on the actuating vacuum line so I can disable it or slow down it's action if I want but I've left that valve open because I like it.
Venting the decel valve to atmosphere also lowered the amount of exhaust popping on deceleration on my car alot. It still burbles and makes some soft popping but no more obnoxious explosive attention getting popping and I was sick of that anyway.
My cars overall fuel curve is different than a stock cars though because of the modified fuel head, and BL adjustable WUR that is set with high control pressure to lean it out on cruise and idle. This combination makes the car run a little richer on deceleration which lowers the exhaust popping.
My AFR gauge says I'm at 10:1 on decel with the decel valve working and vented to atmosphere and it said around 15:1 on decel when the decel valve is removed or disabled. A stock fuel head would be leaner on decel.
You have to try it on your car to see how you like the small change in the engines deceleration rate between upshifts and if there is a change in the deceleration popping.
A local friend tried venting the decel valve on his 930 with stock fuel head and WUR to atmosphere and it seemed to make his pop louder on deceleration, the opposite of mine...
So you have to try it on your car, and it's real easy to change it back if you don't like the result.
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