esses

When the throttle is closed, isn't there actually a buildup of stagnant air (even if it is recycling). It's still the "same" air. Wouldn't that actually create a pressure spike throughout the entire track? The sudden closing of the throttle?

Is it the sudden opening of the throttle that can create pockets of vaccuum that travel all the way back to the air intake?

George @ BostonAudi

Yes... But any time the car is sucking in air, you're getting a partial vacuum.

esses

I can understand a partial vacuum there. I also understand that moving air forms a gradient which has properties similar to a vacuum. But I'm trying to figure out how/when there's a vacuum formed. What causes it?

I think the best analogy is a giant slinky. Right? The air in the intake path is like a giant slinky. You stop it's movement suddently (closing throttle), and you'll get a big spike that travels to the other end and back a couple of times.

You open it up suddenly (open throttle), and you get a low pressure area that quickly moves to the end.... and stays there (as long as the throttle position is maintained anyway).

George @ BostonAudi

Then you'll have pressurized air to the throttle body. When off the gas, you'll get a boost spike (thus the blown TBBs), followed by the recirc valves opening and something at least close to atmospheric pressure throughout the air system... When you get back on the gas, you'll get a vacuum throughout the air intake until the turbos spool back up.