When you run an External Wastegate, which for most is unnecessary, the Wastegate has a vent. This is a pipe which carries the excess bled-off boost pressure and this pipe is often re-introduced into the exhaust making the car no louder than normal. However, most opt for an Atmospheric vent pipe where the excess pressure is dispelled via a separate pipe, which is usually unsilenced and tends to be fooking noisy.

Running an Atmospheric pipe is best for performance as it takes away these gasses from the main exhaust, therefore reducing the strain on it, but since most Ford owners have over-sized exhausts, it is no real problem re-introducing it.

Make sense?