Originally Posted by Mark Shead

But you need to look at how the flap is attached, a internal gate is hinged like a door and is only opened on one side so this limits the flow of the hole,
A external wastage has a flap that moves away the same all round so for the same peak lift it can flow more than a internal gate, unless you use a real big bypass hole you cant flow the same, space and packaging is one of the reasons why they can control the boost better but also the smaller hole helps as theres not a instant drop with a hole thats to big.

Mark