Post mortem on Lumba2's SAP.
It is an interesting design. It's a stacked set of blowers with aluminium blades. The output of the first goes into the input or the second stage. It has to make more pressure than the exhaust at full RPM.
5 series v X5 SAP. I think BMW went to the external filter so they could have a bigger filter to deal with more dust in off road situations.
The fitment was so tight having the filter attached to the pump I had to cut a notch out of the frame around the headlights as it touched and used the headlight like a megaphone.