EW, The pressure bump I am led to believe is that experienced when an aircraft rotates, changing the attitude and therefore the airflow around the OFV.
If the OFV is closed and fuse is pressurised the above cannot happen.
I dont have any experience of the 74 as above regarding selecting packs one at a time time to prevent a pressure surge.I think this maybe be semantics but a slightly different phenomena to a pressure bump eliminated by pre pressurising whilst on the ground.ie the 737 pre press to -387' from memory to prevent the rotation bump.
However, I know of one case where a 320 was having bleed trouble ( nothing new back then, late 80's early 90's) and the crew deselected both bleeds and reslected on.Blew out all the blow out panels but did surely get bleeds flowing
That was a surge!
fred, it is good place to learn.Ram air in and out puts cooler outside air across heat exchangers/precoolers to cool bleed air for use in air con / press. systems.