Intake manifolds never do see positive pressure,
Sorry but there is no such thing as a negative pressure except relative to atmospheric. To a properly trained engineer there is no negative pressure so the manifold is always under positive pressure. Your thinking is pre space age.
I've never seen any engine acheive 30 inches...I'll bet there is an exponential relationship between 25 inches [normally produced] and 30.
I simple gave the perfect vacuum pressure. Did you not read between parenthsis? I did not claim it is a typical engine pressure. 30 is impossibe because no engine can create a perfect vacuum, so I guess itis exponential, never thought about it. I have worked on projects as low as 5 microns of absolute presure, and that is far from the best that can be done. But that had nothing to do with engines.