I think most replies here cover the essential point: under light load and light power requirements, the normal aspiration of a 4-stroke is sufficient to meet all the needs/economy for normal running. This, of course, means that you will only ever register vacuum on the manifold gauge. Atmospheric air pressure is constant at 1 bar (unless you have some wacky ram-air arrangement), so the gauge will only ever register a reading lower than that, by definition: a vacuum.
When you require more power, and therefore open the throttle up, normal aspiration is superseded with boosted air from the charger. This, by definition, is forced in – so air/manifold pressure rises to parity with atmosphere (1 bar) and then to 2 or 4 bar over-pressure or whatever is your system is limited to, or requires.
Point being: under normal aspiration, you can only ever see vacuum (negative pressure) on a gauge, even with the throttle wide open, as external air pressure never changes.
With a charger of some sort, you’re always into a pressure realm above 1 bar, if it is in use. Anything above normal air pressure is always into ‘boost’.
Presumably, you must have some kind of arrangement to be able to mix/route boosted air into the (normally aspirated) manifold stream, when needed. Otherwise, I would anticipate running issues at the transition point between the two states. This would easily achieved if the charger allows unimpeded air flow through it under non-boost conditions, but progressively boosts/mixes as required.
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