Vacuum is when your throttle blade is mostly closed, the outside air can't get in and the engine is still spinning, trying to suck it in, so it forms a vacuum. Once you stomp on the gas the throttle blade opens, air rushes in and fills the vacuum, and your manifold will be close to ambient air pressure (the 0 marker). Once the blower starts to work, up goes the boost.
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Novi 2000 now spinning off the snout of the 331...
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