OK, I'll freely admit to knowing a lot more about physics than I do about engines, and I'm no physics expert, so bear with me.

I recently got a dashdaq, and one of the readouts it has shows the MAP or intake Manifold Absolute Pressure, and that this pressure readout includes atmospheric pressure. Also, I understand that atmospheric pressure at sea level is typically 14.7 psi.

So, to make a poor man's boost gauge, you can simply subtract 14.7 from the MAP readout and that's your boost. Well, I tried this, and I must have goofed something up. So I switched back to just logging/viewing the MAP readout.

The MAP readout confuses me. It ranges from 2.9 all the way up to 27.0.

There's two big issues I don't understand:
1) At full throttle and max RPM what should my MAP be reading? Shouldn't it be higher than 27 psi? At 24 psi that would indicate I was only boosting about 12.3 psi. Isn't that low, especially considering my mods? Doesn't the stock supercharger boost around 14 or 15?

2) How can there be a negative boost? I understand that represents vacuum (or sucking on the part of the engine). But that seems impossible to me. How can the supercharger be spinning and not compressing the air?