Ok, my neighbor has a supercharged pontiac bonnevile. I was riding in it today and noticed a gauge labeled BOOST. It went up and down with acceleration so I assumed that it was the supercharger boost gauge. Well, you are all wondering what the problem is. The thing that I don't understand is that the gauge read from -10 to 10. The thing that really got me is that the gauge stayed int the negative boost range the whole time. I didn't know they had negative boost. could someone please explain (does negative boost actually increase mpg??)

 

That would be manifold vacuum. When the engine is not under load & the throttle blades are not open very far the engine doesn't see any boost. Engines without a supercharger always are in this condition. However, engines that have superchargers will exprience boost if the throttle is depressed enough. Boost is when the super charger is actually moving more air than the engine can inhale.
This varries with elevation but basicaly atmospheric pressure is around 14.something PSI. Without a supercharger if the throttle is fully opened that is about how much pressure is in the intake manifold. With a supercharger & wide open throttle you will have atmospheric pressure + boost pressure added together in the intake manifold. If you have 10 PSI of boost + 14.2 PSI atmospheric pressure you will have a total intake manifold pressure of around 24.2 PSI. "Boost" is the pressure created by an atificial means that is greater than atmospheric pressure. The positive numbers on the gauge are actual boost. The negative numbers are just showing manifold pressure in relation to atmospheric pressure.

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