Rep Power: 528

I always see it on the bottom of a boost gauge. Is it just the opposite of boost? Is it sucking air out? I just drove my friend's new 05 WRX and the gauge had vacuum below 0 and boost above it.

 
Rep Power: 0

Vac is measured in inches or centimeters of Hg (Mercury), and think of that portion of the gauge as an indicator of "reserve power." As you press the gas, the vacuum reading heads toward "0," at which point the engine is working as hard as it possibly can, regardless of RPM's. This means poor gas mileage & high egts.

 
Rep Power: 528

So the turbo psi measure only comes into effect after you're working the engine as hard as you can (ie. vacuum = 0)?

 
Rep Power: 607

it is a machine that u plug in the wall and goes around sucking up dirt

 
Rep Power: 0

^^^^ u smarty pants!

 
Rep Power: 342

OK a vaccuum is basically a lack of air.

Lets do an example. Take a coke bottle, it can be a small one or a 2 liter one, it doesn't matter. Now atmospheric pressure is about 14.7 psi, so at ground level the air around you is exerting a force of 14.7 pounds on every square inch, of course since you lived with this all your life you can't exactly feel it.

Anyways, but your mouth up to the opening and begin to suck the air out. The bottle will begin to implode, this is because the pressure inside the bottle is alot less than the pressure outside the bottle. The pressure outside the bottle will begin to crush the bottle. Basically you can consider a vaccuum to be a pressure below that of atmospheric pressure

Likewise if you blow air into the bottle, then the pressure inside the bottle is greater than the pressure outside the bottle.

Think of a seringine (sp?) it doesn't matter if you hold the needle upside down, but if you put the pointy end in some water and pull back on the plunger, you create a vaccuum inside the seringine and the water rushes against the flow of gravity to fill the vaccuum.

 
Rep Power: 342

Ok lets think of an engine. During the intake stroke the intake valve opens, and the piston begins to move down. This is alot like the seringine example. As the piston moves downward it creates a vaccuum, the air in the intake manifold rushes to fill the newly created volume. A naturally aspirated engine is ALWAYS under a vaccuum.

However on a turbocharged motor the turbo works to pressurize the intake tract. So when the intake valve opens, air rushes in.