im about to get a digital boost gauge. they offer boost/vaccum, and just boost gauges. what is vaccum? why would i need a gauge showing me vaccum?

 

Boost/vacum gauge is good to have so you can monitor your engine while at idle and look for leaks and stuff. At idle your car is making vacum and the vacum drops as you go up into the rpms. You don't make boost until after you hit 0 vacum.

 

Check for boost leaks