Boost Guage --- what's it telling me!?

[LazarusShort]

I have recently acquired a car for track days, anyway it's twin turbo and has two boost guages. It's had a lot of mods done so it has a boost guage as well as the stock one. The stock one goes from -7 to +7 and the bolt on one goes from 30 to 30, I assume that means -30 to +30. Anyway what does this tell me generally, presumably turbo pressure but can anyone give me a more detailed explanation of what's happennning. Thanks!
here's the after market one:
http://www.google.co....:163&biw=1280&bih=649

[fireball87]

Yes, it's turbo pressure which is the air that go's into the engine (inlet manifold).
The inlet manifold is likely where these pressure measurements are taken from.

The gauge will be in units of PSI (pounds per square inch) and the reason is go's from - to + is because when your foot is on the accelerator exhaust gasses pass through the turbo make it spin and on the other side of the turbo there is a compressor that compresses air (charge air) that then generally go's through an intercooler (to cool the compressed air) then into you inlet manifold where you would see a positive pressure. When you lift off the accelerator you create a vacuum in the inlet manifold hence your (-) negative.

Hope thats helps...

[MacKenzieOz]

A boost gauge is a pressure gauge that indicates manifold air pressure or turbocharger or supercharger boost pressure in an internal combustion engine. They are commonly mounted on the dashboard, on the driver's side pillar, or in a radio slot.