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. Turbochargers and superchargers are both engine driven air compressors (exhaust driven or mechanically driven, respectively) and provide varying levels of boost according to engine rpm, load etc. Quite often there is a power band within a given range of available boost pressure and it is an aid to performance driving to be aware of when that power band is being approached, in the same way a driver wants to be aware of engine rpm. A boost gauge is mandatory when boost pressure is being modified to levels higher than OEM standard on a production turbocharged car. Simple methods can be employed to increase factory boost levels, such as bleeding air off the wastegate diaphragm to 'fool' it into staying open longer, or installing a boost controller. To avoid excessive leaning out of the engine (caused by increasing the boost beyond the fuel systems capacity) care must be taken to monitor boost pressure levels, along with oxygen levels in the exhaust gas, using an oxygen sensor. A boost gauge will measure pressure in either psi or bar and many also measure manifold vacuum pressure also in inches of mercury (in. Hg). Can also be displayed as mm of mercury (mm Hg).
