Turbochargers?

Question

i understand the basics of turbos, but what does the gauge mean w/ vacuum and boost?

Answer 1

Most boost gauges read both vacuum and boost. The vacuum side will usually read in inches of mercury (In. Hg.) and the boost side in psi or bar. I idle the car is pulling air in and creating a vacuum which is usually around 18-22 In. Hg. and when the turbo start to push air into the engine it must pressurize which is measured in psi. Most gauges usually read 30-0-30 or up to 30 In. Hg. of vacuum or 30 psi of boost.

Answer 2

Vacuum is the most basic thing when referring to an internal combustion engine. the engine creates a vacuum by displacing a volume in the cylinder by moving the piston  down, as the piston drops in the intake stroke air pulls against a closed throttle blade in the intake manifold, that blade is attached to a shaft and is opened by your right foot ( throttle body). When the blade is closed, a vacuum occurs in the intake manifold, thus you have a vacuum reading of 16-20", this is a standard of measuring the vacuum condition, a negative pressure.On a normally aspirated engine you will have anywhere from 5-20 " of vacuum at idle, depending on the design ( cam, intake manifold, exhaust manifold, etc.). When you open the throttle atmospheric pressure forces air in at what ever the pressure happens to be , at sea level it is more and the higher in altitude you rise the less pressure acts upon it (that's why late model cars use barometric pressure sensors). You have a 0 reading at wide open throttle.No vacuum ,only atmospheric pressure.

During a boost event, when the exhaust pressure has built up enough force to spin the turbine and impeller/compressor wheel fast enough to compress air (turbine is connected to the  impeller) then you start to feed a positive pressure into the intake manifold. This is boost pressure, a turbo can produce a lot of boost pressure, depending on it's size, and design. Boost is limited by diverting the exhaust away from the turbine, by opening a gate valve allowing excessive pressure to bypass the turbine and prevent the pressure from acting on it, in this way a steady level can safely be managed by the engine. Too much pressure and not enough fuel and the engine has catastrophic failure. Boost on gasoline engines is usually limited to below 20 psi. but modified engines can be boosted well above this mark. Daniels can use even more.