Turbo Boost and FE

[GasSavers_Hal9000]

What he means is that the turbo impellers are always spinning and producing some level of positive pressure in the intake system. As long as exhaust if flowing through the turbo housing, the impellers just about have to be turning.

When people talk about a turbo "spooling up" they're describing one of several things because the term has too much of a catchall meaning. The most important meaning for this discussion is when the turbo comes into it's efficient operating range and begins producing large enough quantities of pressure to affect performance... But even when a turbo isn't spooled up (like when cruising on the highway for example) it's still idling so to speak. When this is happening, the turbo only produces a small amount of pressure. That won't affect performance substantially, but what it does do is alleviate the need to the engine to suck air into the engine on it's own (like a vacuum pump). The turbo pushes the air in instead. This can make a difference in the VE and FE of the engine.