Originally Posted by turbo-max
7k rpms at no load (i.e. sitting on the stand) = very little throttle input, there fore very little volume of air/gasses flowing through the engine, riding down the road, you open the throttle up alot more and for alot longer, therfore you have alot more air/gasses flowing this creating the said needed flow/heat energy to spool the turbo.
So, by your explanation of lack of throttle input and time under WOT, if you banged the rev limiter for long enough while its sitting on the stand, you'd build boost? I don't think so.
Load or resistance requires energy to be moved. Its a lot easier to push a ball than it is to push a cube. When the pistons are turning the crank, that is turning the transmission, that is turning the wheel, and moving hundreds of pounds of mass into wind resistance and possible gradients, it requires a considerable amount of energy compared to merely spinning the engine's rotating assembly.
Where does this energy come from? Mostly oxygen, and a little bit of fuel. When this additional oxygen is heated, it expands, like all gases do, and you build pressure between the cylinder and the turbo. The pressure goes the path of least resistance, spins the turbine thus spinning the compressor, and you have boost.