turbo question

does anyone know how much exhaust pressure it takes to spool up a turbo?

It doesn't require a certain pressure to spool a turbo, to reach a certain boost pressure at a certain flow + overcome the inertia of the turbine/impeller assembly will require a certain amount of energy (enthalpy and flow), then it depends how efficient it can convert that.

The greater the pressure ratio over the turbine, aka expansion ratio, the more the gas will expand and hence lose enthalpy, so the less power required by the compressor, the better the efficiency of the turbine and the lower the power loss by the bearings the lower the expansion ratio can be. A free flowing exhaust system will also lower the pressure in the exhaust manifold.