Abstract: A pressure monitor is arranged to calculate a pre turbine pressure (p3). The monitor is programmed to: derive a normalized turbine speed, normalized for a pre turbine temperature, and a heat capacity ratio of exhaust gas before the turbine as first inputs; determine a turbine power normalizing factor as a second input; determine an actual turbocompressor power value as a third input; derive, for said first inputs, a linear relation between a turbine expansion ratio and a turbine power value normalized by the second input; equate the turbine power value to the actual turbocompressor power; thereby deriving the pre turbine pressure from said linear relation, as an output.