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One-dimensional engine simulation codes are frequently used for engine development where turbine performance is in focus for overall engine performance. Turbocharger performance can today not be accurately simulated without adjustments of the turbine efficiency and mass flow multipliers. In spite of the fact that engine exhausts are dominated by unsteady pulsating flow the performance maps provided by the turbo manufacturers are measured under steady conditions and for a range that does not cover the entire turbine operating range. The scope of this investigation is to calculate instantaneous on-engine turbine efficiency from measured and simulated engine data. These calculations are performed as a step towards generating turbine performance maps that will allow predictive engine simulations with higher accuracy than today. Results show an asymmetric behaviour of the twin-entry turbine.