Improving the combustion efficiency and meeting emission regulations from all types of Diesel powertrains is a pressing environmental issue. Particularly, CO2 and soot emissions from heavy duty Diesel engines are expected to increase by 75% the next 2-3 decades. Investigations with 300MPa injection pressures show significant soot reduction, but the effect of such extreme pressures on the in-nozzle flow has not been closely examined. In-nozzle flow dominates primary break-up characteristics and therefore the combustion efficiency. Moreover, the characteristic pressure drops in Diesel injectors may cause the fuel to cavitate, which leads to enhancements in the nozzle outlet velocity, the spray cone angle and the fuel atomisation. However, traditional predictive methods of the internal nozzle flow and its link with the spray characteristics may lead to significant inaccuracies at such extreme conditions. In this work, the fuel property database is modelled using the molecular-based PC-SAFT EoS with a previously studied eight-components surrogate based on a grade no. 2 Diesel emissions-certification fuel. The composition for the surrogate is (in mole fration): 2.7% n-hexadecane, 20.2% n-octadecane, 29.2% heptamethylnonane, 5.1% n-butylcyclohexane, 5.5% trans-decalin, 7.5% trimethylbenzene and 15.4% tetralin. Then, this surrogate is utilised in simulations for a common rail 5-hole tip injector tapered nozzle. The needle is assumed to be still at its maximum lift, i.e. 350µm. The injector operating pressures start from 180MPa and reach 450MPa. The collector back pressure is 5MPa. The density of the bulk fluid is assumed to vary according to a barotropic-like scheme, following an isentropic expansion. Results show an increase in the mass flow rate, following the square root of the pressure difference law and also in the outlet velocity, both as expected. In addition, it is shown how each component cavitates in different amounts according to their respective boiling points. Surprisingly, the cavitation is significantly reduced as the injection pressure increases. A focused study on this particular phenomenon shows a significant decrease in the Reynolds number in the sac, therefore the flow is found to be more stable and the pressure drop along the nozzle is smaller. The reason for the lower Reynolds number is found on the heavy nature of Diesel fuels. While the sac average velocity increases 15% between an injection pressure of 180MPa and 450MPa, the kinematic viscosity increases close to a 70%.