SuperLIGHT-CAR is a collaborative Research & Development project co-funded by the European Commission under the 6th Framework to bring lightweight automotive technologies closer to high volume car production while fulfilling a wide range of automotive requirements in areas such as stiffness, crash performance, fatigue and corrosion resistance. For composites materials to be cost competitive with other material solutions, process development for high fiber volume fraction composites manufacturing in automotive industry tends towards cycle times reduction. In order to reduce cost, prepregs preforms are excluded in the favor of High Speed Resin Transfer Molding (HSRTM) processes which can be considered when highly reactive resin with short curing cycle are used. Mixing of the resin system components is then held at the mold inlet to reduce catalyzed resin life time outside the injection mold. Curing kinetics and induced viscosity changes are thus to be taken into account when injection process simulation is considered. In the case of on-line mixing of resin components, viscosity and degree of cure of the first injected resin droplets are not the same than for the last ones. This topic will be discussed here for mixed constant flow rate and pressure injection schemes. A solution that could be implemented in a RTM simulation package was developed. Resin system viscosity change with time were fully described for different processing temperatures for modeling purposes. Example of the importance of viscosity change was then conducted on a complex shape automotive part demonstrator manufactured by Alcan Composite Structures with Renault partnership.