Upcoming automotive regulations require near-zero emissions, making cold-start catalyst heating a key challenge. This work investigates post oxidation through secondary air injection into the exhaust manifold to oxidise unburned fuel from rich combustion, generating heat and accelerating catalyst warm-up. The study combines experimental tests on a turbocharged four-cylinder engine with 3D-CFD simulations. A constant flow SAI system was installed near the exhaust valves, achieving stoichiometric conditions before the catalyst and increasing exhaust temperature during cold start due to oxidation reactions. Two numerical approaches were employed: a full engine simulation using the 3D-CFD tool QuickSim to extract detailed boundary conditions, and a detailed exhaust manifold model in StarCCM+ using these conditions to resolve mixing and chemical reactions. Results show that secondary air injection effectively increases exhaust temperatures upstream of the catalyst, and optimisation strategies such as higher air temperature, modified manifold design, and fuel post-injection can further improve post-oxidation efficiency.