This paper describes the simulation and experimental work recently carried out during a typical exhaust manifold system development utilizing fabricated stainless steel manifolds. The exhaust manifold bridges the gap between the engine block and the catalytic converter. Bolted tightly to the engine with a gasket in between the manifold and the engine block, the engine's exhaust dispenses spent fuel and air into the manifold at an extremely high temperature. The automotive exhaust manifolds are designed and developed for providing a smooth flow with low/least back pressure and must be able to withstand extreme heating under very high temperatures and cooling under low temperatures. This paper describes all the analytical steps, procedure and tools such as CFD and FEA used in the development of a manifold system. The CFD tool utilizing conjugate heat transfer is used to calculate temperature distribution on the manifold. The manifold system durability is calculated using FEA. The manifold system is subjected to high temperatures and engine pulsations. The thermal damage under alternate heating and cooling cycles results in appearance of cracks after the accumulation of several hundred cycles. The manifold gasket leak is another area of concern that has to sustain the expanding and contracting of the heated metal. A complete lifetime estimation of durability of fabricated stainless steel exhaust manifolds including assessment of gasket leakage is made using the accurate material data. Several different manifold configurations are evaluated and durability deficient manifolds are improved using computer aided analysis. All the relevant steps and design evolution involved in the assessment of both durability issues of manifold and gasket leakage aspects are described in this paper. The simulation results are also compared with the measured experimental thermal shock data.