This study addresses the challenge of ensuring the durability of closed couple exhaust manifolds in the compact engine bays of modern vehicles, focusing on a longitudinally mounted 1.2L 4-cylinder engine. The original sheet metal Exhaust manifold design failed thermal fatigue bench durability test, requiring a complete redesign to improve strength without changing materials. Initial simulation predictions significantly deviated from physical test results, with repeated cracks observed during accelerated thermal fatigue bench testing, despite simulations predicting a higher number of cycles before failure. This difference highlighted the need for a deeper understanding of the manifold's failure modes, primarily thermal fatigue, and mechanical vibration during engine transients. The design of experiment (DOE) approach was used to find the effect of different parameters e.g., gas temperature, surface temperature, air flow, thermal gradient, on the durability result & also to understand these parameters in real life vehicle driving conditions. This revealed the root causes of the cracks and established a dependable match between simulation and actual testing. The redesigned manifold included many key changes: increased wall thickness to enhance structural strength, a reshaped geometry to optimize flow and reduce stress points, and the addition of webbing in the port area to improve heat distribution and provide extra support. Furthermore, testing protocols were refined to replicate real-world driving conditions, including more precise temperature cycling. These refined protocols enabled the identification of design flaws and facilitated the validation of improvements. The final design successfully passed tough accelerated thermal fatigue bench testing, showing significantly improved durability. This research highlights the importance of accurate simulation modelling, targeted design improvements, and refined testing to replicate real world driving condition in overcoming thermal challenges within tight engine spaces, leading to strong and durable exhaust systems.