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An automobile engine discharges more exhaust emission when it is started in a cold state. It is generally known that the catalyst start-up characteristics must be improved to reduce exhaust emissions in that case. We got the relationship between the exhaust gas temperature and the activation of the catalyst by measuring the exhaust gas temperature at each part during the exhaust emission test mode, then found that the improvement of the exhaust manifold is an effective measure to reduce the exhaust emission as the result of the analysis of the temperature distribution of the exhaust gas. Then we developed the exhaust manifolds having light weight and low heat capacity, and we succeeded in improving the catalyst start-up characteristics and reducing exhaust emissions. The details of this study will be explained in this paper.