Previously I have reported a method of using simultaneously measured cylinder pressure of each cylinder in order to separate engine radiation noise measured in the engine acoustic test cell into the contributions of combustion noise and mechanical noise in the time domain. This method was applied to a completed vehicle on the chassis dynamometer, and the results showed that the contribution of the combustion noise can be extracted from the interior noise during acceleration in the time domain. The procedure for this method is as follows. First, a continuous Fourier transform is performed on the measured cylinder pressure of each cylinder and on the vehicle interior sound pressure, and then the structural response functions are calculated using multiple regression analysis. Finally, that structural response function and the cylinder pressure of each cylinder are convoluted, and the contribution of combustion noise is obtained in the time domain. In addition, the method was also used to analyze the combustion noise of Kei small car models, and the results enabled an understanding of the vulnerability of the structural response system, which proved the usefulness of the method.