This study presents the computational results of both automobile intake systems and motorcycle mufflers using the three-dimensional linear acoustics. The results of intake systems show the air cleaner has significant effects on noise reduction. In addition, the acoustic effects are different for different valve openings. Results of motorcycle mufflers show the muffler geometry has important effects on the sound wave properties inside the structure. As the complexity of muffler increases, noise reduction increases, but the exhaust back pressure also increase that could deteriorate the power output.