The intake manifold is located between the throttle valve and the engine intake valve, so it is called the manifold because after the air enters the throttle valve and passes through the manifold buffer system, the air flow channels are different here, corresponding to the number of engine cylinders. For example, there are four channels for a four cylinder engine and five channels for a five cylinder engine, which respectively guide the air into each cylinder. For the fuel injection engine or diesel engine, the intake manifold only distributes clean air to the intake ports of each cylinder. The intake manifold must distribute air, fuel mixture or clean air to each cylinder as evenly as possible, so the length of the gas flow passage in the intake manifold should be as equal as possible. In order to reduce the gas flow resistance and improve the intake capacity, the inner wall of the intake manifold should be smooth.