The intake manifold is located between the throttle and the engine intake valves. The reason why it is called "manifold" is because after the air enters the throttle and passes through the manifold buffer, the air flow paths "diverge" here, which corresponds to the number of engine cylinders, e.g., a four-cylinder engine has four paths, and a five-cylinder engine has five paths to channel the air into the cylinders respectively. In the case of a natural intake engine, since the intake manifold is located behind the throttle, when the engine throttle is open, the cylinder cannot suck enough air, which results in a high vacuum in the manifold; and when the engine throttle is open, the vacuum in the intake manifold will become smaller. Therefore, fuel injection engines are equipped with a pressure gauge on the intake manifold to supply the ECU to determine the engine load and give the right amount of fuel injection.

The intake manifold is located between the throttle and the engine intake valves. The reason why it is called "manifold" is because after the air enters the throttle and passes through the manifold buffer, the air flow paths "diverge" here, which corresponds to the number of engine cylinders, e.g., a four-cylinder engine has four paths, and a five-cylinder engine has five paths to channel the air into the cylinders respectively. In the case of a natural intake engine, since the intake manifold is located behind the throttle, when the engine throttle is open, the cylinder cannot suck enough air, which results in a high vacuum in the manifold; and when the engine throttle is open, the vacuum in the intake manifold will become smaller. Therefore, fuel injection engines are equipped with a pressure gauge on the intake manifold to supply the ECU to determine the engine load and give the right amount of fuel injection.

Manifold vacuum is not only used to supply pressure signals to determine the load on the engine, but it has many other uses as well! For example, brakes need to be assisted by vacuum from the engine, so when the engine is started the brake pedal will be much lighter because of the vacuum assistance. There are also some forms of constant speed control mechanisms that utilize manifold vacuum. And these vacuum tubes once there is a leak or improper modification, will cause engine control disorders, but also affect the brake action, so advise readers to try not to make improper modifications to the vacuum pipe to maintain the safety of driving.