An intake manifold is the engine part that shares incoming air among the cylinders. In a typical gasoline engine, air enters through the throttle body, flows into a larger chamber, then divides into separate runners that lead to each intake port. Good air distribution matters because each cylinder needs the right amount of air to burn fuel efficiently.
When the airflow is balanced, the engine runs smoother, makes more power, and produces fewer emissions.
Inside the manifold, the plenum acts like a shared air reservoir, while the runners guide air toward individual cylinders. As each intake valve opens, the downward motion of the piston creates low pressure that pulls air through its runner. The shape, length, and diameter of the runners affect air speed, torque, and high speed breathing.
Modern engines may use sensors, variable-length runners, and computer control to improve airflow under different driving conditions.
Key Facts
- Air path: air filter to throttle body to plenum to runners to intake ports to cylinders.
- The throttle body controls how much air enters the intake manifold in many gasoline engines.
- Pressure difference drives airflow: air moves from higher pressure toward lower pressure created by the intake stroke.
- Engine displacement for one cylinder: V = pi r^2 h, where r is piston radius and h is stroke length.
- Four-stroke engines complete one intake event per cylinder every 2 crankshaft revolutions.
- Longer, narrower runners usually improve low-speed torque, while shorter, wider runners usually improve high-rpm airflow.
Vocabulary
- Intake manifold
- A set of passages that distributes incoming air from one main inlet to the intake ports of multiple engine cylinders.
- Throttle body
- A valve assembly that regulates the amount of air entering the intake system of many gasoline engines.
- Plenum
- The shared chamber inside an intake manifold that stores and supplies air to the individual runners.
- Runner
- A passage in the intake manifold that carries air from the plenum to one cylinder's intake port.
- Intake valve
- A valve in the cylinder head that opens to let air or air-fuel mixture enter the combustion chamber.
Common Mistakes to Avoid
- Thinking the manifold pumps air into the cylinders. The pistons create low pressure during the intake stroke, and that pressure difference pulls air through the manifold.
- Assuming every runner always gets exactly the same airflow. Runner shape, valve timing, engine speed, and pressure waves can make distribution uneven.
- Ignoring the throttle body when tracing airflow. In many gasoline engines, the throttle body is the main control point for how much air reaches the plenum.
- Believing bigger runners always make more power. Large runners can reduce air speed at low rpm, which can weaken mixing and low-speed torque.
Practice Questions
- 1 An inline 4-cylinder engine has a total displacement of 2.0 L. If the cylinders are equal in size, what is the displacement of one cylinder in liters and in cubic centimeters?
- 2 A 4-cylinder four-stroke engine is running at 3000 rpm. How many intake events occur per minute for the whole engine?
- 3 A car feels weak at low rpm after an intake manifold with very short, wide runners is installed. Explain why this change might reduce low-speed torque even if it improves airflow at high rpm.