What is an Intake Manifold? | Revv.ly Glossary

What is an Intake Manifold?

If we're being precise--and I do try to be--the intake manifold is the component responsible for distributing air (and on port-injected engines, air-fuel mixture) from the throttle body to each cylinder. It's the final stage of the intake system before combustion, and its design profoundly affects how an engine breathes, when it makes power, and how much power it makes.
The name comes from its function: it takes one intake stream and manifolds--divides--it into individual streams for each cylinder. Simple in concept, but the engineering behind an effective manifold is surprisingly sophisticated.

How Design Affects Performance

Runner Length -- The tubes leading to each cylinder are called runners. Their length affects when the engine makes power. Longer runners favor low-end torque through better pulse tuning at lower RPM. Shorter runners favor high-RPM power by reducing restriction when airflow demands are highest. Variable-length intake manifolds, with valves that change effective runner length with RPM, attempt to have it both ways.
Runner Diameter -- Larger diameter means less restriction at high airflow but potentially slower air velocity at lower RPM. Slower velocity can reduce cylinder filling at low speeds. It's a balance.
Plenum Volume -- The common chamber that feeds all runners. Larger plenums store more air, acting as a reservoir for sudden demand, but may reduce throttle response. Smaller plenums respond quickly but may starve cylinders at high demand.
Plenum Shape -- Airflow into each runner should be equal. Poor plenum design can favor some cylinders over others, creating imbalanced mixtures and reduced overall power.

Factory vs. Aftermarket

Factory manifolds optimize for many factors: emissions, fuel economy, drivability across conditions, manufacturing cost, and packaging. Performance is one consideration among many.
Aftermarket manifolds typically prioritize power within a specific RPM range:

• Single-Plane Racing Manifolds -- Short runners, large plenum, maximum high-RPM airflow. Sacrifice low-end drivability for peak power. Appropriate for racing, awful for daily driving.
• Dual-Plane Street Performance -- Divided plenum with longer runners than racing manifolds. Better low-end torque while still improving peak power. The sensible choice for street-driven performance.
• Ported Stock Manifolds -- Factory manifolds professionally smoothed and polished for improved flow. Retain factory appearance and fitting while extracting more potential.

The Installation Considerations

Intake manifold swaps require attention to details:

• Fuel System Compatibility -- Aftermarket manifolds may position injectors differently or require different fuel rails. Ensure compatibility with your fuel system.
• Throttle Body Matching -- Runner diameter should match throttle body size for smooth flow transition. Mismatches create turbulence.
• Sensor Locations -- Modern engines have various sensors mounted to the manifold. Aftermarket options must accommodate these or provide alternative mounting.
• Tuning Requirements -- Changed intake dynamics may require ECU recalibration for optimal results, particularly if the manifold significantly alters airflow characteristics.

The Reality of Gains

On modern engines with well-designed factory manifolds, intake manifold upgrades typically provide modest gains unless part of a larger build. The factory engineers had access to good computational tools and understood airflow quite well.
Where manifold upgrades shine:

• Older engines with genuinely restrictive factory designs
• High-RPM builds that have outgrown factory flow capacity
• Forced induction applications where airflow demands exceed factory specifications
  The Revvly community includes enthusiasts who've tested various manifold options--their data helps separate marketing claims from actual results.
  Related: Throttle Body, Headers, Tune
  Manifold Upgrades Common On: Ford Mustang 5.0, GM LS Platforms, Honda B-Series