The intake manifold distributes air between the cylinders, and the length of its runners determines where the torque is produced: long for low revs, short for the upper end of the rev range. This is a fundamental modification, not an accessory.

Its effect can only be appreciated as part of a complete setup. A short-runner manifold on an engine with the original exhaust and mapping shifts the useful power band without adding anything - and can make the car less pleasant in town. Combined with a free-flow exhaust and remapping, it is one of the parts that genuinely changes the car's character.