The intake manifold distributes air between the cylinders, and the length of its runners determines where torque is delivered: long runners for low revs, short runners 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 usable power band without adding anything - and can make the car less pleasant to drive in town. Combined with a free-flow exhaust system and remapping, it is one of the parts that genuinely changes the car's character.