How is the runner length tuned to provide additional airflow? In addition to providing a simple airflow path to the combustion chamber, the intake runner (combined with the head port) provides additional cylinder filling by way of both an inertial and reflected wave ram tuning. The inertial wave cylinder filling happens when the downward movement of the pistons (and open intake valve) creates a vacuum, which initiates movement of the air column. If timed properly (determined by the length and inside diameter of the inlet tract), the inertia of the air column can improve cylinder filling beyond the vacuum created by the downward moving of the piston.
The reflected wave provides additional cylinder filing as well, but by a different means. The reflected wave happens when the intake valve opens to create a negative pressure wave. This negative pressure wave travels out away from the valve until finally arriving at the common plenum. The negative pressure wave expands out into the open plenum. The expansion of the negative pressure wave creates a low-pressure area that is quickly filled. The filling of the void creates a positive pressure wave, which then travels back down the intake port and into the combustion chamber, improving cylinder filling.
While both inertia and reflected wave ram tuning improve cylinder filling, the key is timing. The pressure waves must arrive at the proper time to maximize cylinder filling. Obviously they must also not overlap or work against each other (where a positive pressure wave is effectively cancelled out by a negative one). The runner length (and diameter) help determine when (in the rpm range) these forms of cylinder filling become effective. Since the pressure waves run through the intake runners (and head port) at the speed of sound, altering the distance changes the travel time through the port. The travel time can also be altered (to a lesser extent) with changes in temperature and pressure. If the runner length is properly tuned, the motor will realize not only the airflow drawn in by way of the vacuum created by the downward moving piston, but also the mild supercharging effect that comes from the inertial and reflected wave tuning.
A third form of wave tuning is called Helmholtz resonance. This deals with the resonance of the common plenum and tuned length of inlet tubing (from the air filter to the intake manifold). Once excited, the common plenum resonates at a given frequency. These resonance waves can be used to bombard the intake valve with additional pressure, once again assuming the tuned frequency coincides with the effective operating speed of the motor. Note that these forms of ram tuning are irrespective of the airflow potential of the port and function even in boosted applications.
While we have simply scratched the surface of the intake manifold, it should be clear that the proper design is much more complicated than simply building runners connected to a common plenum fed by a throttle body. Obviously computer simulations can be used to design an intake manifold for a given application, but given the complex nature and sheer number of pressure waves involved (most intake modeling is done with a single-cylinder motor), the final testing almost always involves actual dyno testing. Though it is possible to determine (or design) the effective operating range of the intake manifold runner length, diameter, and plenum, what effect does the design have at other engine speeds? Is the area under the curve sufficient? Are there any undesirable dips or peak? These are questions that only the dyno can answer effectively. Thus a proper intake design should see plenty of dyno time before being finalized. Obviously it would be nice if the same intake fit properly, bolted up to all the original hardware and looking like a proper performance piece.