Variable Length Intake Manifold (VLIM) is an automobile engine manifold technology. As the name implies, VLIM can vary the length of the intake tract in order to optimize power and torque, as well as provide better fuel efficiency.
There are two main effects of variable intake geometry:
Swirl - Variable geometry can create a beneficial air swirl pattern in the combustion chamber. The swirls help distribute the fuel and form a homogeneous air-fuel mixture which ignites without engine knocking.
At low rpm, the speed of the airflow is increased by directing the air through a longer path with limited capacity, but the shorter and more capable path opens when the load increases so that the greater amount of air can enter the chamber.
In DOHC designs, the air paths are often connected to a separate intake valves so the shorter path can be excluded by inactivating the intake valve itself.
Pressurization - A tuned intake path can have a light pressurizing effect similar to a low-pressure supercharger due to Helmholtz resonance. However, this effect occurs only over a narrow engine speed band. A variable intake can create two or more pressurized hot spots, increasing engine output.
Many automobile manufacturers use similar technology with different names.
Another common term for this technology is Variable Resonance Induction System (VRIS).