Variable length intake manifold: Information from Answers.com

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In internal combustion engines, variable length intake manifold (VLIM), or variable intake manifold (VIM) is an automobile internal combustion engine manifold technology. As the name implies, VLIM/VIM can vary the length of the intake tract in order to optimise power and torque, as well as provide better fuel efficiency.

Lower intake manifold on a 1999 Mazda Miata engine, showing components of a variable length intake system.

There are two main effects of variable intake geometry:

Swirl - Variable geometry can create a beneficial air swirl pattern in the combustion chamber. The swirling helps distribute the fuel and form a homogeneous air-fuel mixture which ignites without engine knocking. At low rpms, the speed of the airflow is increased by directing the air through a longer path with limited capacity (i.e., cross-sectional area), but the shorter and larger path opens when the load increases so that a greater amount of air can enter the chamber. In DOHC designs, the air paths are often connected to separate intake valves so the shorter path can be excluded by de-activating the intake valve itself.
Pressurisation - A tuned intake path can have a light pressurising 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. When the intake air speed is higher, the dynamic pressure pushing the air (and/or mixture) inside the engine is increased. The dynamic pressure is proportional to the square of the inlet air speed, so by making the passage narrower or longer the speed/dynamic pressure is increased.

Many automobile manufacturers use similar technology with different names. Another common term for this technology is Variable Resonance Induction System (VRIS).

Audi - 2.8 litre V6 petrol engine (1991-98); 3.6 and 4.2 litre V8 engines, 1987-present
Alfa Romeo - TwinSpark 16v (1.4, 1.6 120 ps, 1.8 and 2.0 litre) and JTS Engines
BMW - Two Port: BMW M42, Three Port: BMW N52
Dodge - 2.0 A588 - ECH (2001-2005) used in the 2001-2005 model year Dodge Neon R/T
Ferrari - 360 Modena, 550 Maranello
Ford DSI (Dual-Stage Intake) - on their Duratec 2.5 and 3.0 liter V6s and it was also found on the Yamaha V6 in the Taurus SHO.
Ford - The Ford Modular V8 engines sport either the Intake Manifold Runner Control (IMRC) for 4V engines, or the Charge Motion Control Valve (CMCV) for 3V engines.
General Motors - 3.9L LZ8/LZ9 V6, 3.2L LA3 V6
GM Daewoo - DOHC versions of E-TEC II engines
Holden - Alloytec
Honda - Integra, Legend, NSX, Prelude, Civic
Hyundai - XG V6
Isuzu - Isuzu Rodeo Used in the second generation V6, 3.2L (6VD1) Rodeos.
Jaguar - AJ-V6
Lancia VIS
Mazda VICS (Variable Inertia Charging System) is used on the Mazda FE-DOHC engine and Mazda B engine family of straight-4s, and VRIS (Variable Resistance Induction System) in the Mazda K engine family of V6 engines. An updated version of this technology is employed on the new Mazda Z engine, which is also used by Ford as the Duratec.
Mercedes-Benz
Mitsubishi Cyclone is used on the 2.0L I4 4G63 engine family.
Nissan I4, V6, V8
Opel (or Vauxhall) VIM - modern versions of Ecotec Family 1 and Ecotec Family 0 straight-4 engines; a similar technology is used in 3.2 L 54° V6 engine
Peugeot 2.2 L I4, 3.0 L V6
Porsche VarioRam - 964, 993, 996, Boxster
Proton Campro CPS and VIM - Proton Gen-2 CPS and Proton Waja CPS; Proton Campro IAFM - 2008 Proton Saga 1.3
Renault - Clio 2.0RS
Toyota T-VIS - (Toyota Variable Induction System) used in the early versions of the 3S-GE, 7M-GE, and 4A-GE families, and ACIS - (Acoustic Control Induction System).
Volkswagen - 1.6 L I4, VR6, W8
Volvo - VVIS (Volvo Variable Induction System) Volvo B52 engine as found on the Volvo 850 and S70/V70 vehicles, and their successors. Longer inlet ducts used between 1500 and 4100 RPM at 80% load or higher.^[1]

References

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