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

In internal combustion engines, a 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 across the range of engine speed operation, as well as help provide better fuel efficiency.

There are two main effects of variable intake geometry:

Swirl

Variable geometry can create a beneficial air swirl pattern, or turbulence in the combustion chamber. The swirling helps distribute the fuel and form a homogeneous air-fuel mixture - this aids the initiation of the combustion process, helps minimise engine knocking, and helps facilitate complete combustion. At low revolutions per minute (rpms), the speed of the airflow is increased by directing the air through a longer path with limited capacity (i.e., cross-sectional area) - and this assists in improving low engine speed torque. At high rpms, the shorter and larger path opens when the load increases, so that a greater amount of air with least resistance can enter the chamber - this helps maximise 'top-end' power. In double overhead camshaft (DOHC) designs, the air paths may sometimes be 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.

Applications

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 120PS, 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 — Dual-Stage Intake (DSI), on their Duratec 2.5 and 3.0 litre V6s, and it was also found on the Yamaha V6 in the Taurus SHO.
• Ford — The Ford Modular V8 engines use either the Intake Manifold Runner Control (IMRC) for four-valve engines, or the Charge Motion Control Valve (CMCV) for three-valve engines.
• General Motors — 3.9 litre LZ8/LZ9 V6, 3.2 litre LA3 V6, LT5 5.7 Liter
• GM Daewoo — DOHC versions of E-TEC II engines
• Holden — Alloytec
• Honda — Integra, Legend, NSX, Prelude, Civic
• Hyundai — XG V6
• Isuzu — Rodea used in the second generation V6, 3.2 litre (6VD1) Rodeos.
• Jaguar — AJ-V6
• Lancia — VIS
• Mazda — Variable Inertia Charging System (VICS) is used on the Mazda FE-DOHC engine and Mazda B engine family of inline-four engines, and Variable Resistance Induction System (VRIS) 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 2.0 litre inline-four 4G63 engine family.
• Nissan — inline-four engines, V6 engines, V8 engines
• Opel (and/or Vauxhall) — VIM - modern versions of Ecotec Family 1 and Ecotec Family 0 inline-four engines; a similar technology is used in 3.2 litre 54° V6 engine
• Peugeot — 2.2 litre inline-four engine, 3.0 litre 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 2.0 RS
• Rover — Rover 75 and Freelander KV6 engine 2.0 litre and 2.5 litre with Siemens engine control unit.
• Toyota — Toyota Variable Induction System (T-VIS), used in the early versions of the 3S-GE, 7M-GE, and 4A-GE families, and Acoustic Control Induction System - (ACIS).
• Volkswagen — 1.6 litre inline-four engine, V6 engines, VR6 engines, W8 engines, V8 engines
• 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 1,500 and 4,100 rpm at 80% load or higher.^[1]

References

Ford - The 2.0L Split Port engine features an Intake Manifold Runner Control variable geometry intake manifold.