Editing Variable-length intake manifold (section)

{{Short description|Internal combustion engine technology}}
{{Refimprove|date=June 2009}}
[[Image:Lower-intake-manifold.jpg|thumb|Lower intake manifold on a 1999 Mazda Miata [[Mazda B engine#BP-4W|engine]], showing components of a variable-length intake system.]]

In [[internal combustion engine]]s, a '''variable-length intake manifold''' ('''VLIM'''),'''variable intake manifold''' ('''VIM'''), or '''variable intake system''' ('''VIS''') is an [[automobile]] internal combustion engine [[Inlet manifold|manifold]] technology.  As the name implies, VLIM/VIM/VIS can vary the length of the intake tract in order to optimise [[power (physics)|power]] and [[torque]] across the range of engine speed operation, as well as to help provide better [[fuel efficiency]]. This effect is often achieved by having two separate intake ports, each controlled by a valve, that open two different manifolds – one with a short path that operates at full engine load, and another with a significantly longer path that operates at lower load. The first patent issued for a variable length intake manifold was published in 1958, US Patent US2835235 by [[Daimler Benz AG]].<ref>[https://patents.google.com/patent/US2835235 US Patent 2835235]</ref>

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 [[wiktionary:homogeneous|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]] (rpm), 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 rpm, 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 [[overhead camshaft#Double overhead camshaft|double overhead camshaft]] (DOHC) designs, the air paths may sometimes be connected to separate [[poppet valve|intake valve]]s{{citation needed|date=November 2009}} so the shorter path can be excluded by de-activating the intake valve itself.
;Pressurisation: A [[engine tuning|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.