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 | Ford's Duratec 2.5 liters V6 engine Note: Each cylinder has a short / long intake. |
Variable Intake Manifold: Increasingly more popular since the mid-90s. It is employed to boost low to medium speed torque without any drawback in fuel consumption or high speed power, thus improve flexibility of the engine. An ordinary fixed intake manifold has its geometry optimized for high speed power, or low speed torque, or a compromise between them. Variable intake manifolds introduce one or two more stages to deal with different engine speeds.
The result sounds like variable valve timing, but variable intake manifold benefits more low-speed torque than high-end power. Therefore it is very useful for sedans, which are heavier and heavier these days. For better derivability, there are also increasingly more sports cars feature variable intake manifold alongside VVT, these including Ferrari 360 M and 550M.
| Compare with VVT, variable intake manifold is cheaper. What it needs are just some cast manifolds and a few electric-operated valves. In contrast, VVT need some elegant and precise hydraulic actuators, or even some special cam followers and camshafts.
There are two kinds of variable intake manifolds: variable length intake manifolds and resonance intake. Both of them make use of the geometry of intake manifolds to reach the same goal. | Variable length intake manifolds
Commonly used in sedans. Most designs employ 2 intake manifolds with different length to serve each cylinder. The longer one is for low-speed use. The shorter one is for high rev. It is easy to understand why high speed need a short manifold, because it enables freer and straightforward breathing. But why does it need longer pipe for low speed ? because longer pipe results in lower frequency of air mass reaching the cylinder, thus matches the lower rev of engine very much. This provide better cylinder filling, thus improves torque output. Besides, longer intake manifold leads to slower air flow, hence better mixing between air and fuel. | |
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