Variable Intake Manifold

Variable intake manifolding is 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 a variable intake manifold benefits more low-speed torque than high-end power. Therefore it is very useful for saloons which are heavier and heavier these days with better drivability, there are also increasingly more sports cars featuring variable intake manifold alongside VVT, these including Ferrari 360 M and 550M.

Compared with VVT, variable intake manifolding 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

Variable length intake manifolds is commonly used in saloons. Most designs employ 2 intake manifolds with different lengths to serve each cylinder. The longer one is for low-speed use. The shorter one is for high revs. It is easy to understand why high speed need a short manifolds, 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 smoothing the pulses, thus matches the lower revs of engine. This provide better cylinder filling, thus improves torque output. Besides, longer intake manifolds leads to slower air flow, hence better mixing between air and fuel.
 

You can clearly see the manifolds of Ford's Duratec 2.5 litres V6 engine. Each cylinder has a long pipe and a short pipe.

 

Toyota's 2 litres Variable Intake engine also has a manifold longer than another

 

Some systems offer 3 stages of variable length, such as the one used by Audi's V8. How can Audi package all 3 manifolds for each cylinder, and a total of 24 manifolds in one engine? 

In fact, Audi doesn't use separate manifolds. Instead, it uses a rotary intake manifold with the inlet at the center of the rotor. The inlet rotates to different positions to form different lengths of manifold. The whole system recesses in the V-valley.

Resonance intake system

Boxer engines and V-type engines (but not inline engines) may employ resonance intake manifold to boost mid to high rev efficiency. 

Each bank of cylinders are fed by a common plenum chamber through separate pipes. The two plenum chambers are interconnected by two pipes of different diameters. One of the pipes can be closed by a valve controlled by the engine management system. The firing order is arranged such that the cylinders breath alternately from each chamber, creating pressure wave between them. If the frequency of pressure wave matches the revs, it can help in filling the cylinders, thus improving breathing efficiency. As the frequency depends on the cross-sectional area of the interconnecting pipes, by closing one of them at low revs, the area as well as frequency reduces, thus enhancing mid-rev range output. At high rpm, the valve is opened and thus improves high-speed cylinder filling.
 

Porsche 996 GT3's resonance intake system. Note that 2 pipes connect between the 2 plenums.

 

A Resonance intake system has been used in various Porsches starting from 964 Carrera. Since the 993, Porsche combined it with an additional variable length manifolding to form a 3-stage intake system named Varioram. However, it is very space-engaging so that the 996 employs only the resonance intake system. The Honda NSX is another rare and wonderous application of the  resonance intake system.
 

Porsche's VarioRam
Below 5,000 rpm (left A and top right) : long pipes; resonance intake disabled.   5,000-5,800 rpm (left B and middle right) : long pipes plus short-pipe resonance intake, with one of the interconnected pipes of the resonance intake closed.   Above 5,800 rpm (left C and bottom right): long pipes plus short-pipe resonance intake, with both interconnected pipes of the resonance intake opened.