Subaru Impreza RB320oc Owners Club RB320

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Unveiling the hidden beauty of Subaru's mechanical parts

Mechanical Parts

Intake manifold



The intake manifold is the component that delivers air to the engine. Formerly, gasoline was sprayed and mixed with air in the carburetor, after which this air-fuel mixture was delivered into the cylinder. In this set-up, the intake manifold was the path the air-fuel mixture took on its way to the cylinder. Nowadays, the use of fuel injection, which controls the condition of the air-fuel mixture by computer, means that only air flows through the intake manifold. In this system, gasoline is injected from the fuel injector, located next to the intake valve of the cylinder head, and mixed with the air from the intake manifold to become an air-fuel mixture.

In my discussion with Kazuei Itoh of the Subaru Engineering Division he explained, "Delivering air to the cylinder efficiently is the job of the intake manifold. When a lot of power is required, a lot of fuel is burnt. Consequently, even more air is required. In addition, when we keep constant pressure on the accelerator pedal in order to remain at the steady speed, the intake manifold adjusts the amount of air to minimize the amount of fuel burnt, thus enhancing fuel efficiency. Putting engine power and torque out, or in other words, the tuning of engine performance, is determined by the intake manifold."

Kazuei Itoh has been involved in the research and development of engine control for the past eleven years. He designed the intake manifold for the naturally aspirated six-cylinder, Horizontally-Opposed engine mounted on the Outback H6-3.0.

"I had been in charge of intake manifold improvements before that, but this project was the first in which I had to design a manifold from scratch. A new design for the intake manifold is not required unless a new engine is developed. Therefore, this was a rare but valuable opportunity for me. It caused me to realize again the infinitely profound nature of the development process for the intake manifold."

When Mr. Itoh began work as an engineer at Subaru, the age of the carburetor had already ended. He belongs to a new generation, one which knows that fuel injection is much more advantageous for reducing exhaust gases and enhancing fuel efficiency.

At first, after taking charge of the intake manifold for the naturally aspirated engine, Mr. Itoh thought that the design for the intake manifold for the turbo engine would be simpler. With the turbo charger, charged air is delivered forcefully to the cylinder. As a result, he believed that the intake manifold would not play a direct role in engine performance. However, he learned that it would be a tremendous challenge to eliminate turbo lag, which is the great enemy of the turbo engine, in development of the intake manifold for the turbo engine, and he was surprised to rediscover the profound nature of the technology involved in the intake manifold.

"The aim is not only to increase the numerical value of power and torque, but also to determine how regulating power and torque can provide an enjoyable ride for driver and passenger. With this kind of sensual evaluation, we can determine the characteristics of the engine. In this area the role of the intake manifold is very important. Since this approach takes into consideration the human senses, it is a difficult and never-ending area of technical research and development."

The intake manifold of the naturally aspirated, six-cylinder, Horizontally-Opposed engine is positioned on the engine and runs to each cylinder head as if covering the upper part of the engine. The air flows from the air intake located in the front of the engine compartment through the air cleaner and enters the intake manifold. This is where the throttle valve adjusts the total volume of air. When maximum power is required, the valve is fully open.

Once drawn into the intake manifold, the air is first divided into two streams for the right and left laterally-aligned cylinder groups. In this case it is acceptable to think of this as one pipe dividing into two pipes. At Subaru these two pipes are called L2 ports. Air divided into right and left streams is contained in a chamber for each, and from there the air is separated on each side by three pipes, called L1 ports, after which it flows into the cylinder head. There are three pipes leading from each of the two laterally positioned chambers, making a total of six pipes.

The ports have a complicated curvature. The L1 ports -- six pipes of the same length running to each cylinder which are bent slightly to correspond to the dimensions -- are especially complex.

"It is important to determine the diameter and length of the port. That changes the pressure of the air inside the intake manifold, causing waves to occur. Through the intake valves, which open at different times, it is possible to deliver air into cylinders very efficiently."

The diameter and length of the port is initially determined by computer. But the last step is trial-and-error development by hand, manufacturing the prototype of the intake manifold.

The form of each port becomes more complicated, because it is also necessary to share space with other accessories such as the generator.

"When the engine is running at 6000 rpm, the intake valve for the cylinder head is open for about 0.005 seconds. In this fraction of time a 500 cc fuel-air mixture is delivered. By adjusting this volume the character of the engine is determined."

This is a basic function of the intake manifold. However, the intake manifold has two other functions. The first relates to the variable intake valve that links the two chambers. When the valve is open the two chambers are connected. When the valve is closed there is a flow of air for an emphasis on torque, whereas when it is open, the emphasis is on power.

The second function concerns the EGR (exhaust gas recirculation) valve. This function involves mixing exhaust gas and reducing the volume of air, with the aim of enhancing fuel efficiency and thereby restricting the volume of gasoline when cruising at a constant speed. Ideally the air-gasoline mixture that is delivered to the cylinder will maintain a stoichiometric air fuel ratio and be constant. For this reason, when fuel is restricted the volume of air must also be reduced. Since the cylinder has a 500 cc capacity, on occasions when it is unnecessary to have large output such as during cruising, a balance between cylinder capacity and necessary volume of air is achieved by mixing exhaust gas with air.

On viewing the entire air intake manifold system, which distributes air efficiently, a photographer who was with me described its organic form as "strangely sexy."

Certainly the intake manifold does exudes a strong sense of vitality, I realized.

Quick Links

Richard Burns

Subaru rally legend, Richard Burns Britain's only FIA World Rally Champion died in November 2005 of brain cancer at the age of 34.

Click here for more information on the 2001 World Rally Champion.

RB320 Features

  • 6,000 of extra equipment
  • 320PS/450Nm Prodrive tuned
  • build limited to 320 units
  • finished in Obsidian Black
  • bespoke 18" alloy wheels
  • unique numbered tax disc holder
  • Prodrive tuned suspension system with Bilstein dampers, Eibach springs & rear ARB
  • lowered ride height
  • polished s/steel mesh front grille
  • STI front lip spoiler
  • STI quickshift gearchange
  • STI side sill plates
  • RB branded gear knob
  • RB branded car mats
  • RB exterior badging
Click here for full list