Subaru Impreza RB320oc Owners Club RB320

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Unveiling the hidden beauty of Subaru's mechanical parts

Mechanical Parts

Cylinder block & Cylinder head

The box is a term used in the Subaru Engineering Division to refer to the cylinder block and cylinder head.

A silhouette of the cylinder block and cylinder head certainly has a square, box-like appearance. However, looking at them carefully they appear as human organs and have a kind of sensual beauty.

I asked Satoru Tanimoto of the Subaru Engineering Division, who is in charge of the development of boxes for the Horizontally-Opposed Engine, why the cylinder block and cylinder head are called the box.

Both the cylinder block and the cylinder head are parts that do not actually do anything. The pistons and camshafts perform work inside. For this reason, we considered this the box, inside which we put the parts that do the work.

Since the birth of the Subaru's Horizontally-Opposed Engine 36 years ago, the cylinder block and cylinder head have been made of aluminum. When compared to in-line engines with the same number of cylinders, the horizontally opposed and V-type engines have two cylinder heads making them slightly heavier. Subaru successfully reduced weight of its engines by using aluminum. The engine is called all-aluminum to distinguish it from engines in which only the cylinder head is aluminum.

Even now there aren t many all-aluminum engines, but 36 years ago only special aircraft engines and racing cars adopted such material technology.

Mr. Tanimoto, who has the utmost confidence and commented, Subaru has a long history in developing mass-produced, all-aluminum engines. With a wealth of technical knowledge and experience, Subaru is well placed to promote research and development in this area. We are confident that we will have superiority.

The cylinder block, which is the box containing the piston and the conrod, is cast at high pressure called a die cast. This method involves pouring molten aluminum at high pressure into a metal mold. With this manufacturing method a thin precisely finished product is possible. However, it is not possible to cast complicated shapes.

On the other hand, the cylinder head is cast in the traditional manner at a low pressure. A sand core is set in the main metal mold, and molten aluminum poured in. To incorporate many small and varied parts such as camshafts and valves, the shape of the cylinder head is complicated and therefore requires the traditional low-pressure casting method.

I asked Mr. Tanimoto to explain the function of the cylinder block.

The cylinder block, even though we call it a box, has a wide range of functions. If the cooling water jacket and lubricating oil-line does not function well the engine will overheat. In addition, the cylinder block contains the pistons and the crankshafts, which are very high-speed moving parts. This means that if the cylinder block is not rigid and highly precise the engine will not move smoothly. The precision required for parts such as a cylinder is in the order of a few microns. The cylinder head is bolted to the block by six bolts and a force of more than 6 tons is exerted on each bolt, so if they are not of high tensile strength, they will deform.

The cylinders, precisely aligned in the cylinder block, are subjected to heat due to both combustion and piston movement and, although cooled, can be deformed easily. In addition, the attachment of the cylinder head is easily deformed. Even if deformed slightly, the pistons will not move smoothly.

One difficulty in developing a cylinder block is that it must be rigid, but not heavy. Just because a part is not subjected to a load does not mean we can make it thinner. Reducing the thickness contributes to increased vibration and noise. The cylinder block is not a simple box. Complex technology must be focused and developed to satisfy a range of requirements related to the conrod, piston, cooling and lubrication.

During development, simulations are conducted as a matter of course, but the cylinder block is a good example of vehicle engineering which incorporates elements of empirical science. For this reason, Subaru s wealth of technical knowledge plays an important role.

Mr. Tanimoto said that this technical expertise is the source of Subaru s confidence. I continued by asking about the cylinder head.

The cylinder head contains the combustion chamber and a valve system, and therefore has more complicated requirements. These are the key mechanisms for engine performance and therefore the development of the cylinder head, which contains delicate tuning technology, is one of the most complicated parts. Compared to an in-line engine with the same number of cylinders, the Horizontally-Opposed Engine has smaller cylinder head. Therefore this cylinder head is less likely to deform, a significant advantage. However, the complicated layout of air intake and exhaust are major design considerations.

The cylinder head incorporates such parts as valves, the camshafts and a variable valve timing system. Therefore, a high level of rigidity and precision is required. In the combustion chamber, repeated explosions occur, requiring the cylinder head to be precisely fitted to the cylinder block. Since the intake and exhaust ports must allow good flow, the internal wall should not have any irregularities or obstructions.

If enough oil doesn t circulate around the camshafts, the part is worn away. In addition, cooling the cylinder head is of the utmost importance. To increase the effectiveness of cooling, it is necessary to keep the wall of the water jacket thin. However, if it is too thin rigidity may be insufficient.

The cylinder head is where the balancing of each function must be brought together at a high level of precision. In contrast to image of the word box, the part itself far from a simple. I asked Mr. Taniguchi about the development of the complex technology contained in the box.

Many specialists from each department are brought together to design and develop the box. Finally, we build a test model and performs exhaustive tests on it. This is the most important step.

Mr. Taniguchi heads off for the foundry, where he shapes sand molds with his own hands.

Subaru has a tradition of engineers getting their own hands dirty in the pursuit of technological advances. This is one of the secrets to Subaru s success.

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
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