Subaru Impreza RB320oc Owners Club RB320

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Unveiling the hidden beauty of Subaru's mechanical parts

Mechanical Parts

Chain drive system



The chain drive system, which is powered by the rotation of the engine, activates the camshaft and water pump. The chain that transfers this power is called the timing chain. This chain is a well-known mechanism among automobile enthusiasts.

I interviewed the engineer in charge of the chain drive system for the Horizontally-Opposed 6-cylinder engine -- a light, compact engine mounted on the Outback H6-3.0.

To begin with, the timing chain is used in bicycles and motorcycles, and most people probably know what it is. It's a very familiar and easy-to-understand mechanism.

"Like you said, the chain is a very simple mechanism, but it's also a very profound one," says Takumi Ikeda, an engineer at the Subaru Engineering Division. Ikeda conducts research and development on the functionality and durability of all engine parts.

In the Subaru Horizontally-Opposed engine, a rubber belt has been used to drive the camshaft. However, for the new Horizontally-Opposed 6-cylinder engine, Subaru has adopted the chain.

This is why:

"In order to realize comfortable driving with the Horizontally-Opposed 6-cylinder engine, we had to minimize the length of the engine as much as possible. If the length of the engine is too great, the front part of the car becomes too heavy, and the car's overall balance is off. So, we adopted the chain drive system, which allows us to keep the length of the engine at an optimal short size."

What stands out in Ikeda's explanation is that the decision to adopt the chain drive system was made not from the technical point of view of the engine, but rather from the point of view of overall vehicle balance in order to create an enjoyable driving experience. The important thing is the vehicle's overall balance. To ensure superior balance, all aspects of technical development must be thoroughly considered. This is Subaru's philosophy of vehicle development.

In the rubber belt drive system of the former Horizontally-Opposed 6-cylinder engine mounted on the Subaru SVX, a 34mm -wide rubber belt activates the right and left camshafts and the water pump. However, in the current Horizontally-Opposed 6-cylinder engine's chain drive system, two14mm-wide chains are used to perform the same function. In the Horizontally-Opposed 6-cylinder engine, two cylinder heads on the right and left sides have been located slightly out of alignment. The chain drive system utilizes this alignment difference; it is still approx. 16mm shorter than the rubber belt drive system even though it uses two chains. 16mm may not seem like much, but every millimeter must be taken into account in an effort to make the 6-cylinder engine as compact as the 4-cylinder engine. In such technical development as this, 16mm becomes a very significant number. However, there are advantages and disadvantages to any technology. The rubber belt has been used in the 4-cylinder engine because it is lighter and produces less noise and vibration. Because the chain is metal, it is heavier, noisier and vibrates more than a rubber belt. However, the advantage of the chain is that it is much more durable. Depending on the type, there is a possibility of the rubber belt wearing out or breaking after 100,000 to 160,000kilometers (62,000 to 99,200miles), and it must be replaced regularly, but the chain does not ever have to be replaced.

The chain drive system mechanism operates as follows. Power is transferred directly from the engine crankshaft. The sprocket, combined with the crankshaft, drives the chain. The left side chain activates the two camshafts of the left cylinder head, as well as the water pump and the double idler sprocket. The chain, driven by this double idler sprocket, activates the two camshafts of the right cylinder head.

The left side chain measures approx. 1.5m and weighs 619g. The number of links connecting the plates of chain is 148. The right-hand side chain measures approx.1.2m and weighs 552g, and the number of links is 134.

If you read this, the chain may be imagined as a simple mechanism like a bicycle chain, but there is more to it than that.

In the Horizontally-Opposed 6-cylinder engine's chain drive system, the chain moves at speeds of up to 72km/h (44.7miles/h) when the engine is running at 7,000 rpm. Immediately after combustion inside the cylinders, a great deal of power is transferred to the crankshaft. And the camshaft needs a lot of power to lift the valve. In other words, the chain is instantly pulled taut when the crankshaft and the camshaft receive power. When this sharp force catches the chain, the chain gets wiggling or wobbling and doesn't move smoothly.

To put it another way, the entire engine expands when it heats up. As a result, the distance between the sprocket that is attached to the crankshaft and the camshaft, and the double idler sprocket and the sprocket that activates the water pump increases by a few millimeters. The chain must accommodate this increased distance.

This wiggling and slight lack of smooth movement and the expansion of the chain makes difficults to rotate the camshaft smoothly but can also cause vibration and noise. Although the chain itself can bear 1.2tons of weight, if the camshaft rotates unsteadily, there is a possibility that the chain can break.

To prevent this from happening, the chain drive system features a tensioner to regulate the chain's tension and a guide to prevent the chain from wobbling too much. These features enable the chains to move in a controlled, smooth way. However, if the chain's tension is too great, the resulting frictional resistance will cause power wastage and inefficient fuel consumption.

The chain drive system is lubricated with engine oil, and so there is no danger of the oil running dry. However, if the layout is not optimized, it can impair engine performance and lead to other troubles and problems.

"It was the first time for me to be in charge of the development of the chain drive system. I thought it would be difficult to resolve the issues of vibration and noise, but I wanted to take on such a demanding challenge. However, once I had begun development, it was even harder than I thought. Through computer simulation we were able to design only the most basic parts and the other measuring technology had limits. Ultimately, I had to figure it out by observing with my own eyes and find a way out, and it took two years to complete," says Ikeda. For him, the final test of the chain drive system was to drive a prototype car in which the Horizontally-Opposed 6-cylinder engine was mounted.

When I asked him whether he was aware of the movement of the chain, Ikeda laughed. "I couldn't feel it with my body, but because I'd worked so hard to develop it, I could feel it with my mind."

Once again, I am impressed by the devotion and persistence of Subaru engineers.

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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