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Subaru mechanicals

Broadcast dates : 18th July 2004
22nd July 2004


Subaru is one of a handful of manufacturers to employ a boxer engine configuration. The other famous car-builder to use this engine layout is Porsche with its six-cylinder 911 series. And before that, Volkswagen made millions of four-cylinder boxer motors for the long-lived Beetle.

In this type of engine the cylinders are horizontally opposed. In the case of the four-cylinder Subaru engine, two cylinders are laid-out on each side of the crankshaft.

As the crankshaft revolves the pistons travel along their cylinders in a motion resembling a type of counter-punching action, hence the affectionate nickname of "boxer" for these engines.

The Subaru four-cylinder engine is of a very high-performance specification, despite its warbling exhaust note. It uses a turbocharger to boost intake charge to the cylinders and has four valves per cylinder.

The engine casing splits longitudinally over the crankshaft and these casings are made of lightweight aluminum.

The low overall weight of the engine and the low centre of gravity due to the flat, or boxer design, contribute enormously to the Subaru's renowned road-holding.

Aluminum intake manifolds are designed with equal-length runners to all four cylinders to equalise the cylinder charging flow speeds and volumes.

Another interesting design feature is that a single toothed belt is used to drive both sets of camshafts and the water pump.

In the very special 195 kilowatts WRX STi engine, some eighty per cent of the parts are different to the standard WRX engine, in the interests of strength, reliability and performance.

The STi engine uses a steel crankshaft with light counterweights, forged aluminum pistons and sodium-filled exhaust valves.

The combustion chambers are of the pent-roof design, with the pairs of intake and exhaust valves laid out on either side of the centrally located spark plug.

The STi high-performance version of the WRX also features a closed-deck engine casing. This refers to reinforcement in the water-jacket areas where the casing mates with the cylinder heads. This is to contain the extra turbo charging boost levels used in the STi version.

Each cylinder head has a pair of camshafts, the WRX thus being a four-cam engine. The valve actuation from camshaft to the valve stems is carried out by shimless bucket, with variable valve timing on the inlet cams.

The STi engine has an intercooler that is some eleven per cent larger than that of the standard WRX. The intercooler is a type of air-to-air radiator designed to cool the intake charge of the turbocharged engine. The larger intercooler on the STi is said to reduce intake charge temperatures by as much as 60 per cent.

The WRX STi uses a single turbocharger. This is the IHI VF 35 roller bearing turbo unit, designed for a very fast spool-up speed to minimize the dreaded turbo lag. The exhaust-driven turbocharger boosts intake pressure by up to 1,1 bar on the STi model.

Subaru is very proud of its integrated four-wheel-drive layout. It uses a five-speed manual transmission in the WRX and a six-speed manual transmission in the WRX STi. The gearbox casing incorporates the front differential, the transfer case for drive to the rear axle, and the centre differential.

This design is one of the keys to Subaru's famous Symmetrical All-Wheel Drive System.

The forged lightweight pistons on the STi also feature short skirts to reduce internal friction.

The Symmetrical All Wheel Drive system refers to the fact that both pairs of drive shafts on the front and rear axles are of equal length. This enables equal delivery of power to all four wheels without resorting to extra couplings and drive shafts of differing thickness, which causes torque steer.

The power split on the Subaru is fifty per cent to the front, and fifty per cent to the rear axle. However, the viscous coupling incorporated in the centre differential will redirect this power allocation in the case of one or more of the wheels losing traction.

In the STi version of the WRX Sure Track limited slip differentials are also used on both front and rear axles. This reduces under steer in the special model, enabling an aggressive competition driving style.

The front suspension is of the McPherson strut design with beautifully crafted aluminum lower control arms.

The rear suspension is of a multi-link, trailing arm design, with coil-over suspension struts. Anti-roll bars are incorporated on both the front and rear axles to increase stability.

It is engineering of such a high standard that has seen the Subaru name gain worldwide prominence in the past decade. Once a builder of tiny economy cars for the Japanese market, the Subaru is now recognised as one of the world's most desirable road cars.

A cult car with a worldwide following.

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