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| Subaru
mechanicals |
Broadcast
dates : 18th July 2004
22nd July 2004 |
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 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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