Outback H6 – more cylinders, more power

Subaru has released an upgraded version of its Outback wagon, the H6, powered by a completely new six-cylinder `boxer’ engine.

It is only the second six-cylinder engine to be offered by the company in Australia, the first being the 3.3 litre EG33 that powered the Subaru SVX coupe, production of which ended in December 1996.

Representing an engineering feat in itself, the new 3.0 litre engine is 34 per cent more powerful than its four-cylinder 2.5 litre stablemate - yet only 20 mm longer. And its exhaust emissions are nine times lower than Australian 2000 standards.

The new engine produces 154 kW of power at 6,000 rpm and 282 Nm of torque at 4,400 rpm, giving the H6 a 1,600 kg towing capacity.

Outback H6 also features Subaru’s leading edge Vehicle Dynamics Control (VDC), an advanced safety system only recently introduced to Australia in the Liberty Heritage wagon.

The new wagon is also strong on luxury, bringing new levels of comfort and refinement to the highly successful Outback range, with some distinctive changes including a full tan leather interior.

At the heart of things
This newest Boxer engine, the six-cylinder EZ3O, offers all the traditional benefits of Subaru horizontally opposed engines, including low centre of gravity and compactness.

It is just 20 mm longer and approximately 40 kg heavier than Subaru’s four cylinder 2.5 litre engines, achieved by reducing cylinder diameter and the distance between cylinders.

Other innovations include camshaft drive by dual chains, rather than toothed belt. Unlike the standard toothed belt system, extremely durable chains do not require replacement at 100,000 km intervals.

A single multi-ribbed serpentine belt, with an automatic tensioner to also improve serviceability, drives the alternator, power steering pump and air conditioning compressor. Liquid-filled engine mounts also contribute to the smooth, quiet ride.

High output - but Eco-Friendly
Outback H6 meets the world’s strictest emission standard, Europe’s benchmark STEP3.

Cleaner exhaust gas has been achieved by installing three catalytic converters - one in each of the exhaust manifolds and one in the mixing chamber.

However, improved emission does not impact on performance. The variable intake system enables efficient intake in all speed ranges, resulting in high torque output throughout the engine speed range. Assisting this is the dual stage exhaust.

The intake ports (straight tumble ports) promote combustion by increasing air/fuel mixing to optimise efficiency. Air assist injectors are used to further enhance fuel injection, by atomising the fuel more effectively before it enters the combustion chamber.

The induction control valve provides two intake characteristics: at heavy loads below 3,600 rpm, the valve remains closed to create separate intake runners, helping to boost low end and mid-range power. This is achieved by the increase in intake airspeed, creating a ram effect.

At heavy load and high engine speed, above approximately 3,600 rpm, the valve opens, reducing airflow resistance. This creates a "scavenging" effect, allowing each cylinder to catch high-pressure waves of intake air bouncing off the opposite side of the manifold.

A mechanical valve in the rear muffler opens at higher engine loads, when exhaust pressure reaches about 2.2 psi. The increased exhaust volume in these conditions bypasses a section of the muffler, the consequent reduction of exhaust back pressure enhancing power at higher speeds. Muffler valve operation does not affect the noise level.

Combined, these factors produce improved overall engine efficiency. Engine power loss due to friction has also been reduced by improving piston surface contact and connecting rod precision.

Other engine features
Overall power unit rigidity is improved. The crankshaft is supported in seven main bearings and the cylinder block and transmission case are attached with 11 bolts (compared to eight on the four-cylinder).

The engine layout allows an alternating cylinder firing order that reduces inertia, further benefiting the inherent balance of the horizontally opposed engine, and low engine noise levels have been achieved by using upper and lower engine covers and aluminium die cast chain and cam covers.

The all-new EZ3O design has no common components with any current Subaru powerplant. An innovative feature is the separate "oil pan upper," a casting that holds the oil and water pump assemblies, oil filter and cooler. The new engine block structure eliminates the need for a separate coolant pipe casting.

Engine control
The engine control system features sequential fuel injection for a high level of accuracy in the fuel metering process, contributing to low emissions with high power output.

An air/fuel ratio feedback and learning system accurately monitors fuel mixture after combustion, via dual A/F sensors. The engine computer tracks the air/fuel ratio and makes constant mixture adjustments to achieve the lowest possible emissions.

The direct ignition system uses an individual ignition coil for each cylinder, eliminating the need for a distributor and sparkplug wires.

Dual knocksensors allow the engine computer to finely adjust ignition timing in response to combustion noise feedback. Maximum burning efficiency is achieved irrespective of fuel octane rating, though the best efficiency is achieved with 95+ RON fuel, which allows the maximum possible ignition angle to be used.