The new Subaru opposed six-cylinder engine produces 164 kW (220 hp) and 289 Nom (213 lboft) in the Legacy Outback-6.

"Credit must be given to whom it is due," emphasized Masaru Katsurada, "Mr. Legacy" at Fuji Heavy Industries, who has since ascended to the position of Vice President in charge of the company's product planning. "No, the Outback was not invented here. The astute planners at Subaru of America had come up with the idea of this crossover vehicle. Only those Americans, who have a grasp of what the country's love of SUVs is all about, and understand how far we could push the envelope, could come up with such a brilliant concept." So much so that the Outback was made available in the Japanese market with a fancier name: Lancaster.

This is a competitive world, and Subaru could not hope to monopolize the car-derived crossover vehicle market. Already such powerful players as BMW, Audi, and Volvo are entering this lucrative segment.

One item the Outback had been lacking was a six-cylinder engine, although its opposed four-cylinder engine, with its inherently smooth running characteristics, was considered worth two extra cylinders. The number "six" is still a must for a mid-size car that aims upscale. Noriaki Sekine, Manager of power unit development at Fuji Heavy Industries, observed that the Outback is particularly popular among America's female customers and retired couples, and that they would love to have more low-end and mid-speed torque, as well as smooth running.

While the liquid-cooled, opposed four-cylinder engine has been the hallmark of Subaru's compact and mid-size cars since the first Subaru 1000 of 1965, the planners did not limit their search to the "boxer" type, but included inline and vee configurations. Their choice was a new flat six, because it would have to fit in the Outback and because of the company's experience and expertise with the inline all-wheel drivetrain.

Subaru had developed an opposed six-cylinder engine, the type EG33D naturally aspirated, quad-camshaft, 24-valve 3.3-L unit that powered the SVX sports coupe. That engine involved adding two more cylinders to the E-family four, sharing the short 75.0-mm (2.95-in) stroke. The E-family was the second-generation opposed-cylinder engine introduced in 1991, which in its latest specification powers current Subaru models.

The new EZ30D, dubbed Boxer 6 for its three pairs of pistons exchanging punches in the all alloy block, is a new design and represents Subaru's third-generation flat unit. Compact size, especially in length, was an essential criterion, as it would have to fit neatly in the Outback and maintain a similar weight distribution to its four-cylinder sister model. The EZ30 adds only 20 mm (0.8 in) in length to the EJ25 2.5-L four-cylinder unit. This was achieved by a new set of internal dimensions, which obviously called for a new block/head design and manufacturing tooling. "Getting a new bore pitch-the distance between two adjoining cylinder centers-is really a once-in-your-engineering-life occurrence," said Sekine, "so significant is its influence." The EZ30D has a 98.4-mm (3.87-in) pitch versus the EJ family's 113 mm (4.45 in).

The EZ30D has an 89.2-mm (3.51-in) bore and 80-mm (3.15-in) stroke, which is not as "oversquare" as the EJ25 2.5-L four, which has a large 99.5-mm (3.92-in) bore and 79-mm (3.11-in) stroke. The EZ six has a total displacement of 2999 cm3.

Following Subaru's tradition, the aluminum cylinder block has two halves, each accommodating three cylinders, and is secured at the center by 19 bolts, 14 of which are at the seven bearing journals and five at the top of the horizontal block. Cast iron liners are cast in the high-pressure, die-cast, open-deck block. The block's structural rigidity is further enhanced by a die-cast aluminum upper oil pan. The engine is attached to the four-speed automatic transmission via 11 bolts, versus the four-cylinder's eight. Still the inline drivetrain is quite long, as it houses the final drive, transmission proper, and all-wheel-drive transfer unit. Gussets to enhance drivetrain rigidity tie the engine and transmission together.

On the lower end-more precisely, at the mid-height-the forged carbon-steel crankshaft is supported by seven main bearings of 64 mm (2.52 in) diameter. Four different bearing widths are used: the narrowest being 17.6 mm (0.69 in) and the widest 21.2 mm (0.83 in), which is a thrust bearing type, arresting the shaft's fore-aft movement. The crankshaft length is 406.1 mm (15.99 in). The crank pins are evenly spaced at 60°. The opposing left bank-right bank cylinder Nos. are 2-1, 4-3, and 6-5. Main bearing No. 7 is a thrust type, arresting the crankshaft's fore-and-aft movement. The crankshaft carries 12 balance webs, which are so thin that Subaru calls them "razor webs."

The second important factor in achieving the engine's short overall length is the use of two-stage timing chains, instead of a single timing belt used in the EJ four-cylinder engine (and the EG33D six). Each bank's dual overhead camshafts are driven by its own chain. The two-chain, two-stage arrangement uses the opposed six-cylinder engine's offset cylinder layout, squeezing the crankshaft-driven left-hand (LH) chain neatly in the offset space. The LH chain also drives the coolant pump. The right-hand (RH) bank chain is driven by an idler sprocket, which is driven by the LH chain. Each chain drives three cylinders and is subjected to torque fluctuation so it has to be robust, thus the use of a 9.525-mm (0.38-in) pitch chain-"an unusually wide pitch in this day of 8- or 6-mm (0.31- or 0.24-in) pitch," conceded Sekine. "A single-chain drive would have offset the torque fluctuation, but then one must consider the timing system's reliability and durability, especially in North America where long-distance driving at higher speeds is the norm rather than exception."

Dual overhead camshafts in each bank operate four valves per cylinder via bucket tappets with clearance-adjusting shims inserted on top. The camshaft is a lightweight, built type, with sintered lobes pressed onto the carbon-steel shaft. Sekine observed that there are two schools of engine designers in Japan, one preferring relatively low valve lifts and the other high lifts. Subaru belongs to the latter, employing 9.75-mm (0.384-in) lift for intake and 9.2 mm (0.36 in) for exhaust. Valve diameters are 35 mm (1.38 in) for intake and 30.5 mm (1.20 in) for exhaust.

Fuji's engine designers exploit the incoming charge's tumble motion to promote fast burn in the new six-cylinder engine-a strategy first used in the U.S.-bound Legacy's SOHC 2.5-L four-cylinder engine. This is achieved by a pair of "tumble straight" intake ports for each cylinder. It works particularly well, according to Sekine, in Subaru's opposed cylinder engines, presumably because of the incoming charge's inertia mass, which strengthens tumble motion. The engine operates on the leaner side up to about 3000 rpm to improve economy during low- and mid-load conditions. The injector sprays fuel in two directions and is air-assisted to promote fuel atomization. The EZ30D employs a variable-length induction system, using inertia and resonance charge effects to improve cylinder filling.

The six exhaust ports on each bank collect into a single outlet within the cylinder head, leading to a three-way catalytic converter of 1.0-L (60-in3) volume via a front exhaust pipe. Two rear exhaust pipes from the primary converters merge into an underfloor secondary three-way catalytic converter of 0.5-L (30-in3) volume. Each front exhaust pipe, immediately aft of the manifold, is fitted with an air/fuel ratio sensor. A Lambda O2 sensor is positioned after the secondary underfloor catalytic converter.

The EZ30D engine produces 164 kW (220 hp) at 6000 rpm and 289 Nom (213 lboft) at 4400 rpm on a high 10.7:1 compression ratio using premium-grade, unleaded fuel. The Legacy Outback-6 meets U.S. LEV emissions standards.

Jack Yamaguchi

AEI September 2000