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Subaru’s Opposite Cylinders Attract Kudos

Subaru’s Opposite Cylinders Attract Kudos

Subaru’s flat, H-shaped, horizontally opposed-piston “boxer” engine offers low profile, low center of gravity and direct injection.

This is the last in the 2016 “Story Behind the 10 Best Engines” series. Watch for the new series of deep dives on each of the 2017 Wards 10 Best Engines beginning in February.

Turbocharged 2.0L 4-cyl. engines have become almost as ubiquitous as belly buttons in recent years. Nearly everyone has one. But not like this one. And this overachieving Subaru 2.0L turbo proudly carries home its second straight Wards 10 Best Engines Award.

Only Subaru and Porsche offer flat, H-shaped, horizontally opposed-piston “boxer” engines in volume vehicles these days. This configuration has some advantages over inline engines, including good dynamic balance, and a low profile and a low center of gravity, along with such disadvantages as higher cost and the need for unique transmissions.

Subaru’s 268-hp turbocharged, direct-injected 2.0L boxer offers eager response and strong performance in the rally-bred 6-speed manual ‘16 WRX in which the WardsAuto judges tested it, yet it is EPA rated at a respectable 20/27 mpg (11.8-8.7 L/100 km) city/highway.

Driving all four wheels through either the manual or Subaru’s Sport Lineartronic (CVT) transmission and Symmetrical All-Wheel Drive, it generates more power and torque from less displacement than the 2.5L turbo H-4 it replaced, along with better fuel economy, on recommended premium fuel.

Based on the 200-hp Wards 10 Best Engines-winning naturally aspirated 2.0L FA boxer that was new in the ‘13 Subaru BRZ sports car, it also is offered (in slightly detuned 250-hp form) in the ‘16 Subaru Forester. Compared with the Forester engine, it uses hotter camshafts, higher-rate valve springs and other enhancements to generate 18 more horses.

To achieve such excellent balance of performance and efficiency, these new FA-Series engines (unrelated to previous Subaru H-4s), use a “square” architecture with identical 86-mm bore and stroke, and all of their reciprocating components were designed to be as light as possible.

In addition to DI and an intercooled twin-scroll turbocharger, they sport dual-overhead (actually outboard) camshafts, dual variable-valve timing (VVT), which Subaru calls “Dual Active Valve Control System (D-AVCS), and a high 10.6:1 compression ratio.

The WRX version redlines at 6,700 rpm with the manual gearbox and 6,500 rpm with the CVT, but reaches its peak 268-hp (134 hp/L) output at just 5,600 rpm, using 15.9 psi (1.09 bar) of turbo boost. Its broad torque curve tops out at 258 lb.-ft. (350 Nm) from 2,000 to 5,200 rpm.

The FA’s top design priority (like nearly every new passenger-vehicle engine today) was the best achievable balance between performance and efficiency. To achieve that balance, design engineers downsized its displacement (vs. the previous H-4) and upgraded its fuel injection from port to direct to deliver more torque and better fuel efficiency, while the vehicle team  worked to improve aerodynamics and tire rolling resistance, among other things.

Subaru of America car-line manager Todd Hill tells us the engine team’s toughest challenges included developing very high combustion efficiency to optimize both power and fuel economy while retaining high reliability.

One additional challenge unique to boxer engines in performance-oriented cars was maintaining sufficient lubrication to both cylinder banks during the sustained high-lateral-load cornering encountered in racing and rallying, when g-forces drive oil to the cylinders on the outside of the corner while potentially starving the inside cylinders. So to keep oil circulating properly under such conditions, they developed special guides that help the camshaft chain carry oil from the outside cylinders back to the sump.

Three years ago, when the all-new naturally aspirated FA H-4 won its 2013 Wards 10 Best Engines trophy, we asked Nobuo Kyotoku, a project team manager in the Engine Design Department at Fuji’s Subaru Engineering Div., about its design team’s most difficult challenges. One of the toughest was making it compact and short (in height) enough to be mounted low and far back in the BRZ sports car’s chassis for the lowest-possible center of gravity and 52% front, 48% rear weight distribution.

“Because the BRZ is a sports car,” he added, “achieving its power and performance targets without sacrificing fuel efficiency was a challenge. We needed a high-revving engine with sufficient power and enhanced durability and lubrication to withstand high-rpm operation.” Getting sufficient intake air in and exhaust out smoothly and efficiently was another challenge that required a lot of testing and development.

There is no question those original priorities for the FA’s 2013 BRZ debut helped enable later turbocharged versions to achieve such remarkable balances of performance and efficiency in today’s WRX and (tamer but still quick) Forester CUV.

But this turbocharged 2.0L FA does feature some industry firsts.

“We believe it is the world’s-first affordable mass-market horizontally opposed turbocharged engine with direct fuel injection,” Hill says. “The Porsche 911 Turbo has DI, but it is much more expensive and is sold in much more limited numbers.”

WardsAuto editors wrote that this stout-hearted Subaru H-4 rose to the top of a strong field of turbocharged 4-cyl. engines to earn its first 10 Best trophy due to its “neck-snapping gratification, zero turbo lag, surprisingly good fuel economy and a torque curve as flat as the Midwest, with a broad peak that extends all the way from 2,000 to 5,200 rpm.”

As one of just eight 4-cyl. turbos in the 2016 competition, it cruised to a second straight win.

“There’s a little rumble and shake, as appropriate,” editor David Zoia comments. “The exhaust note is a matter of taste – more F1 than Detroit muscle.” Why? Because the WRX’s low-back-pressure exhaust system eliminates one muffler chamber and shortens the tubing.

“Fast, fun, a not-impractical daily driver,” editor Jim Irwin adds. “As good a turbo-4 as any out there.”

In a total 624 miles (1,004 km) of sometimes aggressive, mixed-conditions driving of the hot-performing WRX, the Wards judges observed a range of 24-28 mpg (8.4-9.8 L/100 km) on its trip computer.

As good as this engine is today, we asked, is there room for further improvement down the road? “We keep developing it but cannot say anything about future production,” Hill says.

“With the arrival of high-efficiency hybrids, plug-ins and EVs upon us, “ WardsAuto editors conclude, “there’s something comforting and nostalgic in an engine mated to a 6-speed manual transmission that needs a giant hood scoop to pull in enough air to let the intercooler do its job. That’s kickin’it old-school.”      

TAGS: Powertrain
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