Boxer engines? You can't call them an oddity — not when a top-10 brand in the U.S. uses them — but they're not exactly mainstream, either. The closest the boxer engine has come to that was back in the '60s, when the VW Beetle was the country's super-dominant import and GM offered a larger-scale alternative to the Bug in the elegant shape of the troubled Chevrolet Corvair.
But with Subaru coming off a record year, having sold 263,820 vehicles in the U.S. during 2010, the horizontally opposed engine is enjoying a modest 21st-century renaissance. Now the Japanese automaker is launching its third-generation boxer engine — the first of the company's flat-4s appeared way back in 1966. Designated FB (for "Fuji future" in parent company Fuji Heavy Industries' native tongue), this engine will eventually supersede the outgoing EJ engine, which has been around since 1989.
The FB boxer engine actually debuted last fall in the 2011 Subaru Forester, which uses a 2.5-liter version. Subaru chose not to make a song and dance about it, though, because it produced only modest mpg gains despite providing a broader power band. However, for the smaller 2.0-liter version we're sampling here, the automaker is promising more significant mileage advances. Although we won't be getting this engine in the Forester, this engine is definitely U.S.-bound. Subaru will show a car at the 2011 New York Auto Show that uses it, and the 2.0-liter will see duty in the next-generation Impreza.
Cleaner Breathing, Reduced Friction, Low-Weight Reciprocation
Subaru's wish list for this all-new FB-series engine included improved environmental performance (it claims best-in-class CO2 emissions for the 2.0-liter installed in this European-spec 2011 Forester), better fuel efficiency and enhanced drivability. And the key to all of this is a longer piston stroke, which Subaru was keen to achieve without increasing the physical size of the engine, which would force costly changes on its cars' body structures. And that's no small challenge.
This feat has been achieved by designing more compact cylinder heads (of which a boxer engine has a pair, of course) and redesigning the cylinder block. Shallower pistons, fracture-split connecting rods and amazingly tight clearances between the crankcase help, too, as does a new chain-driven cam drive whose cam sprockets are smaller, further reducing the engine's width. Chain drives reduce maintenance requirements, too.
The longer stroke results in an engine whose bore and stroke are now undersquare in contrast to the very oversquare EJ it replaces. This approach is all about improving efficiency and low-speed torque, as the resulting combustion chambers are more compact and thus suffer less heat loss. In other words, during combustion, a greater proportion of the fuel's energy is converted to useful work in the cylinders. Reducing the size of the heads has forced narrower valve angles and the use of roller-rocker cam followers, and they're aided by a comprehensively rethought variable valve timing system.
Internal friction has been reduced by 28 percent, thanks in part to mass reductions in the rods and pistons. Cylinder distortion has been reduced by an improved manufacturing process, and revisions to the cooling system include independent circuits for the block and head which provide more even cooling, particularly around the cylinder liners. This ensures that the bores remain cylindrical when the engine reaches operating temperature, improving the piston rings' sealing characteristics. The roller-rocker cam gear and an oil pump that more closely matches the engine's lubrication demands further reduce friction and parasitic power consumption.
The combustion process has been sharpened through the introduction of unusual flaps in the intake ports, which induce tumble flow as the air makes its way to the combustion chambers. These flaps work in concert with tumble-generator valves that intensify charge motion during part-load conditions for increased combustion efficiency. A new, water-cooled exhaust gas recirculation system increases EGR density to reduce pumping loss.
All this and plenty of detail work produce a 148-horsepower, naturally aspirated engine that's said to be 10 percent more fuel-efficient than before (Subaru has been selling 2.0-liter Foresters in Europe and Japan for years). Mind you, the fractionally increased 146-pound-foot torque peak still doesn't appear until a lofty 4,200 rpm. And direct injection, increasingly a feature of gasoline engines, is still to come.
The redesign has provided a chance to reduce weight. The intake manifold is plastic rather than aluminum, and the new pistons are 20 percent lighter. The upshot is that this more complex engine is only fractionally heavier than the outgoing EJ unit.
Slick Shifts Produce Smooth Power
We got to try the new FB boxer engine in a Euro-spec 2011 Subaru Forester 2.0 XS, which also gets a fresh grille for 2011, along with new side mirrors and revised 17-inch alloy wheels. Subaru has also fiddled with the suspension tuning to improve cornering stability while calming the ride.
On the road, the discovery that the 2.0 Forester feels pretty similar to the 2.5-liter version is good news — you don't sacrifice a huge amount of pulling power for a worthwhile improvement in fuel consumption. That said, our test vehicle's solid acceleration in the five-speed manual transmission's first two gears is somewhat undermined by rangier gearing in 3rd — the Forester 2.0's performance is flaccid unless the engine is pulling 3,800 rpm or more. Acceleration in 4th and 5th is often leisurely enough to require some slick downshifting if you need to overtake; although the boxer engine's lightly eager warble is compensation. Annoyingly, revs tend to hang when you've released the accelerator, so you need to lift off earlier than you'd expect during gearchanges or while coasting to a stop. This quirk aside, though, the Forester 2.0 is an easy drive, and power delivery is noticeably smoother and more civilized than with the older EJ engine.
They Built a Better Boxer
You need to carry your momentum with you as you drive the 2011 Subaru Forester 2.0, and the compact crossover's well-tuned chassis makes that easy. This year's suspension tweaks allow it to round bends with surprisingly little sway. Factor in steering that's unusually precise for an SUV, quietly confident turn-in and sure-footed all-wheel-drive traction, and you can make rewardingly brisk cross-country progress. The impressive body control doesn't come at the expense of comfort, either, as our Subaru Forester 2.0 took on ice-damaged roads with a calm pliancy.
By the same token, the FB engine brings a new level of sophistication to the Forester without completely muting the endearing boxer soundtrack. We still wish the engine's torque curve peaked at a lower rpm, and we await the day when Subaru releases a direct-injected version that should undoubtedly deliver even more horsepower and torque, while further reducing fuel consumption. And of course, we're curious whether the economy-slanted FB architecture can be successfully tailored for the inevitable turbocharged variants.
For now, though, there's no denying that this is a better boxer engine than the aged EJ engine still found in most U.S.-market Subaru's — and credit is due to the FHI engineers who packaged these improvements within the same mechanical envelope.
By Richard Bremner, European Correspondent | Published Feb 7, 2011