The EJ-series flat-four has ruled the Subaru universe since it debuted in the 1989 Legacy. In today’s most desirable Subarus, a 2.5-liter EJ makes anywhere from 224 to 305 horsepower with belt-driven dual overhead cams and a turbocharger. Lowlier Subie models have a SOHC EJ of identical displacement that makes 170 horses. Of course, those desiring more cylinders can have a 3.6-liter pancake-six making 256 horses in the top-dog Legacy and Outback, and the Tribeca is only available with the six (although we’re not sure anyone desires the Tribeca).

For 2011, only the Forester gets Subaru’s third-generation flat-four. Known as the FB, the new engine looks pretty similar to its predecessor at first glance: 2.5 liters, 170 hp. While the displacement of both the EJ25 and the FB25 round to 2.5 liters, the FB is slightly larger, at 2498 cc versus 2457. This bump in size accounts for the 4 lb-ft increase in torque, to 174.

Subaru says the main motivation for the new engine was improved efficiency, something for which its engines have never been torchbearers—especially when they’re controlled by the right feet in our office. Compared to last year’s car, the 2011 Forester 2.5X gets an extra mpg in the city as rated by the EPA, at 21, while highway consumption is rated at 27 mpg. (Those figures apply to both the manual and the automatic; last year’s Forester carried ratings of 20/27 with the stick and 20/26 with the slushbox.) A 1-mpg improvement in city efficiency may not seem like much, but with the transmission and drivetrain staying the same, five percent is a fairly substantial gain. Subaru claims a 28-percent reduction in friction losses within the engine, with the biggest contributors being lighter pistons and connecting rods, as well as a drop in piston-ring tension. The less the engine has to work to keep spinning, the more efficient it can be.

New Heads on the (New) Block

Despite all the similarity to its predecessor’s specifications, the FB is all-new, including the block and the heads. The biggest fundamental difference between old and new is that the FB’s camshafts are chain driven, with all variations of the engine now carrying twin cams in each head. Going to chain-driven cams allows the valves to be placed at a narrower angle to each other. This let the engine designers shrink the bore, from 99.5 mm to 94. The smaller-diameter cylinder and increased stroke (now 90 mm, up 11) lets the FB suck air in more quickly, and the elongated cylinder also results in less unburned fuel during cold starts. Both intake and exhaust cams are equipped with variable valve timing.

Even though the stroke is slightly longer, the FB’s exterior dimensions are basically unchanged from the EJ’s. Subaru achieved this by employing asymmetrical connecting rods like those it used to increase the stroke of the flat-six in the Tribeca without drastically changing the engine. The lamb-chop-shaped rods also ease assembly of the bottom end. Subaru says 80 percent of the engine’s cooling capacity is routed to the head, and it even added a cooling circuit to the exhaust-gas recirculation channel. By doing this, it was able to reduce NOx emissions.

The intake manifold is plastic, like the outgoing EJ’s (save the cast-aluminum unit on the STI), but this new manifold incorporates four tumble-generation valves, one just upstream of each port. They look like half of a throttle valve and, through electronic control, manipulate the intake flow to maximize efficiency.

More Payoffs Imminent

Doing a complete overhaul of an engine for a one-mpg boost in fuel economy might seem like a waste, but Subaru has its reasons. Performing these same upgrades to the EJ would have resulted in a much heavier engine; as it is, the FB is only marginally heavier than the old mill. We were surprised that the FB doesn’t have direct injection, but Subaru tells us it will add the technology soon. The company says that next-generation transmissions—the Forester’s auto is an old four-speed—more-efficient engine peripherals, and an optimized driveline in conjunction with the FB would (read: will) also enable substantial increases in efficiency.

Expect to see a 148-hp, 2.0-liter version of the FB—dubbed FB20—in the new Impreza when it lands in showrooms this fall. That engine will feature the same stroke as the 2.5-liter, but with a slightly smaller 84-mm bore. The car should bear a strong resemblance to the Impreza concept Subaru showed at the Los Angeles auto show this past year. Subaru won’t comment on transmission options, but a smart bet would put money on a six-speed manual as standard, with the optional four-speed automatic replaced by a CVT. The latter should get close to—if not achieve—40 mpg on the highway.

Another car the FB20 likely will power is the Toyota/Subaru rear-drive coupe that Toyota showed as the FT-86 concept. We expect that a blown FB will power the new WRX, and our fingers are crossed for a turbo edition of at least Subaru’s version of the coupe. Either way, greater efficiency and a looming hot-rod or two will keep Subaru showrooms busy. Didn’t you hear? Subaru sold 263,820 cars in the U.S. last year. That’s a record number for Subaru—a tremendous 21.8-percent increase from 2009—and a lot of flat-fours.