Subaru FB20 – Click above for high-res image gallery
Subaru recently invited us to Budapest to get to know the 2011 European-spec Forester, which was... well, exactly what you'd expect (it was fine), but that's not the point here. What is important is the all-new "FB" engine under the hood – Subaru's third-generation boxer design – the same unit that we'll be seeing in the upcoming Impreza. Essentially, it's a sized-down version of our Forester's 2.5-liter mill, as opposed to the second-gen EJ25 engine in the current Impreza. Got it? Good. Let's get our hands dirty...
The new FB20 engine is a small step forward from its EJ20 predecessor, featuring better power, emissions and economy (Subaru estimates that mpg is up by around 10 percent). Americans will have to take our word on this improvement, though, as it's been years since we've had a Subaru two-liter sans turbo. The engine pulls off the mild improvement without the help of direct injection – the absence of which is disappointing. GDI is just... what you do when you build a new engine these days. Subaru did think outside of the box with some other aspects though, including designing compact, reshaped combustion chambers, chain-driven cams and intriguingly "bent" connecting rods – the latter allowing installation and maintenance without separating the block.
One of Subaru's proudest points when discussing the new engine is an increase in piston stroke without enlarging the dimensions of the boxer, thanks largely to a modified valvetrain and redesigned block and head. In fact, the piston design changed pretty significantly, from the oversquare EJ (92 x 75mm) to a new undersquare layout of 84 x 90mm. This change accounts for the engine's low-end grunt, and achieving such a stroke increase without further widening the horizontally-opposed engine is admittedly impressive. We're still a little mystified why this feat matters, though, given that all current applications already offer a larger 2.5-liter that fits just fine.
While the new, more efficient unit was designed with an emphasis on improved low- and mid-range power delivery, the Forester we drove was still in need of some decent revs to get around briskly. Its full 148 horsepower hits at a fairly average 6,000 rpm, and torque (146 pound-feet) doesn't peak until 4,200 rpm. Combined with rather long gearing and underwhelming transmissions (a four-speed automatic in 2011, really?), the acceleration isn't exactly crushing. But then again, who cares? You won't be buying the engine in a Forester anyway.
So there we have it, Subaru's new boxer four – an interesting, somewhat old-school engine that's technically better than the outgoing version. But isn't it, in some ways, the answer to the question no one asked? While we're inclined to say a lot more could've been done here, we'll have to see how the new entry-level engine handles itself in the Impreza before locking down an opinion.