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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...


  • Image converted using ifftoany

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.

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    • 1 Second Ago
      • 4 Years Ago
      don't want to be a troll, but I read the full text and all I could think is "MEH".
      • 4 Years Ago
      Perhaps there are DI heads in the pipeline that necessitated the redesign of the bottom (middle?) end.
      Begin with your run of the mill injected motor for starters, see how it does reliability wise, then in a year or two offer a DI version of the same motor in more premium models/trims.

      Oh, then turbo the pants off it and put that in every model as well.
        • 4 Years Ago
        Exactly right, the engine starts with the block.

        DI and turbo build off of it. I'd expect to see the turbo next year, and DI after that.

        One step at a time.
      • 4 Years Ago
      I see several extremely positive changes:

      -- top mounted oil filter -- wow!
      -- externally mounted water pump driven by accessory belt instead of the timing belt
      -- timing chain

      If this engine holds water better than EJ25, too... what's not to like?
      • 4 Years Ago
      my guess is that it was designed for their CVT transmission more low-midrange grunt is where it is at considering a cvt never actually bounces off of redline(often) also CVT transmissions arent the most fuel efficient transmissions so 10% is a big deal especially considering all subarus run AWD wich is a hinderence to maximum eficiency. for subaru fuel efficiency is evrything. why they did not go direct injection is beyond me it might be a suplier issue supying new parts for a global market has it's obsticles.

      subaru needs to develop a part-time AWD system next especially for their tribecca replacement this could improve eficiency. this could get 1-3 highway MPG wich is a BIG deal for super large crossovers.
      • 4 Years Ago
      I loves me some engine porn.
      • 4 Years Ago
      The pistons are moving the wrong way, thats what Subaru needs to fix.
        • 4 Years Ago
        I stick to the facts. I have seen A LOT of engine failures in boxer motors. They are overly complicated with the heads being separated for turbo cars. Take an STI and you will dump triple the money into it to make it go as fast as an Evo and it still won't be as reliable.
          • 3 Years Ago
          Meant to vote you up but accidentally clicked down first. Sorry about that :) (Evo IX owner who agrees with you)
        • 4 Years Ago
        A boxer is inherently balanced and will last a long time in NA form. It's difficult to package in a transverse layout, but it's an inherently better engine than any V short of a V-12, or any inline short of an I-6. For a N-S drivetrain, it's hard to do much better than a boxer.

        Straight Sixes are good - they also have inherent balance, like a boxer.

        The only reason for an I-4 or V-6 is transverse packaging.

      • 4 Years Ago
      So is this engine robust enough to be turboed?

      Are we going to go back to the days of 2.0L Turbo WRX instead of the 2.5?

      Minor improves are fine and all, but disappointed that they skipped the DI if they went to the trouble of making a whole new engine anyways.
        • 4 Years Ago
        I'm sure that Subaru designed this engine to be turboed - they go 20 years per engine, so it has to do a lot! The Forester will be fitted with a FB25, and most likely, that will form the basis of the next WRX STi.

        That said, I wouldn't be surprised to see a DI 2.0l turbo WRX down the road, as fuel economy, DI, and turbo technology improve. I also wouldn't be surprised to see this go down to a DI 1.8 or 1.6l for fuel economy reasons.
        • 4 Years Ago
        DI can be added. I am almost positive they accounted for that being added to the cylinder head in the future.

        The turbocharging depends on the short block.

        It seems that the cylinder pitch isn't as small as the EZ series. (cylinder pitch is the distance between one cylinder center and the next adjacent cylinder center, which takes into account the bore diameter, AND the thickness of the interior cylinder walls shared between them.) if that is true, and the wall thickness isn't so narrow as to overheat under pressure, the FB might be good to go, with lower compression robust pistons, and turbo-optimized cam profiles.

        The crank bearings would also need to handle more load, but hopefully those are still as robust as they have been, as well... Cylinder pitch also determines crankshaft journal dimensions, along the overall crankshaft length, and if cylinder pitch hasn't drastically shortened, the crankshaft hasn't either.

        The EZ is much more compact than the EJ... so I am not sure how well it would take to more heat due to more pressure, in terms of being reliable enough for Subaru to bet warranty money on.

        I suppose someone with stats on the engine, or even someone with computer software that can extrapolate dimensions, could check the cylinder wall thinkness, compared to the known bore size, and determine the cylinder pitch measurement... Even just measuring from center to center on the spark-plugs in the cylinder head.

        Even if the cylinder pitch is slightly smaller than EJ, with better cooling it could tolerate a little bit less thickness. But not ultra-compact dimensions and thin cylinder walls.

        But chances are, this engine was designed for the possiblity of turbocharging, to allow the retirement of the EJ turbo engine lineup in the next few years.
      • 4 Years Ago
      Subaru engines are great and all, but what they need to do is divert some of that R&D money into adding another gear or two into the two-pedal transmissions.
        • 4 Years Ago
        Somehow, I think CVT and 5MT are probably going to be the choices for the base NA 4-cylinder engine trims across the lineup.

        The 5EAT, and Legacy GT's 6MT, (and hopefully the 5EAT will be upgraded SOON...) are probably going to become the standard gearbox choices for H6 and Turbo H4 higher power options.

        The CVT will offer less power sapping than a torque converter, and if the ratio range ends with more overdrive than the 4EAT, it will get better cruising fuel economy. The ratio range is what really counts there.

        I guess it just depends on what you consider "bread and butter" to mean...
        • 4 Years Ago
        They'll be subtracting gears, not adding them, as the CVT (zero gears—and what this new Impresa is getting) is the future for bread-and-butter Subies.
      • 4 Years Ago
      This engine may seem underwhelming at face value, but it's just the first step in building Subaru's future. I'm sure one of the motivations was emissions compliance. Extremely large-bore, short-stroke engines (the EJ) have notoriously dirty emissions. A smaller-bore, longer-stroke design will help a lot in reducing unburned hydrocarbons. And as noted, will help with low and midrange torque. This will manifest itself in turbo applications with less lag (which the turbocharged EJs are know for). I would expect turbocharged variants of the FB will be surprising performers, with vastly improved fuel economy and responsiveness. I think the current crop of transmissions is the biggest factor holding back the potential of this new engine platform.
      • 4 Years Ago
      Don't all Subaru's have AWD? uh...loser! Drop it!

      Sill love the boxer engine though.
      • 4 Years Ago
      4 speed auto? If better mileage is the goal, a 6 speed would be a more cost effective change. I'm sure Toyota has one in their parts bin.
      • 4 Years Ago
      "the answer to the question no one asked?"

      seriously? What a stupid comment, why wouldn't anyone want a car that uses less fuel, especially when fuel is so expensive these days.
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