AS NOTED IN THE LAST ISSUE OF DRIVE, 2011 FORESTER NON-TURBOCHARGED MODELS ARE POWERED BY FOUR-CYLINDER SUBARU FB ENGINES. THE FB IS THE FIRST COMPLETELY NEW ENGINE FROM SUBARU IN 20 YEARS. HERE ARE SOME DETAILS.
More specifically, the non-turbocharged Forester engine is the FB25. The 25 nomenclature refers to its displacement of 2.5 liters.
The engine's design and the development of its components and systems are intended to improve both economy and performance. Comparisons in this article are made with the four-cylinder EJ25 engine that previously powered Forester models.
The FB engine's bore and stroke are both different from the EJ's. The comparison chart on page 3 shows a smaller bore and longer stroke. A longer stroke helps to improve fuel efficiency.
Despite the longer stroke, engineers were able to maintain the dimensions of the EJ engine with revisions to connecting rods and valvetrain components. Maintaining engine width was a concern in designing the FB power plant. The new engine was required to fit in the same-sized engine bay as its predecessor.
The change in the combustion chamber's displacement and surface area helps to reduce engine knock.