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.
BASIC DESIGN
The FB engine’s bore and stroke are both different from the EJ’s. The comparison chart below 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.
CYLINDER HEADS/CAM CARRIERS
The cylinder heads and the camshaft carriers consist of separate pieces instead of one metal casting. That allows a reduction in metal thickness, simplification of the engine’s structure, and reduction in weight.
Revisions to the valvetrain include the use of roller rocker arms instead of lifters to actuate valves. These components and their layout contribute to reducing the width of the cylinder heads and overall engine width.
The fuel injectors were moved to the cylinder heads. In the EJ engine, they were mounted in the intake manifolds. The relocation enhances the flow of atomized fuel, helping to improve fuel efficiency and reduce exhaust gas emissions.
Cylinder Head and Cam Carrier
INTAKE AND EXHAUST
Both intake and exhaust systems are designed to optimize the flow of gases and improve performance.
In the intake system, the intake manifold no longer requires a large intake chamber, which reduces the number of parts in the engine. The size and shape of the large and small resonators along the intake manifold are streamlined. These revisions help reduce overall engine weight as well as manufacturing costs.
Another modification resulting in the elimination of parts is the addition of a cooling function to the exhaust gas recirculation system.
Intake and exhaust ports and valves have been revised, too. Valves have been engineered to reduce drops in pressure when they’re open, which helps improve performance. Their design increases tumbling (keeping fuel mixed with air) when closed, which improves fuel efficiency and helps reduce exhaust gas emissions.
The exhaust system features improved tuning. The diameters and lengths of the separate tubes have been modified to improve catalytic converter warm-up (reducing emissions) and to increase power output.
Exhaust System
LIGHTWEIGHT COMPONENTS
Among the components designed to weigh less in the FB engine are:
* Connecting rods shaped to help keep engine width the same as the EJ while allowing longer stroke
* Pistons with a smaller bore, requiring less material and having less reciprocating mass
* Wrist pins with smaller diameters, requiring less material
Along with an efficient and compact oil pump, the lightweight components reduce friction loss by approximately 30 percent, improving fuel efficiency and engine response.
CHAIN DRIVE
The double overhead camshafts are driven by chains rather than toothed belts. The chains are maintenance free and do not require replacement, helping to lower the cost of ownership.
The use of chain drive allows smaller sprocket diameters at the crankshaft and camshafts. That helps to keep engine width within convenient dimensions.
Chain Drive
FB VERSUS EJ
Compared with the previous non-turbocharged EJ25 engine, the FB25 produces:
* The same horsepower, but at lower rpm
* Slightly higher torque at lower rpm
* Improved fuel economy
That makes the Forester’s new FB SUBARU BOXER engine more responsive, especially at lower engine speeds, while allowing the vehicle to travel farther on the same amount of fuel.
