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Subaru gives us a closer look at the new “FB25” Flat 4 motor
Posted: January 6, 2011 by Yukes in Forester, Motor Tech, Subaru Models, TechTags: FB25 Motor, Forester, Subaru Drive Magazine
It’s been some time that our beloved EJ20/25 motor had been “replaced” by a new motor. Yes, the EJ series had many variants but this is the first time that another designated boxer motor took its place and that is the FB25 flat four motor.
The bore and stroke is different from the EJ series. The new longer stroke improves the fuel economy of the FB25 Motor which is what Subaru has been aiming for since they are moving towards “green” campaigns.
Subaru goes in depth with the motor:
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.
One of the most noticeable changes from the EJ to the FB is that the cylinder heads rather than the intake manifold. According to Subaru it improves the fuel economy and reduces emissions.
As for the Cylinder Head itself this is what Subaru has said:
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 FB Intake System has changed with a new intake manifold without a large intake plenum and smoother runners.
The exhaust system has also been revised with a different design from before. The runners before going into the cat have been separated to improve the warm up of the catalytic converter and again reducing emissions.
If you want to read more about this new motor will is featured in the non turbo Forester and the up coming FT86 coupe, check it out on Subaru’s Drive Webpage!
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