Subaru FB25 2.5L: Everything You Need To Know

The Subaru FB25 engine 2.5 L engine boxer engine replaced the EJ253 engine.

It is a member of Subaru’s third-generation FB boxer and FB20, FA20D, FA20F, AND FA20E ENGINES. Subaru FB engines are far more advanced and more efficient than the previous engines.

Apart from that, the engines received several changes that took the engine to the top.  

What is Subaru FB25 2.5L Engines? 

The Subaru FB25 engine is a new generation engine made by Subaru. It was first produced in 2010 at Subaru’s Gunma Oizumi factory, developed exclusively for the FB engines.

The engine is a 2.5 L non-turbo horizontally-opposed (boxer) engine, four-cylinder motor, and occupied the seat of the Subaru EJ25 engine in Subaru Legacy and Forester. 

The engine is a more robust modification of the FV20, the first engine to introduce the FB family to the automotive world. This engine features many similarities from the initial FB20 engine, but Subaru also integrated a lot of new technologies and innovations. 

Some of the notable characteristics of the FB engines are thicker cylinder walls, new cylinder heads, narrower chain-driven cam angles leading to a more efficient machine. It is suited for a compacted low center of gravity vehicles.

Excellent configuration for cornering, absorbing G’s, and putting a brand for a sporty type of vehicle. 

There are two versions for the FB25 engine – the FB25B, which was the initial release, and the FB25D, the latest version and introduced at the 2018 New York International Autoshow.

The latter also featured direct injection, resulting in a slight boost in power and fuel economy. The seventh-generation 2020 Subaru Legacy and Outback adopted the revised direct injection FB25 as its base machine. 

Engine Specifications and Design:

  • Production Run: 2010 – Present
  • Cylinder Head Material: Aluminum
  • Cylinder Block Material: Aluminum
  • Configuration: Flat-4 (Boxer)
  • Bore: 94 mm
  • Stroke: 90 mm
  • Valvetrain: DOHC four valves per cylinder with AVCS
  • Displacement: 2.5 L (2498 cc)
  • Compression Ratio(s): 10.0, 10.3 (2014 – Present), and 12.0 (FB25D)
  • Weight: 500 lbs.
  • Maximum HP: 182 HP at 5,800 RPM
  • Maximum Torque: 176 lb-ft at 4,400 RPM

Engine Design

The Subaru FB25 2.5L engine was based on the smaller capacity FB20 engine. In short, the Subaru FB25 engines are the larger version of the FB20 engine. Though it achieved a larger displacement, the FB25 machines possess some innovations and technologies that the FB20 engine has, including the piston stroke. 

The cylinder block of the Subaru FB25 engine was built on aluminum. It has an open-deck design with a 94 mm cylinder bore and a 90-mm piston stroke. Within the bores, the Subaru FB25 engine has cast-iron liners.

Subaru revised the connecting rods and valvetrain components, making the FB25 engine as large as the EJ25 engine; despite smaller bore and stroke dimensions. 

According to Subaru, the longer piston stroke contributed to much-improved fuel efficiency by enabling faster air induction and reduced unburnt fuel during cold starts. 

The engine also has separate cooling circuits for the cylinder block and head to improve coolant flow and distribution. The flow rate was capped along the block to maintain a high temperature for the cylinder liner oil, thus reducing piston friction during motions. 

The FB25 engine has asymmetrical, diagonally split connecting rods to reduce engine width and allow a longer piston stroke. Compared to the EJ253 engine, the FB25 achieved a twenty-eight percent reduction in frictional losses due to lighter pistons, wrist pins, connecting rods, and lower piston ring tension. 

The Subaru FB25 engine has a plastic intake manifold with reshaped branches to minimize pressure losses resulting in increased power.

The valves are revised to reduce pressure drops acting as a TGV or Tumble Generator Valve to increase air tumble and create vortices within the combustion chambers. Moreover, the size and shape of the intake manifold resonators were streamlined. 

The aluminum FB25 heads comprise two pieces – the cylinder head itself and the camshaft carrier, allowing a reduction in metal thickness and weight. As we mentioned earlier, Subaru enhanced the cylinder head cooling by using separate circuits for the cylinder block and head. 

The engine has a dual overhead camshaft design driven by a maintenance-free chain. Both the intake and exhaust camshafts are equipped with a variable valve timing system AVCS. A chain drive was adopted to enable a narrower valve angle and sprocket diameter reduction as well as reduced camshaft width.

Roller rocker arms actuate the valves, which were valve lifters for the previous EJ253. 

The fuel injectors are relocated to the cylinder head along with the injection system. The system became more complex, but it enhanced the flow of atomized fuel, thus improving fuel efficiency and reducing exhaust gas emissions. 

Moreover, each cylinder has a coil on plug ignition with an integrated igniter. Subaru integrated the ignition coil into the spark plug caps providing contact to the spark plugs. For the Subaru FB25 engine, cooling around the spark plugs improved for increased power, higher knock limit, and fuel efficiency. 

In addition to that, the diameter and lengths of the exhaust manifold were restyled to improve catalytic converter warm-up and increase power output. Moreover, the free-flowing exhaust system contributes to higher power output. 

The exhaust gas recirculation system (EGR) includes a cooling circuit enabling greater exhaust gas volumes to be recirculated. Combustion temperatures remain at the minimum levels through EGR to make the engine less susceptible to knocks and advanced injection timing. 

In 2014, Subaru modified the engine. They reduced the cylinder wall thickness to 3.2mm and changed the compression rating to 10.3. The heads have also been revised. The intake ports, intake valves, intake manifold, and exhaust system are also new. 

FB25 engines are more responsive at lower engine speeds compared to the Subaru EJ25 engine. They do have the same power ceilings, but the fuel efficiency of the FB25 encompasses the EJ25 capabilities.

Making the Subaru FB25 a more practical choice, especially for daily use. 

Applications of the Subaru FB25 Engine:

  • 2011 – 2018 North American Subaru Forester
  • 2013 – 2019 North American Subaru Legacy
  • 2013 – 2019 North American Subaru Outback
  • 2020 North American Subaru Outback
  • 2021 North American Subaru Crosstrek
  • 2019 North American Subaru Forester
  • 2020 North American Subaru Legacy

Engine Tuning, Upgrades, and Modifications

Unfortunately, the FB25 engine is not designed for an aggressive build. It will only aggravate an already excellent engine. It has decent power, though, but if you’re not satisfied with the power it produces, you can buy a Subaru WRX. 

But, if you just want to improve the engine’s reliability, you can purchase an aftermarket cat-back exhaust system and adjust the ECU. This can give you additional reliability and about 190 HP. 

Problems Surrounding Subaru FB25 2.5L Engines:

Good thing that the FB25 engine does not suffer behind the trails of the FB20 engine. The latter has issues regarding high oil consumption, which was addressed by Subaru and improved for the follow-up engine under the FB25 name.

However, there are still issues that might affect the reliability and overall lifespan of the machine, but it might be due to external factors such as age, mileage, and the standard wear and tear items. 

The maintenance-free timing chains and peripheral components are designed to withstand long periods. It tends to last the same as the engine itself. So if your machine is slowly deteriorating, the chances are that the components inside are also under the same sentiments. 

We cannot stress enough the amount of effort and ways to make the engine last longer. Schedule it for check-ups, use quality engine oil and fuel. 


The Subaru FB25 engine has longevity that will contest even the EJ25 engine. It was built to last hundreds of thousands of miles, some owners at a minimum of 250,00 miles. That’s a lot of road coverage, though. 

In addition to that, this engine significantly improved the efficiency of the previous FB20 engine. The restyled block, heads, valves, and other participating components improved engine performance, efficiency, and fuel economy. 

The Subaru FB25 is a successor to the classic modern machine. It was a follow to continue the economical and eco-friendly motor. Enough to fit in current standards. It has its advantages, such as good cornering attributes, and does shy away from power numbers. 

Overall, the engine is solid and probably more reliable than the FB20. 

4 thoughts on “Subaru FB25 2.5L: Everything You Need To Know”

  1. what specific cat back exhaust systems would you suggest? also what ECU tweaks would do best with this engine??


Leave a Comment