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Subaru FB25 specs: 2.498L (152.4 ci) 4-cylinder, 94 x 90 mm (3.7008 x 3.5433 in) bore and stroke, 10.3:1 compression. Rated 170-182 hp. DOHC.
The Subaru FB25 is close to square: 3.7008" bore against 3.5433" stroke, within 4.4%, which splits the difference between a high-revving layout and a torque layout. The 3.5433" stroke puts mean piston speed at 4,000 ft/min around 6,750 RPM, which is where a production bottom end is generally considered at its limit. Each of the 4 cylinders displaces 38.1 ci (624 cc).
10.3:1 on aluminum heads is in the range that usually runs on pump premium; aluminum sheds heat faster than iron, which makes the same number a little more forgiving.
It is one of 4 engines in the FB family, the largest at 152.4 ci. 2 factory variants share these dimensions, rated from 170 to 182 hp. Factory fitments include the Subaru Forester, Subaru Outback, Subaru Legacy.
| Displacement | 152.4 ci / 2.498 L |
|---|---|
| Bore | 3.7008" |
| Stroke | 3.5433" |
| Cylinders | 4 |
| Compression ratio | 10.3:1 |
| Bore/stroke ratio | 1.044 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Advertised HP | 170–182 hp |
| Bore/stroke character | 1.044 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 3,543 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 38.1 ci |
| Static compression | 10.3:1 — in the range that usually runs on pump premium |
Each link opens the calculator with this engine's specs already filled in.
These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| — | 170 hp | FB25 (NA), FB25, FB25D |
| — | 182 hp | FB25 |