See other engines in this family
The same engine also has a main page: Subaru EJ253.
Subaru EJ251 specs: 2.457L (150 ci) 4-cylinder, 99.5 x 79 mm (3.9173 x 3.1102 in) bore and stroke, 10:1 compression. Rated 165 hp, 1999–2004.
The Subaru EJ251 is oversquare — the 3.9173" bore exceeds the 3.1102" stroke by 26.0% — a layout that favors airflow and RPM over leverage at the crank. The 3.1102" stroke does not reach 4,000 ft/min of mean piston speed until about 7,700 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 4 cylinders displaces 37.5 ci (615 cc).
10:1 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 43 engines in the EJ family, sitting between the 91.1 ci and 152 ci members. 6 factory variants share these dimensions, rated at 165 hp across 1999–2004.
| Displacement | 150 ci / 2.457 L |
|---|---|
| Bore | 3.9173" |
| Stroke | 3.1102" |
| Cylinders | 4 |
| Compression ratio | 10:1 * |
| Bore/stroke ratio | 1.26 |
* Compression ratio from a single source; not yet cross-verified.
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | SOHC |
| Fuel type | gas |
| Firing order | 1-3-2-4 |
| Advertised HP | 165 hp |
| Years | 1999–2004 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 2001 | EJ251/EJ253 net | 165 @ 5600 * | 166 lb-ft @ 4000 * | 10:1 * |
| 2002 | EJ251/EJ253 net | 165 @ 5600 * | 166 lb-ft @ 4000 * | 10:1 * |
| 2003 | EJ251/EJ253 net | 165 @ 5600 * | 166 lb-ft @ 4000 * | 10:1 * |
| 2004 | EJ251/EJ253 net | 165 @ 5600 * | 166 lb-ft @ 4000 * | 10:1 * |
Factory specification sheet. * Not yet cross-verified.
| Bore/stroke character | 1.260 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,110 ft/min — conservative for a street engine |
| Displacement per cylinder | 37.5 ci |
| Static compression | 10:1 — 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 |
|---|---|---|
| 1999 | 165 hp | EJ251 |
| 2000 | 165 hp | EJ251 |
| 2001 | 165 hp | EJ251 |
| 2002 | 165 hp | EJ251 |
| 2003 | 165 hp | EJ251 |
| 2004 | 165 hp | EJ251 |
4 factory vehicles, 1999–2005. Each year opens that model's engine table.