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Subaru Ej25 S NA specs: 2.5L (150.2 ci) 4-cylinder, 99.5 x 79 mm (3.9173 x 3.1102 in) bore and stroke, 10:1 compression. Rated 165 hp. SOHC.
The Subaru Ej25 S NA 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 37.6 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 37 engines in the EJ family, sitting between the 91.1 ci and 152 ci members. Factory fitments include the Subaru Legacy, Subaru Outback, Subaru Forester and 3 others.
| Displacement | 150.2 ci / 2.5 L |
|---|---|
| Bore | 3.9173" |
| Stroke | 3.1102" |
| Cylinders | 4 |
| Compression ratio | 10:1 |
| Bore/stroke ratio | 1.26 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | SOHC |
| Fuel type | gas |
| Advertised HP | 165 hp |
| 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 |
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