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Subaru 15 NA specs: 1.5L (91.1 ci) 4-cylinder, 85 x 65.8 mm (3.3465 x 2.5906 in) bore and stroke, 9.5:1 compression. Rated 97 hp. Rod ratio and piston speed.
The Subaru 15 NA is oversquare — the 3.3465" bore exceeds the 2.5906" stroke by 29.2% — a layout that favors airflow and RPM over leverage at the crank. The 2.5906" stroke does not reach 4,000 ft/min of mean piston speed until about 9,250 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 4 cylinders displaces 22.8 ci (373 cc).
9.5:1 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, the smallest at 91.1 ci. Factory fitments include the Subaru Impreza (Japan), Subaru Impreza (JDM), Subaru Impreza.
| Displacement | 91.1 ci / 1.5 L |
|---|---|
| Bore | 3.3465" |
| Stroke | 2.5906" |
| Cylinders | 4 |
| Compression ratio | 9.5:1 |
| Bore/stroke ratio | 1.292 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | SOHC |
| Fuel type | gas |
| Advertised HP | 97 hp |
| Bore/stroke character | 1.292 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 2,591 ft/min — conservative for a street engine |
| Displacement per cylinder | 22.8 ci |
| Static compression | 9.5:1 — in the range that usually runs on pump premium |
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