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Subaru FB20 specs: 2.0L (122 ci) 4-cylinder, 84 x 90 mm (3.3071 x 3.5433 in) bore and stroke, 10.5:1 compression. Rated 148 hp. Rod ratio and piston speed.
The Subaru FB20 is undersquare — the 3.5433" stroke exceeds the 3.3071" bore by 6.7% — a layout that trades peak RPM for leverage and torque low in the range. 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 30.5 ci (500 cc).
10.5:1 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 4 engines in the FB family, sitting between the 97.6 ci and 152.4 ci members. Factory fitments include the Subaru Impreza, Subaru Forester, Subaru Crosstrek.
| Displacement | 122 ci / 2 L |
|---|---|
| Bore | 3.3071" |
| Stroke | 3.5433" |
| Cylinders | 4 |
| Compression ratio | 10.5:1 |
| Bore/stroke ratio | 0.933 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Advertised HP | 148 hp |
| Bore/stroke character | 0.933 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 3,543 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 30.5 ci |
| Static compression | 10.5:1 — in the range that usually runs on pump premium |
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