Bore, stroke, displacement and compression for all 4 Subaru FB engines, compared side by side. Plus rod ratio and head specs where published.
| Displacement | 97.6–152.4 ci |
|---|---|
| Cylinders | 4 |
| Bore | 3.102–3.701" |
| Stroke | 3.228–3.543" |
| Compression | 10.3–10.5:1 |
| Advertised power | 114–182 hp |
| Engine | Displacement | Bore × Stroke | CR | HP |
|---|---|---|---|---|
| FB25 | 152.4 ci | 3.7008" × 3.5433" | 10.3:1 | 170–182 |
| FB20 | 122 ci | 3.3071" × 3.5433" | 10.5:1 | 148 |
| FB20 | 121.7 ci | 3.3071" × 3.5433" | 10.5:1 | 148–152 |
| FB16 | 97.6 ci | 3.1024" × 3.2283" | 10.5:1 | 114 |
Both bore and stroke vary across the family, so these are separate castings rather than variations on one block.
2 of 4 are undersquare — stroke larger than bore, favouring torque low in the range.
Each engine page carries rod ratio, mean piston speed, bore/stroke character and cylinder head specs where published, and links into the calculators with its dimensions already filled in.