See other engines in this family
Mercury 312ci (1956–1960) specs: 312 ci (5.1L) 8-cylinder, 3.80 x 3.44 in (96.5 x 87.4 mm) bore and stroke. Rated 210-260 hp, 1956–1960.
The Mercury 312ci (1956–1960) is oversquare — the 3.8" bore exceeds the 3.44" stroke by 10.5% — a layout that favors airflow and RPM over leverage at the crank. The 3.44" stroke puts mean piston speed at 4,000 ft/min around 7,000 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 39.0 ci (639 cc).
6 factory variants share these dimensions, rated from 210 to 260 hp across 1956–1960.
| Displacement | 312 ci / 5.1 L |
|---|---|
| Bore | 3.8" |
| Stroke | 3.44" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 8–9.75:1 (published figures vary) |
| Bore/stroke ratio | 1.105 |
| Block material | cast iron |
|---|---|
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 210–260 hp |
| Years | 1956–1960 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1956 | gross | 210 @ 4600 | 312 lb-ft @ 2600 | 8:1 |
| 1957 | gross | 255 | — | 9.75:1 |
Factory specification sheet.
| Bore/stroke character | 1.105 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,440 ft/min — conservative for a street engine |
| Displacement per cylinder | 39.0 ci |
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 |
|---|---|---|
| 1956 | 210 hp | — |
| 1956 | 260 hp | — |
| 1957 | 255 hp | — |
| 1958 | — | — |
| 1959 | 210 hp | — |
| 1960 | — | — |
2 factory vehicles, 1956–1960. Each year opens that model's engine table.