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Mercury 256ci (1954) specs: 256 ci 8-cylinder, 3.625 x 3.10 in (92.1 x 78.7 mm) bore and stroke, 7.5:1 compression. Rated 161 hp, 1954.
The Mercury 256ci (1954) is oversquare — the 3.625" bore exceeds the 3.1" stroke by 16.9% — a layout that favors airflow and RPM over leverage at the crank. The 3.1" stroke does not reach 4,000 ft/min of mean piston speed until about 7,750 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 8 cylinders displaces 32.0 ci (524 cc).
7.5:1 is low by modern standards and generally happy on regular pump fuel.
| Displacement | 256 ci |
|---|---|
| Bore | 3.625" |
| Stroke | 3.1" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 7.5:1 * |
| Bore/stroke ratio | 1.169 |
* Compression ratio from a single source; not yet cross-verified.
| Fuel type | gas |
|---|---|
| Advertised HP | 161 hp |
| Years | 1954 |
| Bore/stroke character | 1.169 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,100 ft/min — conservative for a street engine |
| Displacement per cylinder | 32.0 ci |
| Static compression | 7.5:1 — low by modern standards, generally happy on regular pump fuel |
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1 factory vehicle, 1954. Each year opens that model's engine table.