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Mercury Q-code specs: 351 ci (5.8L) 8-cylinder, 4.00 x 3.50 in (101.6 x 88.9 mm) bore and stroke. Rated 255-280 hp, 1971–1974. Used in the Mercury Cougar.
The Mercury Q-code is oversquare — the 4" bore exceeds the 3.5" stroke by 14.3% — a layout that favors airflow and RPM over leverage at the crank. The 3.5" stroke puts mean piston speed at 4,000 ft/min around 6,850 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 43.9 ci (719 cc).
It is one of 24 engines in the Windsor family, the largest at 351 ci. 4 factory variants share these dimensions, rated from 255 to 280 hp across 1971–1974.
| Displacement | 351 ci / 5.8 L |
|---|---|
| Bore | 4" |
| Stroke | 3.5" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.143 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 255–280 hp |
| Years | 1971–1974 |
| Bore/stroke character | 1.143 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,500 ft/min — conservative for a street engine |
| Displacement per cylinder | 43.9 ci |
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These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1971 | 280 hp | Q-code |
| 1972 | 262 hp | Q-code |
| 1973 | 264 hp | Q-code |
| 1974 | 255 hp | Q-code |