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Mercury S-code specs: 390 ci (6.4L) 8-cylinder, 4.05 x 3.784 in (102.9 x 96.1 mm) bore and stroke. Rated 320-325 hp, 1967–1969. Used in the Mercury Cougar.
The Mercury S-code is oversquare — the 4.05" bore exceeds the 3.784" stroke by 7.0% — a layout that favors airflow and RPM over leverage at the crank. The 3.784" stroke puts mean piston speed at 4,000 ft/min around 6,350 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 48.8 ci (799 cc).
It is one of 34 engines in the FE family, sitting between the 332 ci and 428 ci members. 3 factory variants share these dimensions, rated from 320 to 325 hp across 1967–1969.
| Displacement | 390 ci / 6.4 L |
|---|---|
| Bore | 4.05" |
| Stroke | 3.784" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.07 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 320–325 hp |
| Years | 1967–1969 |
| Bore/stroke character | 1.070 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,784 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 48.8 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 |
|---|---|---|
| 1967 | 320 hp | S-code |
| 1968 | 325 hp | S-code |
| 1969 | 320 hp | S-code |