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Mercury 390 specs: 390 ci (6.4L) 8-cylinder, 4.051 x 3.787 in (102.9 x 96.2 mm) bore and stroke, 9.6:1 compression. Rated 266 hp. Rod ratio and piston speed.
The Mercury FE (390) is oversquare — the 4.0512" bore exceeds the 3.7874" stroke by 7.0% — a layout that favors airflow and RPM over leverage at the crank. The 3.7874" 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).
9.6:1 on iron heads is in the range that usually runs on pump premium; iron holds heat, so treat it as the less forgiving side of that range.
It is one of 34 engines in the FE family, sitting between the 332 ci and 428 ci members. Factory fitments include the Mercury Monterey, Mercury Cougar, Mercury Marquis and 2 others.
| Displacement | 390 ci / 6.4 L |
|---|---|
| Bore | 4.0512" |
| Stroke | 3.7874" |
| Cylinders | 8 |
| Compression ratio | 9.6:1 |
| Bore/stroke ratio | 1.07 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 266 hp |
| Bore/stroke character | 1.070 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,787 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 48.8 ci |
| Static compression | 9.6:1 — in the range that usually runs on pump premium |
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