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Mercury 406 specs: 406 ci (6.7L) 8-cylinder, 4.15 x 3.787 in (105.4 x 96.2 mm) bore and stroke, 11.4:1 compression. Rated 385 hp. Rod ratio and piston speed.
The Mercury FE (406) is oversquare — the 4.1496" bore exceeds the 3.7874" stroke by 9.6% — 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 50.8 ci (832 cc).
11.4:1 on iron heads is high enough that pump premium or better is normally required, and iron holds heat, so it is the less forgiving side of that line.
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 S-55.
| Displacement | 406 ci / 6.7 L |
|---|---|
| Bore | 4.1496" |
| Stroke | 3.7874" |
| Cylinders | 8 |
| Compression ratio | 11.4:1 |
| Bore/stroke ratio | 1.096 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 385 hp |
| Bore/stroke character | 1.096 — 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 | 50.8 ci |
| Static compression | 11.4:1 — high enough that pump premium or better is normally required |
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