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Mercury 351W specs: 351 ci (5.8L) 8-cylinder, 4.00 x 3.50 in (101.6 x 88.9 mm) bore and stroke, 10.7:1 compression. Rated 290 hp. Rod ratio and piston speed.
The Mercury 351W 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).
10.7:1 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 24 engines in the Windsor family, the largest at 351 ci. Factory fitments include the Mercury Cougar, Mercury Montego.
| Displacement | 351 ci / 5.8 L |
|---|---|
| Bore | 4" |
| Stroke | 3.5" |
| Cylinders | 8 |
| Compression ratio | 10.7:1 |
| Bore/stroke ratio | 1.143 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 290 hp |
| 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 |
| Static compression | 10.7:1 — in the range that usually runs on pump premium |
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