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Mercury 221 specs: 221 ci (3.6L) 8-cylinder, 3.50 x 2.87 in (88.9 x 72.9 mm) bore and stroke, 8.7:1 compression. Rated 145 hp. Rod ratio and piston speed.
The Mercury Windsor (221) is oversquare — the 3.5" bore exceeds the 2.8701" stroke by 21.9% — a layout that favors airflow and RPM over leverage at the crank. The 2.8701" stroke does not reach 4,000 ft/min of mean piston speed until about 8,350 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 8 cylinders displaces 27.6 ci (453 cc).
8.7:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 24 engines in the Windsor family, the smallest at 221 ci. Factory fitments include the Mercury Comet.
| Displacement | 221 ci / 3.6 L |
|---|---|
| Bore | 3.5" |
| Stroke | 2.8701" |
| Cylinders | 8 |
| Compression ratio | 8.7:1 |
| Bore/stroke ratio | 1.219 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 145 hp |
| Bore/stroke character | 1.219 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 2,870 ft/min — conservative for a street engine |
| Displacement per cylinder | 27.6 ci |
| Static compression | 8.7:1 — low by modern standards, generally happy on regular pump fuel |
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