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Mercury 400ci (1971–1975) specs: 400 ci (6.6L) 8-cylinder, 4.00 x 4.00 in (101.6 x 101.6 mm) bore and stroke. Rated 260 hp, 1971–1975.
The Mercury 400ci (1971–1975) is exactly square — 4" bore and 4" stroke are the same — which splits the difference between a high-revving layout and a torque layout. The 4" stroke puts mean piston speed at 4,000 ft/min around 6,000 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.645 is in the range most production engines fall in. Each of the 8 cylinders displaces 50.0 ci (819 cc).
5 factory variants share these dimensions, rated at 260 hp across 1971–1975.
| Displacement | 400 ci / 6.6 L |
|---|---|
| Bore | 4" |
| Stroke | 4" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 8–9:1 (published figures vary) |
| Rod length | 6.58" |
| Rod ratio | 1.645 |
| Bore/stroke ratio | 1 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Firing order | 1-3-7-2-6-5-4-8 |
| Advertised HP | 260 hp |
| Years | 1971–1975 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1971 | gross | 260 @ 4400 | — | — |
Factory specification sheet.
| Bore/stroke character | 1.000 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 4,000 ft/min — in the range typical of performance street engines |
| Rod ratio | 1.645 — moderate — the range most production V8s fall in |
| Displacement per cylinder | 50.0 ci |
Each link opens the calculator with this engine's specs already filled in.
These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1971 | 260 hp | — |
| 1972 | — | — |
| 1973 | — | — |
| 1974 | — | — |
| 1975 | — | — |
3 factory vehicles, 1971–1975. Each year opens that model's engine table.