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Mercury 429 specs: 429 ci (7.0L) 8-cylinder, 4.36 x 3.59 in (110.7 x 91.2 mm) bore and stroke, 10.5:1 compression. Rated 320-360 hp. OHV.
The Mercury 385 (429) is oversquare — the 4.36" bore exceeds the 3.59" stroke by 21.4% — a layout that favors airflow and RPM over leverage at the crank. The 3.59" stroke puts mean piston speed at 4,000 ft/min around 6,700 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 53.6 ci (879 cc).
10.5:1 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 10 engines in the 385 family, the smallest at 429 ci. 2 factory variants share these dimensions, rated from 320 to 360 hp. Factory fitments include the Mercury Marquis, Mercury Cougar, Mercury Monterey and 6 others.
| Displacement | 429 ci / 7 L |
|---|---|
| Bore | 4.36" |
| Stroke | 3.59" |
| Cylinders | 8 |
| Compression ratio | 10.5:1 |
| Bore/stroke ratio | 1.214 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 320–360 hp |
| Bore/stroke character | 1.214 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,590 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 53.6 ci |
| Static compression | 10.5:1 — in the range that usually runs on pump premium |
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 |
|---|---|---|
| — | 320 hp | 429 |
| — | 360 hp | 429 |