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Buick 401 specs: 401 ci (6.6L) 8-cylinder, 4.188 x 3.64 in (106.4 x 92.5 mm) bore and stroke, 10.25:1 compression. Rated 325 hp. Rod ratio and piston speed.
The Buick Nailhead V8 (401) is oversquare — the 4.1875" bore exceeds the 3.64" stroke by 15.0% — a layout that favors airflow and RPM over leverage at the crank. The 3.64" stroke puts mean piston speed at 4,000 ft/min around 6,600 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 50.1 ci (821 cc).
10.25:1 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 2 engines in the Nailhead V8 family, the smallest at 401 ci. Factory fitments include the Buick Invicta, Buick Electra, Buick Wildcat and 6 others.
| Displacement | 401 ci / 6.6 L |
|---|---|
| Bore | 4.1875" |
| Stroke | 3.64" |
| Cylinders | 8 |
| Compression ratio | 10.25:1 |
| Bore/stroke ratio | 1.15 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 325 hp |
| Bore/stroke character | 1.150 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,640 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 50.1 ci |
| Static compression | 10.25:1 — in the range that usually runs on pump premium |
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