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Buick 400ci specs: 400 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, 1965–1966.
The Buick 400ci is oversquare — the 4.188" bore exceeds the 3.64" stroke by 15.1% — 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. A rod ratio of 1.709 is in the range most production engines fall in. Each of the 8 cylinders displaces 50.0 ci (819 cc).
10.25:1 is in the range that usually runs on pump premium.
It is one of 3 engines in the Buick 400ci family, all of them 400 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated at 325 hp across 1965–1966.
| Displacement | 400 ci / 6.6 L |
|---|---|
| Bore | 4.188" |
| Stroke | 3.64" |
| Cylinders | 8 |
| Compression ratio | 10.25:1 |
| Rod length | 6.22" |
| Rod ratio | 1.709 |
| Bore/stroke ratio | 1.151 |
| Piston deck clearance | 0.01" |
| Induction | 1-4 |
|---|---|
| Advertised HP | 325 hp |
| Years | 1965–1966 |
| Bore/stroke character | 1.151 — 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 |
| Rod ratio | 1.709 — moderate — the range most production V8s fall in |
| Displacement per cylinder | 50.0 ci |
| Static compression | 10.25:1 — in the range that usually runs on pump premium |
| Intake valve diameter | 1.875" |
|---|---|
| Exhaust valve diameter | 1.5" |
| Chamber volume | 123.7 cc |
| Rocker ratio | 1.6:1 |
| Lifter type | hydraulic |
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These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1965 | 325 hp | — |
| 1966 | 325 hp | — |