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Pontiac 389ci specs: 389 ci (6.4L) 8-cylinder, 4.062 x 3.75 in (103.2 x 95.3 mm) bore and stroke, 10.75:1 compression. Rated 318-360 hp, 1961–1966.
The Pontiac 389ci is oversquare — the 4.062" bore exceeds the 3.75" stroke by 8.3% — a layout that favors airflow and RPM over leverage at the crank. The 3.75" stroke puts mean piston speed at 4,000 ft/min around 6,400 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.767 is long by production standards — less side-load on the cylinder wall and gentler piston acceleration around TDC, the geometry that tolerates high RPM. Each of the 8 cylinders displaces 48.6 ci (797 cc).
10.75:1 is in the range that usually runs on pump premium.
It is one of 4 engines in the Pontiac 389ci family, all of them 389 ci — the differences between them are in compression, heads and rating, not displacement. 14 factory variants share these dimensions, rated from 318 to 360 hp across 1961–1966.
| Displacement | 389 ci / 6.4 L |
|---|---|
| Bore | 4.062" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Compression ratio | 10.75:1 |
| Rod length | 6.625" |
| Rod ratio | 1.767 |
| Bore/stroke ratio | 1.083 |
| Piston deck clearance | 0" |
| Induction | 3-2 |
|---|---|
| Advertised HP | 318–360 hp |
| Years | 1961–1966 |
| Bore/stroke character | 1.083 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,750 ft/min — in the range typical of performance street engines |
| Rod ratio | 1.767 — long — less side loading on the cylinder wall, favours high RPM |
| Displacement per cylinder | 48.6 ci |
| Static compression | 10.75:1 — in the range that usually runs on pump premium |
| Intake valve diameter | 1.88" |
|---|---|
| Exhaust valve diameter | 1.6" |
| Chamber volume | 66.8 cc |
| Rocker ratio | 1.5:1 |
| 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 |
|---|---|---|
| 1961 | 318 hp | — |
| 1961 | 333 hp | — |
| 1961 | 348 hp | — |
| 1962 | 318 hp | — |
| 1962 | 333 hp | — |
| 1962 | 348 hp | — |
| 1964 | 325 hp | — |
| 1964 | 330 hp | — |
| 1964 | 348 hp | — |
| 1965 | 335 hp | — |
| 1965 | 338 hp | — |
| 1965 | 360 hp | — |
| 1966 | 335 hp | — |
| 1966 | 360 hp | — |