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Pontiac L78 specs: 400 ci (6.6L) 8-cylinder, 4.125 x 3.75 in (104.8 x 95.3 mm) bore and stroke. Rated 180-330 hp, 1970–1978. Used in the Oldsmobile Cutlass.
The Pontiac 400ci (L78) is oversquare — the 4.125" bore exceeds the 3.75" stroke by 10.0% — 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 50.0 ci (819 cc).
It is one of 18 engines in the Pontiac 400ci family, all of them 400 ci — the differences between them are in compression, heads and rating, not displacement. 6 factory variants share these dimensions, rated from 180 to 330 hp across 1970–1978.
| Displacement | 400 ci / 6.6 L |
|---|---|
| Bore | 4.125" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Configuration | V8 |
| Rod length | 6.625" |
| Rod ratio | 1.767 |
| Bore/stroke ratio | 1.1 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 180–330 hp |
| Years | 1970–1978 |
| Bore/stroke character | 1.100 — 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 | 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 |
|---|---|---|
| 1970 | 330 hp | L78 |
| 1971 | 300 hp | L78 |
| 1972 | 250 hp | L78 |
| 1973 | 230 hp | L78 |
| 1974 | 225 hp | L78 |
| 1978 | 180 hp | L78 |