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Oldsmobile 403ci specs: 403 ci (6.6L) 8-cylinder, 4.351 x 3.385 in (110.5 x 86 mm) bore and stroke, 9.181:1 compression. Rated 185-200 hp, 1977.
The Oldsmobile 403ci is oversquare — the 4.351" bore exceeds the 3.385" stroke by 28.5% — a layout that favors airflow and RPM over leverage at the crank. The 3.385" stroke does not reach 4,000 ft/min of mean piston speed until about 7,100 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. A rod ratio of 1.773 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 50.4 ci (825 cc).
9.181:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
2 factory variants share these dimensions, rated from 185 to 200 hp in 1977.
| Displacement | 403 ci / 6.6 L |
|---|---|
| Bore | 4.351" |
| Stroke | 3.385" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 9.181:1 |
| Rod length | 6.002" |
| Rod ratio | 1.773 |
| Bore/stroke ratio | 1.285 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 185–200 hp |
| Years | 1977 |
| Bore/stroke character | 1.285 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,385 ft/min — conservative for a street engine |
| Rod ratio | 1.773 — long — less side loading on the cylinder wall, favours high RPM |
| Displacement per cylinder | 50.4 ci |
| Static compression | 9.181:1 — low by modern standards, generally happy on regular pump fuel |
| Intake valve diameter | 2" |
|---|---|
| Exhaust valve diameter | 1.5" |
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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 |
|---|---|---|
| 1977 | 185 hp | — |
| 1977 | 200 hp | — |