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Oldsmobile 303ci specs: 303 ci (5.0L) 8-cylinder, 3.75 x 3.438 in (95.3 x 87.3 mm) bore and stroke, 7.25:1 compression. Rated 135 hp, 1950–1951.
The Oldsmobile 303ci is oversquare — the 3.75" bore exceeds the 3.438" stroke by 9.1% — a layout that favors airflow and RPM over leverage at the crank. The 3.438" stroke puts mean piston speed at 4,000 ft/min around 7,000 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.927 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 37.9 ci (621 cc).
7.25:1 is low by modern standards and generally happy on regular pump fuel.
It is one of 3 engines in the Oldsmobile 303ci family, all of them 303 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated at 135 hp across 1950–1951.
| Displacement | 303 ci / 5 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3.438" |
| Cylinders | 8 |
| Compression ratio | 7.25:1 |
| Rod length | 6.625" |
| Rod ratio | 1.927 |
| Bore/stroke ratio | 1.091 |
| Induction | 1-2 |
|---|---|
| Advertised HP | 135 hp |
| Years | 1950–1951 |
| Bore/stroke character | 1.091 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,438 ft/min — conservative for a street engine |
| Rod ratio | 1.927 — long — less side loading on the cylinder wall, favours high RPM |
| Displacement per cylinder | 37.9 ci |
| Static compression | 7.25:1 — low by modern standards, generally happy on regular pump fuel |
| Chamber volume | 99.3 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 |
|---|---|---|
| 1950 | 135 hp | — |
| 1951 | 135 hp | — |