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Plymouth 303ci (1956) specs: 303 ci 8-cylinder, 3.813 x 3.313 in (96.8 x 84.1 mm) bore and stroke, 9.25:1 compression. Rated 240 hp, 1956.
The Plymouth 303ci (1956) is oversquare — the 3.8125" bore exceeds the 3.3125" stroke by 15.1% — a layout that favors airflow and RPM over leverage at the crank. The 3.3125" stroke does not reach 4,000 ft/min of mean piston speed until about 7,250 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 8 cylinders displaces 37.9 ci (621 cc).
9.25:1 is low by modern standards and generally happy on regular pump fuel.
| Displacement | 303 ci |
|---|---|
| Bore | 3.8125" |
| Stroke | 3.3125" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 9.25:1 |
| Bore/stroke ratio | 1.151 |
| Fuel type | gas |
|---|---|
| Advertised HP | 240 hp |
| Years | 1956 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1956 | gross | 240 @ 4800 | 310 lb-ft @ 2800 | 9.25:1 |
Factory specification sheet.
| Bore/stroke character | 1.151 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,313 ft/min — conservative for a street engine |
| Displacement per cylinder | 37.9 ci |
| Static compression | 9.25:1 — low by modern standards, generally happy on regular pump fuel |
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1 factory vehicle, 1956. Each year opens that model's engine table.