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Plymouth A (1957–1966) specs: 318 ci (5.2L) 8-cylinder, 3.91 x 3.31 in (99.3 x 84.1 mm) bore and stroke. Rated 230-290 hp, 1957–1966.
The Plymouth A (1957–1966) is oversquare — the 3.91" bore exceeds the 3.31" stroke by 18.1% — a layout that favors airflow and RPM over leverage at the crank. The 3.31" 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 39.8 ci (651 cc).
It is one of 2 engines in the A family, all of them 318 ci — the differences between them are in compression, heads and rating, not displacement. 14 factory variants share these dimensions, rated from 230 to 290 hp across 1957–1966.
| Displacement | 318 ci / 5.2 L |
|---|---|
| Bore | 3.91" |
| Stroke | 3.31" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 9–9.25:1 (published figures vary) |
| Bore/stroke ratio | 1.181 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Firing order | 1-8-4-3-6-5-7-2 |
| Advertised HP | 230–290 hp |
| Years | 1957–1966 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. | Duration int / exh | Valve lift int / exh | LSA |
|---|---|---|---|---|---|---|---|
| 1957 | gross | 290 @ 5400 | 325 lb-ft @ 4000 | 9.25:1 | 256° | — | 109° |
| 1958 | gross | 290 @ 5200 | 330 lb-ft @ 3600 | 9.25:1 | 260° / 252° | — | 109° |
| 1959 | gross | 230 @ 4400 | 340 lb-ft @ 2400 | 9:1 | 244° | — | 109° |
| 1959 | gross | 260 @ 4400 | 345 lb-ft @ 2800 | 9:1 | 248° | — | 109° |
| 1960 | gross | 230 @ 4400 | 340 lb-ft @ 2400 | 9:1 | — | — | — |
| 1960 | gross | 260 @ 4400 | 345 lb-ft @ 2800 | 9:1 | — | — | — |
| 1961 | gross | 230 @ 4400 | 340 lb-ft @ 2400 | 9:1 | — | — | — |
| 1961 | gross | 260 | — | 9:1 | — | — | — |
| 1962 | gross | 230 @ 4400 | 340 lb-ft @ 2400 | 9:1 | — | — | — |
| 1962 | gross | 260 @ 4400 | 345 lb-ft @ 2800 | 9:1 | — | — | — |
| 1963 | gross | 230 @ 4400 | 340 lb-ft @ 2400 | 9:1 | — | — | — |
| 1964 | gross | 230 @ 4400 | 340 lb-ft @ 2400 | 9:1 | — | — | — |
| 1965 | gross | 230 @ 4400 | 340 lb-ft @ 2400 | 9:1 | 244° / 240° | 0.397" / 0.403" | 111° |
Factory specification sheet. Durations are the factory's advertised figures at the basis printed on the sheet, not duration at .050" lift; lobe separation is calculated from the printed valve timing.
| Bore/stroke character | 1.181 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,310 ft/min — conservative for a street engine |
| Displacement per cylinder | 39.8 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 |
|---|---|---|
| 1957 | 290 hp | — |
| 1958 | 290 hp | — |
| 1959 | 230 hp | — |
| 1959 | 260 hp | — |
| 1960 | 230 hp | — |
| 1960 | 260 hp | — |
| 1961 | 230 hp | — |
| 1961 | 260 hp | — |
| 1962 | 230 hp | — |
| 1962 | 260 hp | — |
| 1963 | 230 hp | — |
| 1964 | 230 hp | — |
| 1965 | 230 hp | — |
| 1966 | 230 hp | — |
2 factory vehicles, 1957–1966. Each year opens that model's engine table.