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Plymouth A 318 (poly) specs: 318 ci (5.2L) 8-cylinder, 3.91 x 3.31 in (99.3 x 84.1 mm) bore and stroke. OHV. Rod ratio and piston speed worked out.
The Plymouth A 318 (poly) 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.
| 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 |
| 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 |
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