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Plymouth J specs: 340 ci (5.6L) 8-cylinder, 4.032 x 3.311 in (102.4 x 84.1 mm) bore and stroke. Rated 290 hp, 1970. Used in the Dodge Challenger. OHV.
The Plymouth LA (J) is oversquare — the 4.0315" bore exceeds the 3.311" stroke by 21.8% — a layout that favors airflow and RPM over leverage at the crank. The 3.311" 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 42.5 ci (696 cc).
It is one of 20 engines in the LA family, sitting between the 273 ci and 360 ci members.
| Displacement | 340 ci / 5.6 L |
|---|---|
| Bore | 4.0315" |
| Stroke | 3.311" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.218 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 290 hp |
| Years | 1970 |
| Bore/stroke character | 1.218 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,311 ft/min — conservative for a street engine |
| Displacement per cylinder | 42.5 ci |
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