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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, 10.5:1 compression. Rated 275-290 hp. 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).
10.5:1 on iron heads is in the range that usually runs on pump premium; iron holds heat, so treat it as the less forgiving side of that range.
It is one of 20 engines in the LA family, sitting between the 273 ci and 360 ci members. 2 factory variants share these dimensions, rated from 275 to 290 hp. Factory fitments include the Plymouth Duster 340, Plymouth Barracuda, Plymouth 'Cuda 340 and 2 others.
| Displacement | 340 ci / 5.6 L |
|---|---|
| Bore | 4.0315" |
| Stroke | 3.311" |
| Cylinders | 8 |
| Compression ratio | 10.5:1 |
| Bore/stroke ratio | 1.218 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 275–290 hp |
| 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 |
| Static compression | 10.5:1 — in the range that usually runs on pump premium |
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 |
|---|---|---|
| — | 275 hp | J |
| — | 290 hp | J |