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Plymouth LM specs: 318 ci (5.2L) 8-cylinder, 3.909 x 3.311 in (99.3 x 84.1 mm) bore and stroke, 9:1 compression. Rated 230 hp. Rod ratio and piston speed.
The Plymouth LA (LM) is oversquare — the 3.9094" bore exceeds the 3.311" stroke by 18.1% — 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 39.8 ci (651 cc).
9:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
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 at 230 hp. Factory fitments include the Plymouth Satellite, Plymouth Fury, Plymouth Duster and 4 others.
| Displacement | 318 ci / 5.2 L |
|---|---|
| Bore | 3.9094" |
| Stroke | 3.311" |
| Cylinders | 8 |
| Compression ratio | 9:1 |
| Bore/stroke ratio | 1.181 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 230 hp |
| Bore/stroke character | 1.181 — 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 | 39.8 ci |
| Static compression | 9:1 — low by modern standards, generally happy on regular pump fuel |
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 |
|---|---|---|
| — | 230 hp | LM |
| — | 230 hp | LA-318, 318, E, LM |