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Plymouth A specs: 273 ci (4.5L) 8-cylinder, 3.63 x 3.311 in (92.2 x 84.1 mm) bore and stroke, 8.8:1 compression. Rated 180 hp. Rod ratio and piston speed.
The Plymouth LA (A) is oversquare — the 3.6299" bore exceeds the 3.311" stroke by 9.6% — 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 34.1 ci (559 cc).
8.8:1 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, the smallest at 273 ci. Factory fitments include the Plymouth Valiant, Plymouth Barracuda.
| Displacement | 273 ci / 4.5 L |
|---|---|
| Bore | 3.6299" |
| Stroke | 3.311" |
| Cylinders | 8 |
| Compression ratio | 8.8:1 |
| Bore/stroke ratio | 1.096 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 180 hp |
| Bore/stroke character | 1.096 — 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 | 34.1 ci |
| Static compression | 8.8:1 — low by modern standards, generally happy on regular pump fuel |
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