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Buick LM9 specs: 231 ci (3.8L) 6-cylinder, 3.80 x 3.40 in (96.5 x 86.4 mm) bore and stroke. Rated 200 hp, 1984. Used in the Buick Regal. OHV.
The Buick 231ci (LM9) is oversquare — the 3.8" bore exceeds the 3.4" stroke by 11.8% — a layout that favors airflow and RPM over leverage at the crank. The 3.4" stroke does not reach 4,000 ft/min of mean piston speed until about 7,050 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. A rod ratio of 1.753 is long by production standards — less side-load on the cylinder wall and gentler piston acceleration around TDC, the geometry that tolerates high RPM. Each of the 6 cylinders displaces 38.5 ci (631 cc).
It is one of 12 engines in the Buick 231ci family, all of them 231 ci — the differences between them are in compression, heads and rating, not displacement.
| Displacement | 231 ci / 3.8 L |
|---|---|
| Bore | 3.8" |
| Stroke | 3.4" |
| Cylinders | 6 |
| Configuration | V6 |
| Rod length | 5.96" |
| Rod ratio | 1.753 |
| Bore/stroke ratio | 1.118 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 200 hp |
| Years | 1984 |
| Bore/stroke character | 1.118 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,400 ft/min — conservative for a street engine |
| Rod ratio | 1.753 — long — less side loading on the cylinder wall, favours high RPM |
| Displacement per cylinder | 38.5 ci |
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