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GMC LG9 specs: 305 ci (5.0L) 8-cylinder, 3.75 x 3.48 in (95.3 x 88.4 mm) bore and stroke. Rated 130-140 hp, 1978–1981. Used in the GMC Jimmy. OHV.
The GMC Small Block Chevy (LG9) is oversquare — the 3.75" bore exceeds the 3.4799" stroke by 7.8% — a layout that favors airflow and RPM over leverage at the crank. The 3.4799" stroke puts mean piston speed at 4,000 ft/min around 6,900 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 38.1 ci (625 cc).
It is one of 23 engines in the Small Block Chevy family, sitting between the 265 ci and 400 ci members. 2 factory variants share these dimensions, rated from 130 to 140 hp across 1978–1981.
| Displacement | 305 ci / 5 L |
|---|---|
| Bore | 3.75" |
| Stroke | 3.4799" |
| Cylinders | 8 |
| Configuration | V8 |
| Bore/stroke ratio | 1.078 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 130–140 hp |
| Years | 1978–1981 |
| Bore/stroke character | 1.078 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,480 ft/min — conservative for a street engine |
| Displacement per cylinder | 38.1 ci |
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 |
|---|---|---|
| 1978 | 140 hp | LG9 |
| 1981 | 130 hp | LG9 |