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Pontiac LB9 specs: 305 ci (5.0L) 8-cylinder, 3.736 x 3.484 in (94.9 x 88.5 mm) bore and stroke. Rated 190-230 hp, 1985–1992. Used in the Pontiac Firebird.
The Pontiac 305ci (LB9) is oversquare — the 3.736" bore exceeds the 3.484" stroke by 7.2% — a layout that favors airflow and RPM over leverage at the crank. The 3.484" 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. A rod ratio of 1.636 is in the range most production engines fall in. Each of the 8 cylinders displaces 38.1 ci (625 cc).
It is one of 14 engines in the Pontiac 305ci family, all of them 305 ci — the differences between them are in compression, heads and rating, not displacement. 10 factory variants share these dimensions, rated from 190 to 230 hp across 1985–1992.
| Displacement | 305 ci / 5 L |
|---|---|
| Bore | 3.736" |
| Stroke | 3.484" |
| Cylinders | 8 |
| Configuration | V8 |
| Rod length | 5.7" |
| Rod ratio | 1.636 |
| Bore/stroke ratio | 1.072 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 190–230 hp |
| Years | 1985–1992 |
| Bore/stroke character | 1.072 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,484 ft/min — conservative for a street engine |
| Rod ratio | 1.636 — moderate — the range most production V8s fall in |
| 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 |
|---|---|---|
| 1985 | 210 hp | LB9 |
| 1987 | 205 hp | LB9 |
| 1988 | 215 hp | LB9 |
| 1989 | 190 hp | LB9 |
| 1989 | 215 hp | LB9 |
| 1989 | 225 hp | LB9 |
| 1990 | 200 hp | LB9 |
| 1990 | 225 hp | LB9 |
| 1992 | 205 hp | LB9 |
| 1992 | 230 hp | LB9 |