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Pontiac 305ci specs: 305 ci (5.0L) 8-cylinder, 3.736 x 3.484 in (94.9 x 88.5 mm) bore and stroke, 11.38:1 compression. Rated 170 hp, 1988–1991.
The Pontiac 305ci 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).
11.38:1 on iron heads is high enough that pump premium or better is normally required, and iron holds heat, so it is the less forgiving side of that line.
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. 3 factory variants share these dimensions, rated at 170 hp across 1988–1991.
| Displacement | 305 ci / 5 L |
|---|---|
| Bore | 3.736" |
| Stroke | 3.484" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 11.38:1 |
| 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 | 170 hp |
| Years | 1988–1991 |
| 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 |
| Static compression | 11.38:1 — high enough that pump premium or better is normally required |
| Intake valve diameter | 1.845" |
|---|---|
| Exhaust valve diameter | 1.505" |
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 |
|---|---|---|
| 1988 | 170 hp | — |
| 1989 | 170 hp | — |
| 1991 | 170 hp | — |