See other engines in this family
Oldsmobile 231ci specs: 231 ci (3.8L) 6-cylinder, 3.80 x 3.40 in (96.5 x 86.4 mm) bore and stroke, 9.8:1 compression. Rated 165-205 hp, 1988–1993. OHV.
The Oldsmobile 231ci 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).
9.8:1 on iron heads is in the range that usually runs on pump premium; iron holds heat, so treat it as the less forgiving side of that range.
It is one of 6 engines in the Oldsmobile 231ci family, all of them 231 ci — the differences between them are in compression, heads and rating, not displacement. 5 factory variants share these dimensions, rated from 165 to 205 hp across 1988–1993.
| Displacement | 231 ci / 3.8 L |
|---|---|
| Bore | 3.8" |
| Stroke | 3.4" |
| Cylinders | 6 |
| Configuration | V6 |
| Compression ratio | 9.8:1 |
| 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 | 165–205 hp |
| Years | 1988–1993 |
| 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 |
| Static compression | 9.8:1 — in the range that usually runs on pump premium |
| Intake valve diameter | 1.71" |
|---|---|
| Exhaust valve diameter | 1.5" |
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 | 165 hp | — |
| 1990 | 165 hp | — |
| 1991 | 165 hp | — |
| 1992 | 170 hp | — |
| 1993 | 205 hp | — |