See other engines in this family
Buick 307ci specs: 307 ci (5.0L) 8-cylinder, 3.80 x 3.385 in (96.5 x 86 mm) bore and stroke, 9.611:1 compression. Rated 140-145 hp, 1985–1986. OHV.
The Buick 307ci is oversquare — the 3.8" bore exceeds the 3.385" stroke by 12.3% — a layout that favors airflow and RPM over leverage at the crank. The 3.385" stroke does not reach 4,000 ft/min of mean piston speed until about 7,100 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. A rod ratio of 1.773 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 8 cylinders displaces 38.4 ci (629 cc).
9.611:1 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 2 engines in the Buick 307ci family, all of them 307 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated from 140 to 145 hp across 1985–1986.
| Displacement | 307 ci / 5 L |
|---|---|
| Bore | 3.8" |
| Stroke | 3.385" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 9.611:1 |
| Rod length | 6.002" |
| Rod ratio | 1.773 |
| Bore/stroke ratio | 1.123 |
| Piston deck clearance | 0.03" |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 140–145 hp |
| Years | 1985–1986 |
| Bore/stroke character | 1.123 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,385 ft/min — conservative for a street engine |
| Rod ratio | 1.773 — long — less side loading on the cylinder wall, favours high RPM |
| Displacement per cylinder | 38.4 ci |
| Static compression | 9.611:1 — in the range that usually runs on pump premium |
| Intake valve diameter | 1.75" |
|---|---|
| Exhaust valve diameter | 1.502" |
| Chamber volume | 59.85 cc |
| Rocker ratio | 1.6:1 |
| Lifter type | hydraulic |
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 | 140 hp | — |
| 1986 | 145 hp | — |