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Opel 1975 115 81HP specs: 1.9L (115 ci) 4-cylinder, 93 x 69.9 mm (3.6600 x 2.7500 in) bore and stroke, 8.02:1 compression. Rated 81 hp, 1975. OHC.
The Opel 1975 115 81HP is oversquare — the 3.66" bore exceeds the 2.75" stroke by 33.1% — a layout that favors airflow and RPM over leverage at the crank. The 2.75" stroke does not reach 4,000 ft/min of mean piston speed until about 8,750 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. A rod ratio of 1.833 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 4 cylinders displaces 28.8 ci (471 cc).
8.02:1 on aluminum heads is low by modern standards and generally happy on regular pump fuel.
| Displacement | 115 ci / 1.9 L |
|---|---|
| Bore | 3.66" |
| Stroke | 2.75" |
| Cylinders | 4 |
| Configuration | I4 |
| Compression ratio | 8.02:1 |
| Rod length | 5.04" |
| Rod ratio | 1.833 |
| Bore/stroke ratio | 1.331 |
| Piston deck clearance | 0.028" |
| Block material | cast iron |
|---|---|
| Head material | aluminum |
| Valvetrain | OHC |
| Fuel type | gas |
| Advertised HP | 81 hp |
| Years | 1975 |
| Bore/stroke character | 1.331 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 2,750 ft/min — conservative for a street engine |
| Rod ratio | 1.833 — long — less side loading on the cylinder wall, favours high RPM |
| Displacement per cylinder | 28.8 ci |
| Static compression | 8.02:1 — low by modern standards, generally happy on regular pump fuel |
| Chamber volume | 46 cc |
|---|---|
| Rocker ratio | 1.4:1 |
| Lifter type | mechanical |
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