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Porsche 931 specs: 2.0L (121 ci) 4-cylinder, 86.5 x 84.4 mm (3.4055 x 3.3228 in) bore and stroke. Rated 143-154 hp, 1980–1982. Used in the Porsche 924.
The Porsche 121ci (931) is close to square: 3.4055" bore against 3.3228" stroke, within 2.5%, which splits the difference between a high-revving layout and a torque layout. The 3.3228" stroke does not reach 4,000 ft/min of mean piston speed until about 7,200 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 4 cylinders displaces 30.3 ci (496 cc).
It is one of 3 engines in the Porsche 121ci family, all of them 121 ci — the differences between them are in compression, heads and rating, not displacement. 3 factory variants share these dimensions, rated from 143 to 154 hp across 1980–1982.
| Displacement | 121 ci / 2 L |
|---|---|
| Bore | 3.4055" |
| Stroke | 3.3228" |
| Cylinders | 4 |
| Configuration | I4 |
| Compression ratio | 7.5–8.5:1 (published figures vary) |
| Bore/stroke ratio | 1.025 |
| Fuel type | gas |
|---|---|
| Advertised HP | 143–154 hp |
| Years | 1980–1982 |
| Bore/stroke character | 1.025 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 3,323 ft/min — conservative for a street engine |
| Displacement per cylinder | 30.3 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 |
|---|---|---|
| 1980 | 143 hp | 931 |
| 1981 | — | 931 |
| 1982 | 154 hp | 931 |
1 factory vehicle, 1980–1982. Each year opens that model's engine table.