Mercedes-Benz 121ci specs: 83 x 92 mm (3.2677 x 3.6220 in) bore and stroke, shared by all 5 versions. Displacement, compression and factory ratings.
| Displacement | 121 ci |
|---|---|
| Cylinders | 4 |
| Bore | 3.268" |
| Stroke | 3.622" |
| Compression | 9.0–10.5:1 |
| Advertised power | 188–416 hp |
| Years | 2015–2026 |
| Engine | Displacement | Bore × Stroke | CR | HP |
|---|---|---|---|---|
| M274 | 121 ci | 3.2677" × 3.622" | 9.8:1 | 188–241 |
| M274 + e-motor | 121 ci | 3.2677" × 3.622" | 9.8:1 | — |
| M139l | 121 ci | 3.2677" × 3.622" | 9:1 | 416 |
| M139l + rear e-motor | 121 ci | 3.2677" × 3.622" | 9:1 | — |
| M264 | 121 ci | 3.2677" × 3.622" | 10.5:1 | 255 |
Compression spans 9:1 to 10.5:1, so fuel requirements differ meaningfully between members — worth checking before assuming parts interchange.
All are undersquare — stroke larger than bore, favouring torque low in the range.
Each engine page carries rod ratio, mean piston speed, bore/stroke character and cylinder head specs where published, and links into the calculators with its dimensions already filled in.