See other engines in this family
BMW M10B20 specs: 2.0L (121 ci) 4-cylinder, 89 x 80 mm (3.5039 x 3.1496 in) bore and stroke, 8.1:1 compression. Rated 110 hp, 1977–1979.
The BMW M10B20 is oversquare — the 3.5039" bore exceeds the 3.1496" stroke by 11.2% — a layout that favors airflow and RPM over leverage at the crank. The 3.1496" stroke does not reach 4,000 ft/min of mean piston speed until about 7,600 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).
8.1:1 is low by modern standards and generally happy on regular pump fuel.
It is one of 2 engines in the BMW 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 at 110 hp across 1977–1979.
| Displacement | 121 ci / 2 L |
|---|---|
| Bore | 3.5039" |
| Stroke | 3.1496" |
| Cylinders | 4 |
| Configuration | I4 |
| Compression ratio | 8.1:1 * |
| Bore/stroke ratio | 1.112 |
* Compression ratio from a single source; not yet cross-verified.
| Fuel type | gas |
|---|---|
| Advertised HP | 110 hp |
| Years | 1977–1979 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 1977 | M10B20 net | 110 | 112 lb-ft | — |
| 1978 | M10B20 net | 110 | 112 lb-ft | — |
| 1979 | M10B20 net | 110 | 112 lb-ft | — |
Factory specification sheet.
| Bore/stroke character | 1.112 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,150 ft/min — conservative for a street engine |
| Displacement per cylinder | 30.3 ci |
| Static compression | 8.1:1 — low by modern standards, generally happy on regular pump fuel |
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 |
|---|---|---|
| 1977 | 110 hp | M10B20 |
| 1978 | 110 hp | M10B20 |
| 1979 | 110 hp | M10B20 |
1 factory vehicle, 1977–1979. Each year opens that model's engine table.