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BMW M21D24 specs: 2.4L (149 ci) 6-cylinder, 80 x 81 mm (3.1496 x 3.1890 in) bore and stroke, 22:1 compression. Rated 115 hp, 1985–1986.
The BMW M21D24 is close to square: 3.1496" bore against 3.189" stroke, within 1.2%, which splits the difference between a high-revving layout and a torque layout. The 3.189" stroke does not reach 4,000 ft/min of mean piston speed until about 7,550 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 6 cylinders displaces 24.8 ci (407 cc).
At 22:1 this is a compression-ignition engine: the ratio is the heat source that lights the fuel, not a knock limit to be managed.
2 factory variants share these dimensions, rated at 115 hp across 1985–1986.
| Displacement | 149 ci / 2.4 L |
|---|---|
| Bore | 3.1496" |
| Stroke | 3.189" |
| Cylinders | 6 |
| Configuration | I6 |
| Compression ratio | 22:1 * |
| Bore/stroke ratio | 0.988 |
* Compression ratio from a single source; not yet cross-verified.
| Fuel type | diesel |
|---|---|
| Advertised HP | 115 hp |
| Years | 1985–1986 |
| Bore/stroke character | 0.988 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 3,189 ft/min — conservative for a street engine |
| Displacement per cylinder | 24.8 ci |
| Static compression | 22:1 — high enough that pump premium or better is normally required |
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 | 115 hp | M21D24 |
| 1986 | 115 hp | M21D24 |
1 factory vehicle, 1985–1986. Each year opens that model's engine table.