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Jeep AMC 242 MPI specs: 242 ci (4.0L) 6-cylinder, 3.875 x 3.41 in (98.4 x 86.6 mm) bore and stroke. Rated 173-177 hp, 1987–1990. Rod ratio and piston speed.
The Jeep AMC 242 MPI is oversquare — the 3.8752" bore exceeds the 3.4098" stroke by 13.6% — a layout that favors airflow and RPM over leverage at the crank. The 3.4098" stroke does not reach 4,000 ft/min of mean piston speed until about 7,050 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 6 cylinders displaces 40.3 ci (661 cc).
It is one of 14 engines in the AMC Straight-6 family, the smallest at 242 ci. 4 factory variants share these dimensions, rated from 173 to 177 hp across 1987–1990.
| Displacement | 242 ci / 4 L |
|---|---|
| Bore | 3.8752" |
| Stroke | 3.4098" |
| Cylinders | 6 |
| Configuration | I6 |
| Bore/stroke ratio | 1.136 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 173–177 hp |
| Years | 1987–1990 |
| Bore/stroke character | 1.136 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,410 ft/min — conservative for a street engine |
| Displacement per cylinder | 40.3 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 |
|---|---|---|
| 1987 | 173 hp | AMC 242 MPI |
| 1988 | 177 hp | AMC 242 MPI |
| 1989 | 177 hp | AMC 242 MPI |
| 1990 | 177 hp | AMC 242 MPI |