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GMC LL8 specs: 256 ci (4.2L) 6-cylinder, 3.661 x 4.016 in (93 x 102 mm) bore and stroke, 10:1 compression. Rated 275-291 hp. DOHC. Rod ratio and piston speed.
The GMC Vortec 4200 (4.2L I6) (LL8) is undersquare — the 4.0157" stroke exceeds the 3.6614" bore by 8.8% — a layout that trades peak RPM for leverage and torque low in the range. The 4.0157" stroke puts mean piston speed at 4,000 ft/min around 6,000 RPM, which is where a production bottom end is generally considered at its limit. Each of the 6 cylinders displaces 42.7 ci (699 cc).
10:1 on aluminum heads is in the range that usually runs on pump premium; aluminum sheds heat faster than iron, which makes the same number a little more forgiving.
2 factory variants share these dimensions, rated from 275 to 291 hp. Factory fitments include the Chevrolet TrailBlazer, GMC Envoy, Oldsmobile Bravada and 3 others.
| Displacement | 256 ci / 4.2 L |
|---|---|
| Bore | 3.6614" |
| Stroke | 4.0157" |
| Cylinders | 6 |
| Compression ratio | 10:1 |
| Bore/stroke ratio | 0.912 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Advertised HP | 275–291 hp |
| Bore/stroke character | 0.912 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 4,016 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 42.7 ci |
| Static compression | 10:1 — in the range that usually runs on pump premium |
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 |
|---|---|---|
| — | 275 hp | LL8 |
| — | 291 hp | LL8 |