See other engines in this family
The same engine also has a main page: Chevrolet LF4.
GMC LF4 specs: 400 ci (6.6L) 8-cylinder, 4.125 x 3.75 in (104.8 x 95.3 mm) bore and stroke. Rated 175-185 hp, 1977–1979. OHV. Rod ratio and piston speed.
The GMC 400ci (LF4) is oversquare — the 4.125" bore exceeds the 3.75" stroke by 10.0% — a layout that favors airflow and RPM over leverage at the crank. The 3.75" stroke puts mean piston speed at 4,000 ft/min around 6,400 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.485 is short — higher piston acceleration and more side-load, typical of stroker combinations and a reason to watch piston speed closely. Each of the 8 cylinders displaces 50.0 ci (819 cc).
3 factory variants share these dimensions, rated from 175 to 185 hp across 1977–1979.
| Displacement | 400 ci / 6.6 L |
|---|---|
| Bore | 4.125" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Configuration | V8 |
| Rod length | 5.57" |
| Rod ratio | 1.485 |
| Bore/stroke ratio | 1.1 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 175–185 hp |
| Years | 1977–1979 |
| Bore/stroke character | 1.100 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,750 ft/min — in the range typical of performance street engines |
| Rod ratio | 1.485 — short — more side loading and piston dwell at TDC, typical of stroker combinations |
| Displacement per cylinder | 50.0 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 |
|---|---|---|
| 1977 | 175 hp | LF4 |
| 1978 | 175 hp | LF4 |
| 1979 | 185 hp | LF4 |