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GMC 400ci (1976–1978) specs: 400 ci (6.6L) 8-cylinder, 4.125 x 3.75 in (104.8 x 95.3 mm) bore and stroke. Rated 175 hp, 1976–1978. Rod ratio and piston speed.
The GMC 400ci (1976–1978) 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).
It is one of 2 engines in the GMC 400ci family, all of them 400 ci — the differences between them are in compression, heads and rating, not displacement. 3 factory variants share these dimensions, rated at 175 hp across 1976–1978.
| Displacement | 400 ci / 6.6 L |
|---|---|
| Bore | 4.125" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 8.3–8.5:1 (published figures vary) |
| 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 |
| Firing order | 1-8-4-3-6-5-7-2 |
| Advertised HP | 175 hp |
| Years | 1976–1978 |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. | Duration int / exh | Valve lift int / exh | LSA |
|---|---|---|---|---|---|---|---|
| 1976 | LT4 net | 175 @ 3600 | 305 lb-ft @ 2000 | 8.5:1 | 280° / 288° | 0.390" / 0.410" | 113° |
| 1977 | net | 175 @ 3600 | 290 lb-ft @ 2800 | 8.5:1 | 280° / 288° | — | 113° |
| 1978 | net | 175 @ 3600 | 290 lb-ft @ 2800 | 8.3:1 | 280° / 288° | — | 113° |
Factory specification sheet. Durations are the factory's advertised figures at the basis printed on the sheet, not duration at .050" lift; lobe separation is calculated from the printed valve timing.
| 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 |
|---|---|---|
| 1976 | 175 hp | — |
| 1977 | 175 hp | — |
| 1978 | 175 hp | — |