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GMC 400 specs: 400 ci (6.6L) 8-cylinder, 4.12 x 3.75 in (104.6 x 95.3 mm) bore and stroke, 10.75:1 compression. Rated 360 hp. OHV. Rod ratio and piston speed.
The GMC Pontiac V8 (400) is oversquare — the 4.12" bore exceeds the 3.75" stroke by 9.9% — 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. Each of the 8 cylinders displaces 50.0 ci (819 cc).
10.75:1 on iron heads is in the range that usually runs on pump premium; iron holds heat, so treat it as the less forgiving side of that range.
It is one of 9 engines in the Pontiac V8 family, sitting between the 287 ci and 455 ci members. Factory fitments include the Pontiac GTO, Pontiac Firebird, Pontiac Trans Am and 1 other.
| Displacement | 400 ci / 6.6 L |
|---|---|
| Bore | 4.12" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Compression ratio | 10.75:1 |
| Bore/stroke ratio | 1.099 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 360 hp |
| Bore/stroke character | 1.099 — 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 |
| Displacement per cylinder | 50.0 ci |
| Static compression | 10.75:1 — in the range that usually runs on pump premium |
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