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GMC 396 specs: 396 ci (6.5L) 8-cylinder, 4.094 x 3.76 in (104 x 95.5 mm) bore and stroke, 10.25:1 compression. Rated 325 hp. OHV. Rod ratio and piston speed.
The GMC Big Block Chevy (396) is oversquare — the 4.094" bore exceeds the 3.76" stroke by 8.9% — a layout that favors airflow and RPM over leverage at the crank. The 3.76" 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 49.5 ci (811 cc).
10.25:1 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 5 engines in the Big Block Chevy family, the smallest at 396 ci. Factory fitments include the GMC C/K Series, GMC Suburban.
| Displacement | 396 ci / 6.5 L |
|---|---|
| Bore | 4.094" |
| Stroke | 3.76" |
| Cylinders | 8 |
| Compression ratio | 10.25:1 |
| Bore/stroke ratio | 1.089 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 325 hp |
| Bore/stroke character | 1.089 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,760 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 49.5 ci |
| Static compression | 10.25:1 — in the range that usually runs on pump premium |
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