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GMC 400 specs: 400 ci (6.6L) 8-cylinder, 4.125 x 3.76 in (104.8 x 95.5 mm) bore and stroke, 8:1 compression. Rated 150 hp. OHV. Rod ratio and piston speed.
The GMC Small Block Chevy (400) is oversquare — the 4.1248" bore exceeds the 3.7598" stroke by 9.7% — a layout that favors airflow and RPM over leverage at the crank. The 3.7598" 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).
8:1 on iron heads is low by modern standards and generally happy on regular pump fuel.
It is one of 23 engines in the Small Block Chevy family, the largest at 400 ci. Factory fitments include the GMC C/K Series, GMC Suburban, GMC Sprint.
| Displacement | 400 ci / 6.6 L |
|---|---|
| Bore | 4.1248" |
| Stroke | 3.7598" |
| Cylinders | 8 |
| Compression ratio | 8:1 |
| Bore/stroke ratio | 1.097 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 150 hp |
| Bore/stroke character | 1.097 — 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 | 50.0 ci |
| Static compression | 8:1 — low by modern standards, generally happy on regular pump fuel |
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