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GMC 427 specs: 427 ci (7.0L) 8-cylinder, 4.25 x 3.76 in (108 x 95.5 mm) bore and stroke, 11:1 compression. Rated 335 hp. OHV. Rod ratio and piston speed.
The GMC Big Block Chevy (427) is oversquare — the 4.25" bore exceeds the 3.76" stroke by 13.0% — 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 53.4 ci (875 cc).
11:1 on iron heads is high enough that pump premium or better is normally required, and iron holds heat, so it is the less forgiving side of that line.
It is one of 5 engines in the Big Block Chevy family, sitting between the 396 ci and 502 ci members. Factory fitments include the GMC C/K Series, GMC Medium Duty Trucks.
| Displacement | 427 ci / 7 L |
|---|---|
| Bore | 4.25" |
| Stroke | 3.76" |
| Cylinders | 8 |
| Compression ratio | 11:1 |
| Bore/stroke ratio | 1.13 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 335 hp |
| Bore/stroke character | 1.130 — 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 | 53.4 ci |
| Static compression | 11:1 — high enough that pump premium or better is normally required |
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