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GMC 428 specs: 428 ci (7.0L) 8-cylinder, 4.12 x 4.00 in (104.6 x 101.6 mm) bore and stroke, 10.75:1 compression. Rated 390 hp. Rod ratio and piston speed.
The GMC Pontiac V8 (428) is close to square: 4.12" bore against 4" stroke, within 3.0%, which splits the difference between a high-revving layout and a torque layout. The 4" stroke puts mean piston speed at 4,000 ft/min around 6,000 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 53.5 ci (877 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 Grand Prix, Pontiac Bonneville, Pontiac Catalina.
| Displacement | 428 ci / 7 L |
|---|---|
| Bore | 4.12" |
| Stroke | 4" |
| Cylinders | 8 |
| Compression ratio | 10.75:1 |
| Bore/stroke ratio | 1.03 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 390 hp |
| Bore/stroke character | 1.030 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 4,000 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 53.5 ci |
| Static compression | 10.75:1 — in the range that usually runs on pump premium |
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