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Pontiac 428 specs: 428 ci (7.0L) 8-cylinder, 4.189 x 3.902 in (106.4 x 99.1 mm) bore and stroke, 10.5:1 compression. Rated 375 hp. Rod ratio and piston speed.
The Pontiac V8 (428) is oversquare — the 4.189" bore exceeds the 3.9016" stroke by 7.4% — a layout that favors airflow and RPM over leverage at the crank. The 3.9016" stroke puts mean piston speed at 4,000 ft/min around 6,150 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).
10.5:1 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 13 engines in the Pontiac V8 family, sitting between the 326 ci and 455 ci members. Factory fitments include the Pontiac Grand Prix, Pontiac Bonneville, Pontiac Catalina and 3 others.
| Displacement | 428 ci / 7 L |
|---|---|
| Bore | 4.189" |
| Stroke | 3.9016" |
| Cylinders | 8 |
| Compression ratio | 10.5:1 |
| Bore/stroke ratio | 1.074 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 375 hp |
| Bore/stroke character | 1.074 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,902 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 53.5 ci |
| Static compression | 10.5:1 — in the range that usually runs on pump premium |
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