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Pontiac 421 specs: 421 ci (6.9L) 8-cylinder, 4.189 x 3.811 in (106.4 x 96.8 mm) bore and stroke, 10.75:1 compression. Rated 370 hp. OHV.
The Pontiac V8 (421) is oversquare — the 4.189" bore exceeds the 3.811" stroke by 9.9% — a layout that favors airflow and RPM over leverage at the crank. The 3.811" stroke puts mean piston speed at 4,000 ft/min around 6,300 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 52.6 ci (862 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 13 engines in the Pontiac V8 family, sitting between the 326 ci and 455 ci members. Factory fitments include the Pontiac Catalina, Pontiac Grand Prix, Pontiac Bonneville.
| Displacement | 421 ci / 6.9 L |
|---|---|
| Bore | 4.189" |
| Stroke | 3.811" |
| Cylinders | 8 |
| Compression ratio | 10.75:1 |
| Bore/stroke ratio | 1.099 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 370 hp |
| Bore/stroke character | 1.099 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,811 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 52.6 ci |
| Static compression | 10.75:1 — in the range that usually runs on pump premium |
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