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Pontiac 421 specs: 421 ci (6.9L) 8-cylinder, 4.091 x 4.00 in (103.9 x 101.6 mm) bore and stroke, 10.75:1 compression. Rated 376-405 hp. OHV.
The Pontiac V8 (421) is close to square: 4.0906" bore against 4" stroke, within 2.3%, 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 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. 2 factory variants share these dimensions, rated from 376 to 405 hp. Factory fitments include the Pontiac Grand Prix, Pontiac Catalina 2+2, Grand Prix and 2 others.
| Displacement | 421 ci / 6.9 L |
|---|---|
| Bore | 4.0906" |
| Stroke | 4" |
| Cylinders | 8 |
| Compression ratio | 10.75:1 |
| Bore/stroke ratio | 1.023 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 376–405 hp |
| Bore/stroke character | 1.023 — 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 | 52.6 ci |
| Static compression | 10.75:1 — in the range that usually runs on pump premium |
Each link opens the calculator with this engine's specs already filled in.
These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| — | 376 hp | 421 |
| — | 405 hp | 421 |