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GMC 421 specs: 421 ci (6.9L) 8-cylinder, 4.094 x 4.00 in (104 x 101.6 mm) bore and stroke, 10.75:1 compression. Rated 370 hp. OHV. Rod ratio and piston speed.
The GMC Pontiac V8 (421) is close to square: 4.09375" 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 9 engines in the Pontiac V8 family, sitting between the 287 ci and 455 ci members. Factory fitments include the Pontiac Catalina, Pontiac Grand Prix, Pontiac GTO.
| Displacement | 421 ci / 6.9 L |
|---|---|
| Bore | 4.09375" |
| 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 | 370 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 |
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