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Pontiac 428 H.O. specs: 428 ci (7.0L) 8-cylinder, 4.125 x 4.00 in (104.8 x 101.6 mm) bore and stroke. Rated 390 hp, 1968–1969. Used in the Pontiac Grand Prix.
The Pontiac 428 H.O. is close to square: 4.125" bore against 4" stroke, within 3.1%, 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. A rod ratio of 1.656 is in the range most production engines fall in. Each of the 8 cylinders displaces 53.5 ci (877 cc).
It is one of 4 engines in the Pontiac 428ci family, all of them 428 ci — the differences between them are in compression, heads and rating, not displacement. 2 factory variants share these dimensions, rated at 390 hp across 1968–1969.
| Displacement | 428 ci / 7 L |
|---|---|
| Bore | 4.125" |
| Stroke | 4" |
| Cylinders | 8 |
| Configuration | V8 |
| Rod length | 6.625" |
| Rod ratio | 1.656 |
| Bore/stroke ratio | 1.031 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 390 hp |
| Years | 1968–1969 |
| Bore/stroke character | 1.031 — 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 |
| Rod ratio | 1.656 — moderate — the range most production V8s fall in |
| Displacement per cylinder | 53.5 ci |
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These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| 1968 | 390 hp | 428 H.O. |
| 1969 | 390 hp | 428 H.O. |