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Pontiac 421ci specs: 421 ci (6.9L) 8-cylinder, 4.093 x 4.00 in (104 x 101.6 mm) bore and stroke, 10.5:1 compression. Rated 320-338 hp, 1964–1966.
The Pontiac 421ci is close to square: 4.093" 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. A rod ratio of 1.656 is in the range most production engines fall in. Each of the 8 cylinders displaces 52.6 ci (862 cc).
10.5:1 is in the range that usually runs on pump premium.
It is one of 3 engines in the Pontiac 421ci family, all of them 421 ci — the differences between them are in compression, heads and rating, not displacement. 3 factory variants share these dimensions, rated from 320 to 338 hp across 1964–1966.
| Displacement | 421 ci / 6.9 L |
|---|---|
| Bore | 4.093" |
| Stroke | 4" |
| Cylinders | 8 |
| Compression ratio | 10.5:1 |
| Rod length | 6.625" |
| Rod ratio | 1.656 |
| Bore/stroke ratio | 1.023 |
| Piston deck clearance | 0" |
| Induction | 1-4 |
|---|---|
| Advertised HP | 320–338 hp |
| Years | 1964–1966 |
| 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 |
| Rod ratio | 1.656 — moderate — the range most production V8s fall in |
| Displacement per cylinder | 52.6 ci |
| Static compression | 10.5:1 — in the range that usually runs on pump premium |
| Intake valve diameter | 1.923" |
|---|---|
| Exhaust valve diameter | 1.663" |
| Chamber volume | 71.3 cc |
| Rocker ratio | 1.5:1 |
| Lifter type | hydraulic |
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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 |
|---|---|---|
| 1964 | 320 hp | — |
| 1965 | 338 hp | — |
| 1966 | 338 hp | — |