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American Motors X specs: 390 ci (6.4L) 8-cylinder, 4.165 x 3.574 in (105.8 x 90.8 mm) bore and stroke. Rated 315 hp, 1969. Used in the AMC AMX.
The American Motors 390ci (X) is oversquare — the 4.165" bore exceeds the 3.574" stroke by 16.5% — a layout that favors airflow and RPM over leverage at the crank. The 3.574" stroke puts mean piston speed at 4,000 ft/min around 6,700 RPM, which is where a production bottom end is generally considered at its limit. A rod ratio of 1.62 is in the range most production engines fall in. Each of the 8 cylinders displaces 48.8 ci (799 cc).
It is one of 4 engines in the American Motors 390ci family, all of them 390 ci — the differences between them are in compression, heads and rating, not displacement.
| Displacement | 390 ci / 6.4 L |
|---|---|
| Bore | 4.165" |
| Stroke | 3.574" |
| Cylinders | 8 |
| Configuration | V8 |
| Rod length | 5.79" |
| Rod ratio | 1.62 |
| Bore/stroke ratio | 1.165 |
| Fuel type | gas |
|---|---|
| Advertised HP | 315 hp |
| Years | 1969 |
| Bore/stroke character | 1.165 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,574 ft/min — in the range typical of performance street engines |
| Rod ratio | 1.62 — moderate — the range most production V8s fall in |
| Displacement per cylinder | 48.8 ci |
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