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Plymouth R specs: 426 ci (7.0L) 8-cylinder, 4.25 x 3.75 in (108 x 95.3 mm) bore and stroke, 10.25:1 compression. Rated 425 hp. Rod ratio and piston speed.
The Plymouth 426 Hemi (R) is oversquare — the 4.25" bore exceeds the 3.75" stroke by 13.3% — a layout that favors airflow and RPM over leverage at the crank. The 3.75" stroke puts mean piston speed at 4,000 ft/min around 6,400 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 53.3 ci (873 cc).
10.25:1 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 2 engines in the 426 Hemi family, all of them 426 ci — the differences between them are in compression, heads and rating, not displacement. Factory fitments include the Plymouth Hemi 'Cuda, Plymouth Road Runner Hemi, Plymouth GTX.
| Displacement | 426 ci / 7 L |
|---|---|
| Bore | 4.25" |
| Stroke | 3.75" |
| Cylinders | 8 |
| Compression ratio | 10.25:1 |
| Bore/stroke ratio | 1.133 |
| Block material | cast iron |
|---|---|
| Head material | cast iron |
| Valvetrain | OHV |
| Fuel type | gas |
| Advertised HP | 425 hp |
| Bore/stroke character | 1.133 — oversquare — bore exceeds stroke, which favours higher RPM and larger valves |
|---|---|
| Mean piston speed at 6,000 RPM | 3,750 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 53.3 ci |
| Static compression | 10.25:1 — in the range that usually runs on pump premium |
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