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Plymouth 25 K I4 specs: 153 ci (2.5L) 4-cylinder, 3.445 x 4.095 in (87.5 x 104 mm) bore and stroke, 9:1 compression. Rated 100 hp. Rod ratio and piston speed.
The Plymouth 25 K I4 is undersquare — the 4.0945" stroke exceeds the 3.4449" bore by 15.9% — a layout that trades peak RPM for leverage and torque low in the range. The 4.0945" stroke puts mean piston speed at 4,000 ft/min around 5,850 RPM, which is where a production bottom end is generally considered at its limit. Each of the 4 cylinders displaces 38.3 ci (627 cc).
9:1 on aluminum heads is low by modern standards and generally happy on regular pump fuel.
It is one of 4 engines in the K-Engine family, the largest at 153 ci. Factory fitments include the Acclaim, Sundance, Voyager and 3 others.
| Displacement | 153 ci / 2.5 L |
|---|---|
| Bore | 3.4449" |
| Stroke | 4.0945" |
| Cylinders | 4 |
| Compression ratio | 9:1 |
| Bore/stroke ratio | 0.841 |
| Block material | cast iron |
|---|---|
| Head material | aluminum |
| Valvetrain | SOHC |
| Fuel type | gas |
| Advertised HP | 100 hp |
| Bore/stroke character | 0.841 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 4,095 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 38.3 ci |
| Static compression | 9:1 — low by modern standards, generally happy on regular pump fuel |
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