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Subaru EK31 specs: 0.356L (21.7 ci) 2-cylinder, 61.5 x 60 mm (2.4213 x 2.3622 in) bore and stroke, 6.5:1 compression. Rated 16 hp. Rod ratio and piston speed.
The Subaru EK31 is close to square: 2.4213" bore against 2.3622" stroke, within 2.5%, which splits the difference between a high-revving layout and a torque layout. The 2.3622" stroke does not reach 4,000 ft/min of mean piston speed until about 10,150 RPM, so piston speed is not what limits this engine's revs — valvetrain and breathing are. Each of the 2 cylinders displaces 10.9 ci (178 cc).
6.5:1 on aluminum heads is low by modern standards and generally happy on regular pump fuel.
Factory fitments include the Subaru 360, Subaru Sambar.
| Displacement | 21.7 ci / 0.356 L |
|---|---|
| Bore | 2.4213" |
| Stroke | 2.3622" |
| Cylinders | 2 |
| Compression ratio | 6.5:1 |
| Bore/stroke ratio | 1.025 |
| Block material | cast iron |
|---|---|
| Head material | aluminum |
| Valvetrain | 2-stroke |
| Fuel type | gas |
| Advertised HP | 16 hp |
| Bore/stroke character | 1.025 — square — bore and stroke are within 5%, a balanced compromise |
|---|---|
| Mean piston speed at 6,000 RPM | 2,362 ft/min — conservative for a street engine |
| Displacement per cylinder | 10.8 ci |
| Static compression | 6.5:1 — low by modern standards, generally happy on regular pump fuel |
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