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Honda R18A specs: 1.8L (109.8 ci) 4-cylinder, 81 x 87.3 mm (3.1890 x 3.4370 in) bore and stroke. Used in the Honda Civic. SOHC. Rod ratio and piston speed.
The Honda R18A is undersquare — the 3.437" stroke exceeds the 3.189" bore by 7.2% — a layout that trades peak RPM for leverage and torque low in the range. The 3.437" stroke puts mean piston speed at 4,000 ft/min around 7,000 RPM, which is where a production bottom end is generally considered at its limit. Each of the 4 cylinders displaces 27.5 ci (450 cc).
It is one of 4 engines in the R-series family, all of them 109.8 ci — the differences between them are in compression, heads and rating, not displacement.
| Displacement | 109.8 ci / 1.8 L |
|---|---|
| Bore | 3.189" |
| Stroke | 3.437" |
| Cylinders | 4 |
| Compression ratio | 10.5–10.6:1 (published figures vary) |
| Bore/stroke ratio | 0.928 |
| Valvetrain | SOHC |
|---|---|
| Fuel type | gas |
| Bore/stroke character | 0.928 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 3,437 ft/min — conservative for a street engine |
| Displacement per cylinder | 27.4 ci |
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1 factory vehicle, 2006–2015. Each year opens that model's engine table.