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Audi BNS specs: 4.2L (254 ci) 8-cylinder, 84.5 x 92.8 mm (3.3268 x 3.6535 in) bore and stroke, 12.5:1 compression. Built 2008. Used in the Audi A4 / S4. DOHC.
The Audi EA824 (BNS) is undersquare — the 3.6535" stroke exceeds the 3.3268" bore by 8.9% — a layout that trades peak RPM for leverage and torque low in the range. The 3.6535" stroke puts mean piston speed at 4,000 ft/min around 6,550 RPM, which is where a production bottom end is generally considered at its limit. Each of the 8 cylinders displaces 31.8 ci (520 cc).
12.5:1 on aluminum heads is high for a production gasoline engine; running a ratio like that on pump fuel depends on chamber design and fuel delivery rather than the number alone.
It is one of 11 engines in the EA824 family, sitting between the 244.1 ci and 317.4 ci members.
| Displacement | 254 ci / 4.2 L |
|---|---|
| Bore | 3.3268" |
| Stroke | 3.6535" |
| Cylinders | 8 |
| Configuration | V8 |
| Compression ratio | 12.5:1 * |
| Bore/stroke ratio | 0.911 |
* Compression ratio from a single source; not yet cross-verified.
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Years | 2008 |
| Bore/stroke character | 0.911 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 3,654 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 31.8 ci |
| Static compression | 12.5:1 — high enough that pump premium or better is normally required |
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1 factory vehicle, 2008. Each year opens that model's engine table.