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Audi 42 V8 BXA specs: 4.2L (256.3 ci) 8-cylinder, 84.5 x 93 mm (3.3268 x 3.6614 in) bore and stroke, 12.5:1 compression. Rated 420 hp. DOHC.
The Audi 42 V8 BXA is undersquare — the 3.6614" stroke exceeds the 3.3268" bore by 9.1% — a layout that trades peak RPM for leverage and torque low in the range. The 3.6614" 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 32.0 ci (525 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. Factory fitments include the Audi R8 (Type 42).
| Displacement | 256.3 ci / 4.2 L |
|---|---|
| Bore | 3.3268" |
| Stroke | 3.6614" |
| Cylinders | 8 |
| Compression ratio | 12.5:1 |
| Bore/stroke ratio | 0.909 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Advertised HP | 420 hp |
| Bore/stroke character | 0.909 — undersquare — stroke exceeds bore, which favours low-end torque |
|---|---|
| Mean piston speed at 6,000 RPM | 3,661 ft/min — in the range typical of performance street engines |
| Displacement per cylinder | 32.0 ci |
| Static compression | 12.5:1 — high enough that pump premium or better is normally required |
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