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Audi 42 V8 FSI 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. Rated 420 hp. DOHC.
The Audi 42 V8 FSI 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 9 engines in the EA824 family, sitting between the 244.1 ci and 317.4 ci members. Factory fitments include the RS4 B7, R8 V8, Q7 and 12 others.
| Displacement | 254 ci / 4.2 L |
|---|---|
| Bore | 3.3268" |
| Stroke | 3.6535" |
| Cylinders | 8 |
| Compression ratio | 12.5:1 |
| Bore/stroke ratio | 0.911 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Advertised HP | 420 hp |
| 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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