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Audi EA824 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 420-450 hp. DOHC.
The Audi EA824 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 9 engines in the EA824 family, sitting between the 244.1 ci and 317.4 ci members. 3 factory variants share these dimensions, rated from 420 to 450 hp. Factory fitments include the Audi RS4 (B7), Audi R8 (Type 42), Audi RS5 (B8).
| 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 | 420–450 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 |
Each link opens the calculator with this engine's specs already filled in.
These records share identical bore, stroke, displacement and compression. They differ by model year and factory power rating.
| Year | Advertised HP | Code |
|---|---|---|
| — | 420 hp | BNS |
| — | 420 hp | BXA |
| — | 450 hp | CFSA |