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Audi BFM specs: 4.2L (256.3 ci) 8-cylinder, 84.5 x 93 mm (3.3268 x 3.6614 in) bore and stroke, 11:1 compression. Rated 335 hp. Rod ratio and piston speed.
The Audi EA824 (BFM) 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).
11:1 on aluminum heads is high enough that pump premium or better is normally required.
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 A8 (D3).
| Displacement | 256.3 ci / 4.2 L |
|---|---|
| Bore | 3.3268" |
| Stroke | 3.6614" * |
| Cylinders | 8 |
| Compression ratio | 11:1 |
| Bore/stroke ratio | 0.909 |
* Conflicting reports on the stroke: 92.8 mm (4,163 cc) or 93.0 mm (4,172 cc).
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Advertised HP | 335 hp |
| Year | Version | HP @ rpm | Torque @ rpm | Comp. |
|---|---|---|---|---|
| 2003 | BFM | 335 @ 6500 | 317 lb-ft @ 3500 | 11:1 |
| 2004 | BFM | 335 @ 6500 | 317 lb-ft @ 3500 | 11:1 |
| 2005 | BFM | 335 @ 6500 | 317 lb-ft @ 3500 | 11:1 |
| 2006 | BFM net | 335 @ 6500 | 317 lb-ft @ 3500 | 11:1 |
Factory specification sheet.
| 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 | 11:1 — high enough that pump premium or better is normally required |
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