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Audi BKH, BFM, BFL 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 350 hp. DOHC.
The Audi BKH, BFM, BFL 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.
Factory fitments include the Audi A6 (C6), Audi A8 (D3), Audi Q7 (4L) and 1 other.
| Displacement | 256.3 ci / 4.2 L |
|---|---|
| Bore | 3.3268" |
| Stroke | 3.6614" |
| Cylinders | 8 |
| Compression ratio | 11:1 |
| Bore/stroke ratio | 0.909 |
| Block material | aluminum |
|---|---|
| Head material | aluminum |
| Valvetrain | DOHC |
| Fuel type | gas |
| Advertised HP | 350 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 | 11:1 — high enough that pump premium or better is normally required |
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