AUDI A8 S8 4E D3 Q7 4M Big brake brake calipers 400mm

400mm Big Brake Kit (BBK) utilizing components from the Audi A8/S8 (D3/4E) and the Q7 (4M) represents the absolute upper limit of braking power for the VAG platform. While both use massive rotors, they represent two different generations of braking technology.

1. The Audi S3/S8 (D3/4E) Setup

This setup was famously one of the first “super-sedan” brakes to exceed 380mm.

• Caliper: Brembo 6-piston (often referred to as “19Z” or similar large-casting Brembos).

• Rotor: $400\text{mm} \times 38\text{mm}$ vented discs.

• Fitment: These calipers are massive and heavy. They were designed for the heavy D3 chassis. On smaller platforms (like a Golf or A3), they require custom radial-to-axial adapters and significant wheel clearance.

2. The Modern Audi Q7 (4M) / Akebono Setup

Most modern 400mm conversions now use the Akebono 6-piston calipers found on the current Q7 (4M), SQ7, and RS6/RS7 (C8).

• Caliper: Akebono 6-piston aluminum monoblock.

• Rotor: $400\text{mm} \times 38\text{mm}$ (Standard) or the 420mm Carbon Ceramic option.

• Benefit: These calipers are significantly lighter than the older D3 Brembo units and offer better piston retraction, reducing drag and heat.

Critical Technical Requirements

Wheel Clearance

• Minimum Size: 20-inch wheels are almost always required to clear the 400mm rotor diameter and the massive caliper body.

• Exception: Some very specific 19-inch “flow-formed” motorsport wheels (like Apex or certain BBS models) can clear a 400mm rotor, but the clearance to the inner barrel is often less than $3\text{mm}$, which is risky if road debris gets caught.

Hydraulic Displacement

The total piston area of a 6-piston Akebono or D3 Brembo is much larger than a standard GTI or S3 single-piston setup.

• The “Woody” Pedal: If you run these on a standard master cylinder ($22.2\text{mm}$), the pedal will travel very far before the brakes bite.

• Recommendation: You almost certainly need to upgrade to an RS3 or TTRS master cylinder ($23.8\text{mm}$ or $25.4\text{mm}$) to displace enough fluid to move all six pistons effectively.

Comparison Table: 400mm Setup