In Part 5 of the StealthBuilt Audi A6 Widebody Build, Sam Kimmel takes us to the rear of the car, where a full custom diffuser and widebody-friendly exhaust system are being hand-built from scratch. If you’re into fabricating body panels, welding your own exhaust, or making mock-ups using cardboard and particleboard, this chapter delivers serious insight into grassroots fabrication.

Laying Out the Rear Bumper and Diffuser

Sam starts by trimming the bottom of the OEM bumper using an air saw, setting the stage for a fully custom rear diffuser. He uses:

• Cardboard for the initial mock-up
• A yardstick + permanent marker combo for marking curves
• A jigsaw and sander to shape and refine the final panel

Once the shape feels right, it’s transferred to particleboard to form a rigid base structure for the diffuser build. Attention to details like backup sensor clearance ensures functionality isn’t sacrificed for aesthetic fiberglass work. Sam saves time and ensures symmetry across the car by pre-cutting both sides at once.

Testing Designs — The Power of Prototyping

Sam demonstrates an important builder’s mindset: test, revise, repeat. He experiments with additional flare blades and lower bumper elements made from cardboard, reminding us:

“Sometimes the ideas you think are dumb turn out cool — and the cool ideas flop. That’s why prototyping is essential.”

This flexible approach saves time, money, and frustration as the project evolves.

Using a Plastic Welder to Rebuild Cut Sections

To support the new diffuser structure, Sam reattaches parts of the cut bumper using a plastic stapler/welder with Meldon clips. This provides:

• A sturdy structure for mounting
• Flexibility in shaping and reworking
• A professional method to fix ABS plastic

After installing staples, he clips off the sharp ends and smooths the surface with a grinder — a clean and fast way to restore structural strength to trimmed plastic.

Mocking Up the Exhaust: Symmetry, Spacing, and Flow

Next comes the rear exhaust layout. With the car’s wide stance, Sam fabricates symmetrical exhaust cutouts using templates and a mix of spare materials — including 4″ tubing from another project. He uses:

• Blue tape for visual layout
• A band saw for clean angled cuts
• A jig to hold exhaust tips in place while welding

He starts by TIG welding the mock-up pipes to dial in the perfect angle, then transitions to custom pie-cut tubing for tight radius bends and a precise fit.

TIG Welding the Exhaust with Pie Cuts

Using a TIG welder, Sam forms and fuses the custom piping, prioritizing:

• Clean welds
• Tight fitment
• Structural support with minimal vibration

To prevent damage and noise, he adds rubber-isolated exhaust hangers, ensuring the system stays secure and quiet under load.

Key Fabrication Highlights in This Video

• DIY rear diffuser baseboard layout using cardboard and particleboard
• Plastic welding techniques for reattaching bumper pieces
• Exhaust cutout shaping for symmetrical rear flow
• Pie-cut tubing TIG welding for precise exhaust angles
• Rubber-mounted hangers for anti-vibration exhaust support

Final Thoughts and What’s Next

This episode wraps up the early stages of the rear diffuser and exhaust — two major components of any functional widebody project. With the rear section now roughed in, the next phase will include finishing the diffuser shape, reinforcing the bumper, and moving toward bodywork prep.

🔧 Questions about the process? Leave them in the YouTube comments. Sam often responds and shares follow-up tips in later videos.

📺 Watch the full video on YouTube
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