by Stephen Rivers
- B5 Audi A4 features a VR6 swap and an aircraft-derived turbine boost system.
- Kelby Krahn’s latest test exposed another drivetrain weak point under massive power.
- The project continues proving the boost system isn’t the part struggling to survive.
If there were ever a project that perfectly embodied the phrase “because race car,” it’s Kelby Krahn’s Jet-boosted VR6 Audi. The B5-generation Audi A4 was already far removed from its factory roots thanks to a very clean VR6 engine swap, but Krahn wasn’t content to stop there. Instead, he added one of the most unconventional parts imaginable: an aircraft-derived gas turbine system designed to feed boost into the engine. The only issue is that it just keeps breaking things.
First, let’s outline exactly how this works. The turbine is not a turbocharger. Turbos use exhaust gas to spin up a compressor that feeds compressed air into the engine. Add some extra fuel, and you’ll get extra power. Krahn’s turbine is a genuine auxiliary power unit (APU) from an airplane, and unlike most “jet cars” you’ve seen, this one isn’t creating thrust to power the car.
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Instead, the turbine simply builds boost that Krahn then feeds into the VR6 itself. Why do this? It scratches an itch in his brain, Krahn says. Well, that and, as he tells Carscoops, to “eliminate turbo lag altogether.” Since the turbine doesn’t run on exhaust gas, it can create boost regardless of whatever the engine might be doing.
The Opposite of Turbo Lag
Krahn could technically just add an output shaft and run the car with the turbine, but as he points out, “the turbine weighs 320lbs and produces 60hp, but provides 1500hp worth of air.” In other words, this B5 A4 has no such thing as turbo lag in any sense. In fact, it has the opposite issue. Even with the valve open all the way, the throttle is too small, and the charge pipe maintains 12 psi.
In his latest update, Krahn once again found himself dealing with the consequences of having enormous power. While the turbine-assisted boost system continues to do its job, the rest of the Audi has been forced to endure levels of stress that its original engineers almost certainly never imagined. Most recently, he snapped an axle. Before that, it was a subframe bolt that ripped itself out of the frame, and before that, it was second gear that decided it was done.
The Real Challenge Isn’t the Jet
Here’s maybe the wildest bit of it all. Even without the turbine setup, this Audi would be noteworthy. Swapping a VR6 into a B5 A4 is a substantial undertaking on its own, requiring significant fabrication and engineering. Most builders would call it a day after getting the engine installed and running properly. Of course, the biggest challenge may simply be keeping the rest of the car alive.
Projects like this often fail because the experimental technology doesn’t function as intended. In Krahn’s case, the opposite appears to be happening. The jet-inspired boost system keeps doing its job, while the Audi continues discovering new mechanical weak points one component at a time.