Hello everyone, Justin here, sharing my journey of transforming my 2001 Audi S4, also known as the B5 S4 within the Audi community. As a car enthusiast and mechanical engineer, I’ve owned this Audi S4 for eleven years, constantly evolving its performance. The most ambitious project to date has been the compound turbo system I designed and installed. For those unfamiliar with the B5 S4 2001 model, it’s an all-wheel-drive vehicle known for its potential, despite weighing around 3600 lbs.
My aim was to surpass my previous 700whp setup without sacrificing responsiveness, which led me to explore a compound turbo configuration. I selected a PTE PT5858 BB with a .64AR hotside as the high-pressure turbo and a BW S372SXE as the low-pressure unit.
Understanding pressure ratios is crucial in compound turbo setups. With calculations, running the small turbo at 16psi and the larger one at 6psi yields a total manifold boost of approximately 28.7psi. Cranking both turbos to 16psi escalates this to around 49.5psi. This compounding effect is key to achieving both high power and quick spool.
The project demanded significant fabrication skills, which motivated me to learn TIG welding. Using a Chinese AHP201 TIG welder and 304 stainless steel tubing for the hot side, along with standard eBay intercooler kits for the cold side, I began piecing together the system.
To create ample space for the turbos and associated plumbing, I removed the secondary firewall, significantly opening up the engine bay.
Working with a spare engine and transmission assembly allowed me to develop the new turbo setup off the car, ensuring precise fitment.
Positioning the turbos optimally was critical to minimize hot side dead volume and maximize efficiency.
Fabricating the hot side piping to fit perfectly, avoiding any rubbing or interference, was a challenging yet rewarding part of the build.
Ensuring weld integrity, I back-purged all welds to prevent oxidation and ensure maximum strength and longevity.
The initial test fit revealed a clearance issue with the steering rack.
To resolve this, I modified the piping, incorporating a flex joint to provide the necessary clearance.
This project showcases the detailed work involved in a DIY compound turbo upgrade for a 2001 Audi S4, balancing performance goals with fabrication challenges.
