Billet Bypass Valve: Billet Bypass Valve: The original diaphragm bypass valve can fail under high boost. The Turbo XS part will never fail fully adjustable to hold higher boost levels. Th makes your peak boost horsepower useable for longer periods. Application- Beetle / Golf & Jetta 1.8t (all) Audi A4 (S4 requires 2- parts) & Passat 1.8t ( all)