Exhaust Y-Pipe for GMC Sierra (1999-2006)High Flow Exhaust Y Pipe for 1999 2006 GMC Sierra (Gen III Vortec GMT800) Replaces the restrictive factory Y pipe to eliminate exhaust bottlenecks, reduce backpressure, and improve exhaust scavenging for better throttle response and increased horsepower. The Stock Sierra Exhaust Bottleneck The factory GMT800 exhaust system, particularly the OEM Y pipe, features crushed bends and narrow piping diameters designed primarily for mass production cost