I know I'm probably in the minority here, but I had the factory exhaust coming off the headers and located my Spartan 2 in the factory crossover tube. I always saw wild swings in O2 reading during cruising/idle, but seemed to be more stable on accel. Apparently that's just a poor location for the O2 sensor as now that it's been moved to the turbo downpipe an in the actual direct exhaust flow my readings are super stable. Just some info for potential installs moving forward.