A resonator is similar to a muffler but its primary purpase is to reduce the noise hear in the vehical. However, it also restricts the exaust which robs power, not to mention it make the exaust note quieter. Not all cars have them but many of the newer cars do such as the vettes and the chargers. On the vette it is located about a 2 feet below the exaust manafolds and about a foot away from the cats.
