Abstract: An engine exhaust accessory unit comprises a tubular housing adapted to be fitted over the end of an exhaust header. The outer end of the tubular housing is partially obstructed by a pivotable rigid perforated plate. The plate is pivotally mounted about a transverse axis offset below the central longitudinal axis of the housing. The larger upper section of the plate causes it to swing outward in response to passage of exhaust gases through the housing. The perforated plate automatically modifies engine noises and varies exhaust back pressure as a function of its angular position about the transverse axis. A yieldable spring biases the plate to a perpendicular position across the housing, and resists pivotal movement of the butterfly plate in response to passage of exhaust gases. Stops in the path of movement of the plate limit its movement between a perpendicular position across the housing and a position parallel to the longitudinal central housing axis.