JT8D hush kits consist of a mixer, inlet guide vane spacer case, No. 1 disk and hub, accoustic treatment of the fan ducts, and tubes.
The mixer is installed just behind the turbine exhaust case. The mixer breaks up the core engine flow and mixes it with the fan flow. The high velocity core engine exhaust flow is the primary component of jet noise.
The inlet guide vane (IGV) spacer case moves the IGV case about 4 inches forward. This moves the wakes of the air going through the IGVs forward of the fan blades. The fan blades cutting through those wakes is another major component of jet noise.
The new No. 1 disk and hub has a longer nose piece for the No. 1 bearing journal to account for the IGV spacer case and the repositioned No. 1 bearing support.
The fan ducts have panels on the interior surfaces that have holes drilled in them. The holes are intended to absorb specfic sound frequencies of the compressor and combustor noise.
The tubes are the oil and antiice tubes that had to be replaced because of the respaced inlet case.
Because the 727, 737, and DC9 airplanes were certificated engines that generated specific levels of thrust, the hush kits could not and did not effect engine performance.