This feature is supported as of BeamNG.tech and .drive version 0.6.0 to latest.
Exhaust modifiers are additional parameters used on exhaust nodes to modify exhaust noise and afterfire.
Their modifier is applied if they are part of an exhaust tree that is connected to the engine, allowing you to have an exhaust that can be removed in crashes, causing the car to become louder.
You can have multiple nodes with exhaust modifiers on the same tree, with their coefficients getting multiplied. This means that if you have, for example, 2 nodes with an afterfire visual coefficient of 0.5 on the same exhaust will result in a total afterfire visual coefficient of 0.25.
["exh6",0.82,0.60,0.2, {"selfCollision":false,"afterFireAudioCoef":0.0,"afterFireVisualCoef":0, "exhaustAudioMufflingCoef": 0.2, "exhaustAudioGainChange":-6}],