If my memory serves me right, before 0.18 the backfire had some more variation to it, but as of 0.18 it's pretty much the same sound for every car, which is somewhat confusing as to why they did it
Can't promise anything like that, access to afterfire sounds is extremely limited, not much we can do unfortunately.
What surprised me is that even the devs are having a very hard time accessing the sound files. We should help each other to find(or make) a libre sound pack.