just make the chassis heavier in the main.cfg
Added 2 minutes later:
And if you want to add a muffler, just use this slot. Play with coordinates to move it around or simply move it ingame if u have the movable parts script.
slot -0.07 0.046 1.389 0.000 0.000 0.000 789 ; L muffler
attach 0x00040047 1