Which super 8 rally do you have? Is it the one based on the inferno or the MP7.5?
If it's the inferno, check out the on-road version's exhaust. It has an expansion chamber behind the rear shock tower with dual exhaust tips exiting under the body in the rear.
Tower Hobbies still sells them for about $23.
http://www2.towerhobbies.com/cgi-bin...78&FVPROFIL=++