in some situations, it messes with the geometry of the exhaust system...putting strain on the joints. Sometimes this stress is great enough to cause leaks. I tried them with my APS 3.5" and 1) it looked terrible...2)it was way too close to the ground...3)It put strains on the mid-pipe hanger and the axle back joint that I was not comfortable with...on the other hand, I had to use the extended Kartboy hangers with an axleback TurboXS RFL-Ti....that thing would have melted my bumper it got so hot...

I just put on the Kartboy std length hangers with my APS and everything is real tight (~1/2" clearance)...and since I am running either my stock bypass valve or an HKS 100% recirc, I don't run so rich anymore...so I get less backfires and carbon deposits on my bumper...keeps the flaming down...heh heh.
I noticed though that my APS doesn't get nearly as hot as some exhaust systems...probably due to the bigger diameter piping...

BTW...I am assuming that those that mentioned black marks on their bumpers above (not burn marks) are running 100% Atmospheric BOV's...yeah, you will get those black, carbon residues...and the occasional "Fireball" if your exhaust is a straight-through design...

 

ya im venting and i know it makes for a rich mixture causing the black marks and backfires etc etc. i was just curious if the hangers would aid in not leaving as much of a mark as it does. like i said before it comes right off when i wash the car with a little elbow grease. no melting ive got a little over 3/4 inch gap between bumper and tip. so not worring about melting. if i get some extra cash to toss around i might get the CF one. but for now ill just wash it