Originally Posted by Nate@IND
When our friends at Fall Line ran an OEM exhaust system, the CV boots did not melt, but after installing their competition exhaust system with no heat shields, they experienced quite a bit of trouble with melted CV boots. Since then, Fall Line has added heat shields to their competition exhaust, and it is this experience that leads me to believe that heat shields are necessary to protect the rubber CV boots.
Thanks for the info. So, it sounds like this happened on an E92 M3 racecar pushed hard without a shield in that area of the exhaust. There are sevaral things going on at the same time: heat is being transferred to the joint via conduction from the diff; heat is being transferred to the joint via convection from the exhaust pipe (can be significant when the car really slows down and/or comes to a stop after running hard and there is no forced convection but a really hot pipe briefly, or in any other situation when flow is interrupted); heat is being transferred to the joint via radiation (although the magnititude of this effect appears small, when combined with the other heat transfer modes in extreme driving conditions, it seems that it indeed does make a difference in overheating a rubber part). Placing a shield obviously does not do anything to reduce conduction from the diff, but blocks radiation and reduces convection from that section of the pipe.