Maybe it's no mystery to others but I've been wondering about it for a few years. I watch a lot of rally video and often spot this glowing inside the exhaust tips. Like so. I used to see it all the time in both of the dual tips on the MINIs when they were running in WRC but can't find a pic just now. After Craig Scarborough posted today that he wondered about all the honeycombs he'd seen in exhausts at a RallyCross event ... ... I decided to do a little hunting and it looks like it's because that's where they usually put their catalytic converters. According to this article, pretty much all racers have to run cats in Europe, but in the states it's mostly just rally cars because they use public roads to transit between stages, etc, while others get to run straight pipes. I've been wondering about this for years and would never have guessed converters at the end of the exhaust pipe.