The problem is the exhaust system, not possible holes as 68fb400 mentions. Just because they dump out the back doesn't mean that the ehxaust clears the chassis. There is a "backwash" turbulence behind the car, and it's a safe bet that the exit of the pipes are located in this area. (Most rear exit exhausts, imo, I have seen are defective. If the engine is running clean enough, you won't smell fumes.)
Effective systems are tucked tightly under the valance and downturned to ensure full exit of spent gas, and I would refuse any other designed system, meaning that most of the current systems, including the pos pypes, wouldn't meet my specifcations.
