As cities grow and racetracks start to be closer and closer to the cities, mufflers will become more and more the norm and not the exception. Mufflers present a whole new set of issues. Just reducing noise does not always equate to a loss of power. When constructed as a system, a well-designed muffler can reduce sound without making your engine run like it's breathing through a restrictor. The real issue is that most racing headers were not designed to support the additional weight of a muffler, which can cause structural failure of the header even if the muffler has additional supports and brackets mounting it to the chassis.