the cat acts as a collector for emissions and also collects a great deal of the heat that is diffused from the header. backpressure is what keeps the exhaust flowing smooth and keeps gas milage at an optimum. When u dont have back pressure you get back-firing and worse gas milage. the decrease in back pressure results in the hot air staying up by the headers for a longer time...not like seconds but more like nanoseconds basically enough to cause the headers to get hotter faster.