Perhaps you mean "True Bend" exhaust?
Usually stock exhaust systems have poorly bent pipe, they have all kinds of kinks and areas where the pipe diameter is reduced to accomodate production time and costs. Elimnate the bends and kinks in most stock systems and your vehicle will have less back pressure working on the engine and should perform better.
When most shops bend pipe they have to bend it around a roller which causes pipe to pinch in a little at that bend reducing the diameter of the pipe which will effect performance.
That's why there are so many aftermarket exhaust manufacturers, that offer "true bend" exhaust systems. True Bend pipe will have no pinch points in the exhaust system, "the diameter of the pipe will be the same all the way through the bend."
This is the preferred way to bend pipe so there are no kinks or reduction in pipe diameter at a bend in the exhaust pipe. You will also here the term "mandrel bent pipe" for exhaust systems.
With a true mandrel bent exhaust system it's not unheard of to hear a increase of 15-20 hp.
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