i had a leak on mine, yes the o/r x with the factory manifolds will make a sound up front that sounds like a leak but put your hand around the flange and you will feel air if its leaking, also there will be a carbon track on the manifold where the exhaust is leaking out. if you do have a leak the problem is the flanges on the aftermarket x/pipes and the down tube are bigger than factory and dont make a perfect seal on the ball on the manifold. what i did was cut the flange and like 1 inch of the pipe off a set of crown vic catalytic converters to make a reducer and weld them onto the x pipe. no leaks now and the vic flanges are the same as the mustang. if you wanted to canibalize your factory h-pipe you could cut your flanges off and do the same thing but i didnt want to ruin mine. pipes and other companies make reducers as well if your not up to fab work (like most of the deep pocket people on this forum)
