I'm doing some work on a '96 beretta with the 3.1L motor. I had to cut the down pipe today and it's coaxial!?!? Why? The only reason I can see is to prevent heat transfer. The inner pipe isn't welded to the flang or to the resonator. I can't get the inner pipe out of the downpipe because of the bend. Should I just take the car to an exhaust shop or buy some pipe and bend it and weld it to the collector? Anyone know the reason for this and any other cars that use it?
which pipe has the exhaust soot.. if it in the internal, you probably have it right. no doubt it was done to keep heat from the exhaust from hurting other parts of the car
It sounds like a double wall pipe commonly used by oems for motorcycles and atvs, used reduce heat and noise on those machines. I've never heard of that on a car though. 
festivas have double wall exhaust pipes too. I don't remember if it's the whole length or just between manifold and cat.
Maybe just manifold to cat...
The strange thing is that it's not welded to the flange or the resonator. It doesn't rattle around or anything but it's not welded. Very strange. So, I think I'm going to remove the inner pipe and weld the outer pipe back together. Maybe I should use header wrap? This is the downtube so there is plenty of room between it and the toe board and there is plenty of heat shielding.
GM did that kind of thing to reduce NVH, if I remember correctly.
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