Double wall exhaust pipe?

[ericsmith32]

I cut the exhaust off of the parts car today so I could experiment with the cat. The exhaust pipe between the cat and resonator and resonator and muffler had another pipe within it. I've never seen exhaust pipe like that before, and hard to cut also. The funny thing was the cat was already gutted.. makes me wonder about the engine I pulled from it.

[lilred]

my 66 f250 had double wall pipe on it. Get this, at one of the bends the inner wall callapsed closing the pipe up 85% of the way... it ran like crap and would backfire like crap through the carb when you floored it.