A "test pipe" is just a hollow pipe that goes in place of the catalytic converter, ostensibly so that one could "test" the cat by seeing if the car ran better without it. Then people would just leave it on the car instead of the cat, which is one of the reasons why you hardly (if ever) see them anymore. The benefit would technically be a less restrictive exhaust. An "off-road" H-pipe accomplishes the same thing and lots of people sell them. Benefit of one vs a modern high-flow cat is subject to debate but they sure as hell SOUND better than cats.
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