Is the test pipe suppose to be the same size as the cat back or is it suppose to be the same size as the collector? for example dc 4-1 header 2.25 collector and cat back 2.5. Do you need the test pipe that is 2.25 or a 2.5?

 

you could get a test pipe that is 2.25 on one side and 2.5 on the other. kteller.com has them