will a catalyic converter off a GSR work for a 2000 honda civic si?

 

if you are referring to the stock catalytic converter, then it depends on the length and diameter of the stock civic cat. 94-97 gsr cats are around 13" long and 00-01 gsr cats are around 17" long, all gsr cats have around 2" diameter piping. i assume the civic si have close to the same diameter piping, not sure about the length of the civic cat though, you should ask someone in the civic forum.