It does matter. I don't know about the kit you bought, but the Flowmaster kits are made to fit either headers or stock x-mans (w/ down-pipes). If the pipes/headers you attached it to are too long, the H-pipe will be too far back. You are supposed to do it from the back to the front, and you have to adapt the front pipes to your headers or make/supply your own down-pipes (for x-mans).
John Nasta -----Original Message----- Yes, I installed the headpipes first but I don't think the install order matters in this case - with the pipes installed, the mufflers are still going to end up in the same location. I just wanted to see if this pipe kit had the mufflers in the general location as other kits (OE or Flowmaster).
