It has been a while but if I remember right I used 1 1/2" tubing inside the 6" piece. If you go to http://www.spintechmufflers.com/oval-round-tubing/cat_97.html you will find oval exhaust tubing.......it is a bit pricey and my friend did not want to spend the money on it. We only needed two 6" long pieces. We used a normal GM air filter housing so we needed the oval tube to be no taller than it. Does that make any more sense of it?? If not, let me know, I will have him email me some pictures of it and I will post them.......