I purchased a 4" turndown to put on my 2000 Cummins from Geno's and I went to a local muffler shop to have about 4" cut off of my stock tailpipe so the turndown would not stick out to far and the guy at the muffler shop said that you don't want to put a turndown on a Cummins because it won't allow the exhaust system to receive air and will make the truck not run right. Has antbody ever heard of this or is the guy full of Elcrapo. 
