Mopar
Mopar Mudder,
All of our system is 4 inch mandrel bent aluminized or stainless steel depending on the system you choose to purchase. I'm going to step out on a limb here and maybe make some folks upset but here goes. We have dynoed 4 and 5 inch systems and we have seen a loss of performance at the low end of the rpm scale with the 5 inch systems. The appearance is very appealing to some folks but the loss of performance is a negative to us. That's why we have stayed away for the 5 inch diameter tubing. Now the opposite is true as well, if you're running the hottest injectors, propane or nitrous, a performance box etc etc, then a 5 inch will help with the performance at the top end of the scale. Look at some of the 800-1000 inch work horse diesels in day to day commercial use they have a 4-5 inch tubing size, and even the guys that dress them out stay under 6 inches. Now do the math in CFM flow between a 4 inch and a 5 or 6 inch tube based on the CFM output of the engine at wide open throttle, it's just not necessary. Too big a pipe will hurt your performance, it real critical on Ford's, we even offer a 3. 5 inch down pipe as well as a 4 inch for them. The 3. 5 is for stock or real lightly modified trucks while the 4 inch works best for strongly modified trucks.
Our system is 4 inch from the turbo back and is totally mandrel bent, with our Spiral Flow Muffler that helps to evacuate exhaust gasses as they cool down from leaving the turbo. We weld on stainless steel hangers even on our aluminized systems that snap into the factory eurethane body hangers and we include a 5 inch, rolled edge, polished stainless tip with each system. Getting the rest of the factory system replaced even though the tail pipe is 4 inches works together and drops EGT's and ups HP and torque etc.
Hope this answers the question, if not call us we'll be glad to help you.
Mark Craig
Diesel Performance Parts Inc.