It's funny how times change opinions... I bent custom pipe for a couple years back in the early 90s and as lucky enough to have some really sharp guys to talk to. Back then any dual was better than single, and the hot setup was 2.5" custom bent. I have seen as large as 4" mandrel bent thru the factory valance on a cuda. I couldn't do it, but my buddy has. As far as mandrel vs compression bending... There are two main types of compression benders. Those that use a notched die, and those that dont. The notched die is the one that leaves the little kinks every 1" or so along the ID of a bend. It also tends to stretch the metal thinner. Walker exh used to be that way, and had a tendency to rot quickly because of it. The other one does reduce the diameter very slightly because it is still compressing the pipe. You will find very little gain by going from the std compression bent pipe to a DOM tubing. That's Drawn Over Mandrel, or "mandrel bent". There is a small bit of power, but it's going to be less than 2% on a 400hp application. That's 8hp on a 400hp engine. There is a significant amount to be gained by going to larger diameters AS NEEDED. The factory used pipe size changes to control noise too. A smaller pipe will be louder and higher pitch than a larger. A decrease in diameter will dampen the low frequency waves similar to a deflector based welded case muffler, but cause back pressure. The Cuda pipe go from 2 1/4 to 2", and then 1 7/8 into the factory tips, along with using the resonators just to keep things very quiet. Increasing pipe size will help limit higher frequencies while being less restrictive. H pipes and X pipes will lower the noise level and fre up power, providing they are placed exactly where they need to be. The further forward or rearward they are of that perfect spot, the less effective they get. What's hysterical is I know very few people who know how to place them, and endeavor to get them in the right spot, as opposed to sticking them where they fit and look the best. Every car is different. Bigger pipe also gets heavy. Beyond 2.5", the thickness of the pipe goes up... my 4" pipe that exited before the tires (think AAR on steroids) with the flowmasters weighed close to 40lbs per side. And that's a total length of about 4.5 feet from header to rocker. If it were me, on anything less than 500hp, you will be more than fine with 2.5". Those that say theres a huge difference IMO need to prove it. The sound is much cooler, but back to back, the cars do not typically go faster as a result of that extra 1/2". The engines simply dont need the extra flow, aspecially as the exhaust cools as it goes thru the pipes. The factory tips can be bought in any inlet size now. Or, a savvy custom guy can replace the 1 7/8" section with any size you want. I prefer a good custom bent system to TTi. I've had too many that hit stuff on E, and B bodys even with careful installation. My own 4406bbl car at that time had no trouble with 2.5". I like the sound of the rounds in factory locations with no resonators and modified tips. I also like flowmasters...lol. Loud ones.... but that's me.
