I have a 71 duster with a 440 that I currently have hooker fender well headers on. I am going to be changing to TTI headers, have anyone done this in the past and what performance loss or gain did they get?

I'd doubt you'll see any gain. And they aren't going to be as friendly. Under chassis? I though my Hooker Super comps installed like a dream. They fit better than the 1 3/4" cheapo's I had.

Add collector extensions so you end up with about
16" to 18" total .. this will help the torque.. dont
expect any HP gain

If the fenderwell headers are the type with the really long (~40") primaries, they tend to show a definite tradeoff of less peak HP for increased peak torque compared with standard-length primaries of the same diameter. Whether that means the TTIs will perform better depends on the total combination.

I know that going from hookers to TTI I lost alittle ! IIRC it was .02 to .03 ! Went from TTI to pro-parts and we were .01 to .02 slow than the hookers !

Thanks for the input guys. The only main reason I am switching is so I can put a taller tire on the front to give me more rollout. I guess the next question is which Mr. P Body has touch on is how much collector including a muffler should I add to the header? I'm plan on installing Schoenfeld Headers mufflers, they come in 6, 8 and 12" length. I can add a collector and muffler, but what length should I do?