I have a 2000 gt auto with vortech want to install long tube headers but I do not know what size to use 1 5/8 x 2 1/2 or 1 3/4 x 3 any help would be grate
i'd go 1 5/8 x 2 1/2. you don't want a 3" exhaust all the way back, so i don't see the need for a 3" collector. it would probably just cause turbulance.
1 5/8 is more than enough for 95% of the 4.6L cars out there, you would only go up to 1 3/4 if you had a wild forced induction setup or something like that also keep the 2 1/2" exhaust, the 3" is too big and will cause a loss in tq, which is something we dont need to happen.