I have built my own headers. And did research in this matter some time ago. In reguards to equal lenght and tube sizeing,HP,torque.And collector size and lenght.
My headers are stepped up from a 1 5/8 to 1 3/4
the last 10-12 inch is 1 3/4 But also my header tubes are with in less then 1 inch of each other.
anything with in one inch ( give or take) of each other is considered to be a equal length header.
If the header /exhaust is to Big you lose back pressure Which in turn hurts over all performance.
As exhaust cools down it also slows down. So the Idea was to down size the Pipes as they got further away from the mufflers to help keep heat in the exhaust & to keep it flowing.
but like i said before I never tested this so I honestly have no clue as to it's effects exhaust flow.
Seeing that I do not run a full lenght exhaust.And
Not offen do i have mufflers on the car.
On a stock/Motor about the only thing that will make any difference is doing away with the stock exhaust manifold. If you build your own headers
No tube bigger the 1 5/8 For the hp that a stock motor has a 1 1/2 is fine The collector 2 1/4 The main exhaust doesn't need to be any bigger then 2 1/2. (2 1/4 is Fine)
Even for a healthy built 6 ( 200-300hp) the 1 5/8 header tubes and a 2 1/4 collector is all It needs
especialy if nothing has been done to the head to help it's performance.(breathing ability).
God doesn't like ugly.
