collector size
 

How much difference would there be between going with my crapped up thorleys with whats left of the 2.5" collector to 2.5" pipe that Y's into a 3.5" collector as opposed to some Pacesetters with a 3" collector?
My setup is a 402 with 14-15# (hopefully) from a T trim with FMIC and meth.
How much difference would there really be from a 2.5" collector and pipe to a 3" collector and pipe??

some of you guys had to have gotten into this when you put together your exhaust systems. At what point, if any, do you start losing power by not having a larger exhaust system?

You might think about leaving the inlet to the cats and the collectors at 2.5" It's not going to be an advantage really to go to 3" for that short of a distance. As air inside a tube makes a transition from a smaller diameter to a larger one its velocity decreases. In a collector you already have decelleration happening in the transition from the primary tubing diameter to the collector diameter. The next reducution in velcoity is really the catalytic converters. At the exit end of the cats it funnels back down to the tubing size. That's where the larger tubing needs to begin. It's possible to modify the stock cats with larger piping. If your are a can-within-a-can design as mine were you'll see that the inner can is what retains the elements. When you hack the small ends off one of those cats that inner shell gets loose and has to be welded into place to keep the element secure. It's hard to explain in text but it will become apparant as the end gets chopped off. Also, the y-pipe should be sized appropriately - this is another point of exhaust gas decelleration.

From what Im gathering, it wouldnt be much of a gain by going to 3" collectors as the 2 3" pipes would be going to the same 3.5" as the 2.5" collectors on my thorleys currently as the distance isnt all that long?? I wont have any cats at all. We dont have emissions here and I dont pretend to think that any cats would last any period of time with that kind of airflow.
If thats the case than I am very happy :) I would love to not have to replace them and throw them in the trash when I have so many other things that still need to installed :devil:
Thanks James B.
BTW Im keeping tabs as to what it takes to get into the low 12's and dare I say it... I think your previous estimate was very near $$$:cry:

WIth a 408 and blower, you'll definitely see gains going from 2.5" collectors to 3" collectors as long as the remainder of your exhaust system is sized properly as well. I customized my ASMs with a 3" collector and 3" front pipes into an X. My Radix loves it.

So you can have a different collector welded on there?? Where did you go to get the collector?? Is this something that requires the headers to be removed?? Im going to see what an exhaust shop would charge for it (welding on a new collector and piecing together the exhaust system) since there is no chance I can get under there to everything without a lift. Im going to supply the bends and all of the parts and I dont mind paying for good work as long as it isnt going to be like $600 just for labor.

The collector on most of these headers are plain 3-bolt flanges. I cut off the flange, expanded the collector with an exhaust tube expander, then welded on another flange. I've done two sets - one with a 3-bolt flange, the other with a ball and socket flange.

Here is what the end result is:
http://z71.net/flange.jpg

http://z71.net/exh1.jpg
http://z71.net/exh2.jpg

Tex thats a beautiful exhaust I love the way it glistens do you clean the underside to the truck also

HA! I wish. To be honest, my truck gets neglected. That was when it was new. It was sprayed with rattle-can high heat metallic silver. It not quite as shiny any more.