looking at cars with highflow dual cats, i see that the cats are 2 1/2"s for a total of 5" of flowing exhaust. ok so then 2, 2.5 tubes dump into a single 3" system. so in theory you are actually only letting 60% of the exhaust gas to go through as freely, as the 2, 2.5 tubes.

how about instead of dumping into a single 3", what about if those 2, 2.5 tubes just continue all the way to the back, making a true dual system, flowing at max capacity. for mufflers you can use whatever, im not worried about that part,

question #1. is there enough room for 2, 2.5 tubes under the body?(course they would have to be pretty close or angled diagonaly)

question #2, is it worth doing?

question #3 is this actually flowing better than a 3" since you have an additional 2" of space?

question #4 anyone done this yet?

i already searched but did not find this particular topic.

 

Pi * r^2

2 x 2 1/2" pipes, is no where near the same flow as a 5"

basicly the dueal 2.5" = 7.95 in^2 of area.

single 3" = 7.06

so you really only have about a 12% diffrence. Inless you are running some sort of X pipe, I think i would go with 3" for easy. with an X pipe the duals should out perform the single.

 

There are still problems with that math.

Factory duals were 2-1/4". If you put high flow 2-1/4"-ers back in, the flow area would be a little less. If you use 2-1/2" dual cats, the flow would probably be restrained by the down pipes from manifold to exhaust.

Also, pipe diameters are given as OD unless otherwise specified. The cat size is typically ID since it slips over the pipe OD. However, the 3" cat-back would be OD, so you need to subtract the wall thickness from the diameter in order to get the true flow area.

There are also flow dynamics issues that come into play that can get complicated. Effective flow area is reduced by boundry layer conditions, which is a function of wall area (amoung other things) in whatever flow area you are talking about. Duals will have more wall area, so will therefore have more boundry layer within the flow. So, the "loss" going from dual 2-1/2's to 3" is even less than what was calculated above.