I take no credit for this information, I did not make it up. I just figured out how to use it. It may or may not be correct.
I have found some interesting statements about exhaust flow and horsepower.
1. Straight pipe is said to flow 115 CFM per square inch of area
2. To avoid exhaust restriction the exhaust pipe should flow 2.2 CFM per horsepower. (I am assuming crankshaft HP)
These statements may not apply to all vehicles under all conditions, but they are interesting.
The statements are used with this formula.
The area of a pipe X 115 CFM / 2.2 CFM = Horsepower that the exhaust pipes can support.
(1/2 x True Pipe ID) ² x 3.14 = Area of pipe
To find area of exhaust pipe you need to know the thickness of the pipe wall.
16 gauge is .065
There are 2 walls for a total of .13
Subtract that from the pipe diameter.
2.5 inch OD - .13 = 2.37 ID
2.37 x .5 = 1.185 (half of pipe ID)
1.185 x 1.185 = 1.404225 (squared)
1.404225 x 3.14 = 4.4092665 (Pie'd) lol.
Area of 2.5 inch pipe with .065 thick walls = 4.4092665
(4.4092665 x 115) / 2.2 = 230.4843852272727 HP
Rounded 230 HP per 2.5 inch exhaust pipe per side.
Duals with 2.5 inch pipes should be good for 460HP
The below numbers would be with .065 wall thickness
So if my math is correct.
3 inch can handle 337HP per side
676 HP Total with duals
2.5 inch can handle 230 HP per side
460 HP Total per side
2.25 inch can handle 184 HP per side
368 HP Total with duals
2 inch can handle 143HP per side
286 HP Total with duals
I last tested my truck on the dyno a while back with 290 HP at the wheels when I was running 13.34, at the crank maybe 350HP according to 20% drivetrain loss.
Now with water meth running 12.9. I have not been able to test on the dyno but.... online calculators...
Maybe 380 HP at the crank.
Remember it takes 75 HP to drive the GT500 supercharger.
380+75= 455HP at Crankshaft.
Kind of puts things in perspective.
Wednesday I drop my truck off at the exhaust shop.
If that truck was producing that kind of airflow, based on those assumptions, then it would need 455HP worth of exhaust sizing. The single 3" in that posting suggests supporting 338HP.
That is of course based on throughout parts sizing being at least that 3".
From the hardcore performance forums I've seen, tons of real cars making over 350HP at the wheels, NA, virtually all have 3" dual systems. Rarely does one of those manage hit those power levels with 2.5" duals. Now of course there are tons of examples of people with blown cars making 500HP+ with anything from stock 2.5" duals to 2.5" etc. Those cars are choking off a lot of power with those exhaust sizes.
The real point I'm suggesting is that tons of people see real benefits to going to a full 3" dual exhaust.
I picked the 350RWHP level as an example, which is tough benchmark to hit with a NA 302 or 4.6 based Mustang. I also noticed people doing very well with 3" systems on mild cars, mostly real 3.0" stroke 302's with no boost. The ones worth noting like that have no losses of idle power, off idle torque etc. They can leave in 2nd and 3rd gear easily. One is a 289 in a 1965 Mustang and 7000+ rpm shift point, but I don't recall every detail of his car, exhaust etc.
The point is with actual tuning of the A/F ratio and timing, which you guys are doing, you cannot make the exhaust too big. There is nothing to gain from any back pressure(which is restriction).
I'd make it at least a true dual system given the space. There is plenty of room for dual mufflers and tail pipes(use both sides). Bringing the exhaust into one muffler only creates the bottleneck of those HP limitations. Look at the exhaust flow available at every bottleneck of the system, front to back, and the HP each can support.
