I was wondering if there is any benefit at all to running a dual three inch exhaust. It seems like two three inchers would be better than one four but then again I know absolutely nothing. Maybe it is a flow or restriction issue, there must be somthing or people would be doing it. I was just thinking that with all the three inch factory systems laying around it would be a cheap way to go if there was any benefit.

factory exhaust is 3 1/2", the cheap way out is cut out the stock muffler & straight pipe it.

go to mbrp they have a "cool duals" that your might like

I was wondering if there is any benefit at all to running a dual three inch exhaust. It seems like two three inchers would be better than one four...Assuming the restriction is directly proportional to the area of the circle (diameter of the exhaust)
3.5" = 9.6 sq.in. - stock
4.0" = 12.6 sq.in. -> 31% larger than stock
3.0" = 7.06 sq.in.-:t
3.0"*2=14.1 sq.in. -> 47% larger than stock and 12.5% larger than 4.0":)
5.0" = 19.6 sq.in. ->104% larger than stock (more than twice the size :ro)) and 56% larger than 4.0"

I don't know where you would get a 3.5"-> 2*3.0" Y-pipe, but it looks like it would be an improvement. Also give it sort of a 'sleeper';) look. I wonder how it would sound??

daurand, you must be a math major. Good info.:cool:

I don't mean to offend anyone, but you are almost right. It is actually worse than you think. :mad:

I pulled out my good old Machinery's Handbook and checked into it.
I know engine exhaust flow changes as you press the go pedal, but as an example, here is what I found.

At a flow velocity of 20 feet per second - which corresponds to 2000 RPM on a 6.6 Liter engine, the flow for a pipe is: (In Cubic Feet Per Minute)
1" Pipe - 6.55 CFM
2" Pipe - 26.27 CFM (431% more flow than 1")
3" Pipe - 58.91 CFM (124% more flow than 2")
4" Pipe - 104.64 CFM (77% more flow than 3")
5" Pipe - 163.47 CFM (56% more flow than 4")
6" Pipe - 235.40 CFM (44% more flow than 5") Censored

So adding bigger pipe will only help more air exit the engine to a certain point. After that, adding bigger pipe won't do much for you.

The other thing you have to remember, with a dual exhaust system, there are more bends and fittings than a single system. Each of these bends will slightly restrict the flow. So it is possible, that a simple 4" single system could flow as much or more than a dual 3" system. It all depends on the shape of the pipe.

Sounds like we need someone to put this to a dyno test.

Assuming the restriction is directly proportional to the area of the circle (diameter of the exhaust)
3.5" = 9.6 sq.in. - stock
4.0" = 12.6 sq.in. -> 31% larger than stock
3.0" = 7.06 sq.in.-:t
3.0"*2=14.1 sq.in. -> 47% larger than stock and 12.5% larger than 4.0":)
5.0" = 19.6 sq.in. ->104% larger than stock (more than twice the size :ro)) and 56% larger than 4.0"

I don't know where you would get a 3.5"-> 2*3.0" Y-pipe, but it looks like it would be an improvement. Also give it sort of a 'sleeper';) look. I wonder how it would sound??

The flow rates are not additive. Two 3 inch exhausts will not flow the equivalent of a 6 inch system.

A good estimation formula for flow is x (flow) = SQRT(2*(E*E)). Taking the square inch numbers from above and solving for x = SQRT(2*(7.06*7.06)) yields 9.98 square inches of flow, just a small improvement over the 3.5 inch system.

A single four inch system should outperform the 3-inch duals.

The flow rates are not additive. Two 3 inch exhausts will not flow the equivalent of a 6 inch system.

A good estimation formula for flow is x (flow) = SQRT(2*(E*E)). Taking the square inch numbers from above and solving for x = SQRT(2*(7.06*7.06)) yields 9.98 square inches of flow, just a small improvement over the 3.5 inch system.

A single four inch system should outperform the 3-inch duals.
Not to offend anyone but as a basic rule anytime you double pipe size you increase the flow area 4x. Area = 3.14 x Radius squared. Daurand and BigStu128 is dead on with their numbers. Dual 3" will flow slightly more than 4" pipe. :cool:

The flow rates are not additive. Two 3 inch exhausts will not flow the equivalent of a 6 inch system.

A good estimation formula for flow is x (flow) = SQRT(2*(E*E)). Taking the square inch numbers from above and solving for x = SQRT(2*(7.06*7.06)) yields 9.98 square inches of flow, just a small improvement over the 3.5 inch system.

A single four inch system should outperform the 3-inch duals.

Got it, pie R Square, or is Pie R Round, cornbread R Square? I'm so confused
:grd: :grd: :grd: :grd: :grd: :grd:

All that I know is 3 1/2" is not a stock pipe size in an older truck. Maybe we should ask Roy what he is driving before we go into the big debate of what will help him..;-)
I have a 93 6.5TD(unlike you guys with the newer more expensive models) and mine was 2.5 or 2.75" stock and I went with custom dual 3" pipes with 4" x 12" SS Magnaflow tips. My only fault is that I have not gotton the mandrel bent downpipe yet. It will come but I have other issues that just jumped in(IP and injectors).
Aside from all of that, the truck sounds real good with the duals and have had many compliments.
Mark

While I find this subject interesting, it reminds me of why I hated high school math. That said, since it is coming back 3.5 before you put a y in it, how could it flow more when only half the pipe is dual?

I am thinking about removing my muffler and making a y pipe there, and running another 3.5 inch tailpipe out the driver's side. Not for more flow, I just want to be different and make smoke come out both sides.:ro)

Ok guys,What I meant to say in my first post was dual 3.5inchers not dual threes sorry about that . I have a 2004 lly crew cab long bed.it sounds like some of you think it would flow better and others think it would flow less I wonder if anyone has done any kind of dyno or flow test to confirm this. It would be interesting to see and would be a more economical way to get the cool sound and look of duals.

[QUOTE=roy]Ok guys,What I meant to say in my first post was dual 3.5inchers not dual threes sorry about that . I have a 2004 lly crew cab long bed.QUOTE]

Oh sure, now you tell us ;)

I believe that the closer you can get the duals to the turbo, the better flow. Putting them after the muffler wouldn't do much. The stock down pipe (yes-the one attached to the turbo) is only 3.5", but many have shown that going larger after that does improve performance.

(math minor - physics major and M.S. in radiological physics)

All this sounds good in theory, but, if your down pipe is only 3.5" and your pipes could be 10" and it ain't gonna matter!!

Obviously 4" and 5" systems work to relieve the back pressure with a good flowing muffler or strait pipe.

Think of it like this, its kinda like p!ssin' through the small end of a funnel, ya think more p!ss is gonna come out?
You can push and push.... well you get the picture!;)
Now try to hold two funnels and "Y" it, just think of the mess!!!!!!!):h

All this sounds good in theory, but, if your down pipe is only 3.5" and your pipes could be 10" and it ain't gonna matter!!

Obviously 4" and 5" systems work to relieve the back pressure with a good flowing muffler or strait pipe.

Think of it like this, its kinda like p!ssin' through the small end of a funnel, ya think more p!ss is gonna come out?
You can push and push.... well you get the picture!;)
Now try to hold two funnels and "Y" it, just think of the mess!!!!!!!):h
:funnypost