Flow in a 2.5" is indeed half that of a 3", but the dual 2.5" doubles it making it = to 3". To answer the question of why true duals add power, its because this formula assumes flow through a perfectly straight 3" pipe vs. two perfectly straight 2.5" pipes, no mufflers etc. A well designed 3" single exhaust showed flow about the same as a well designed 2.5 true dual.

 

rise again

 

Since you do not seem to be reading what has been presented here, I will
summarize the key point:

>>>>>>>>>2.5" dual = 3.4" single<<<<<<<<<<

with all other items equal (bends, length, muffler[s], etc.)

 

After some more research of my own it looks like dual 2.5" will flow marginally more air than a single 3" but I still cant find any dyno comparisons of a FULL true dual vs a single 3" with the same mufflers. From some of the numbers ive seen they arent that far off. + or - 4-5 hp. does anyone have any dyno sheet of there before and after exhaust setups? I think the only REAL benfit of dual 2.5s is the sound and that was enough for me to go dual. after all this reseach tho im doing dual 3" over the axles.

 

And you never will. Each system requires a different muffler. I gained 12 rwhp
with similar design (flowmaster) mufflers.

 

Ok maybe i should have cleared that part up a bit. I ment flow master to flow master or other same manufacturer. do you have a dyno sheet for the single exhaust and were you using the same dyno? what other differences were there when you tested. temps, elevation etc....

 

Same dyno, different day, all runs STD corrected. The only difference was
the exhaust system. Look at the true dual pics link in my sig and scroll down
to the PDF of the dyno comparison. 310 vs. 298 rwhp. Duals made power all
across the powerband.

 

is your exhaust 2 1/2 or 3" duals???


kris

 

You and others on this board have mistakenly assumed that flow is proportional to the cross sectional area of the pipe. Pick up any physics text. Flow is proportional to radius to the power of 4, NOT to the power of 2 (i.e. NOT to cross sectional area). That means if you double the diameter of a pipe, it will flow 16x the volume of air, not 4x.

Ive already proven mathematically that a 3" straight pipe will flow more air at a given pressure difference and piple length than two 2.5" pipes put together. If you assume that flow is proportional to cross sectional area, then you mistakenly conclude that 2.5" duals flow like a 3.4" single

part of my job is calculating relative flow through coronary arteries based on degree of stenosis, so im not making this stuff up.

 

I've been designing commercial fluid (liquid and gas) handling systems for more than 40 years, and I have never seen an analysis that equates flow capacity at equal pressure drop to the radius raised to the 4th power. I even took the time to check a couple of the available online pressure loss calculators for compressible fluids, and they confirmed that the flow capacity can be approximated by raising the radius (or diameter, it doesn't make a difference) to an exponent ranging between 2.0 and 2.5. Ran a quick check using my Crane tables.... same results.

Perhaps your studies of a fluid with the viscous and surface tension properties of human blood makes wetted perimeter a more important part of the equation, but with low viscosity fluids, like water and exhaust gasses, the impact of wetted perimeter can best be approximated by varying the exponent in the range of 2.0 to 2.5

 

We use Poiseuille’s principle to estimate the effect of radius on flow which assumes, among other things, an incompressible fluid and contact with the entire internal perimeter of the "pipe".

It wont apply to systems where part of the pipe is occupied by fluid, and Im prob wrong in having assumed it approximates flow of a gas through an exhaust pipe. Your 40+yrs vs. my 6months...

 

Just to add something, my 3" setup has better ground clearance that what my Flowmaster cat-back did. Clearance should not be an excuse.

 

Joe, are those 3" over the axle or dumps?

 

Dumps, but are setup that I can go over the axle w/ out changing anything.