To start things off, I do not consider myself single-minded about exhaust systems, from what I have learned a single would be just as good, or better than, duals for making low-end to midrange torque.

Second, this business about a single larger pipe radiating more heat than 2 smaller pipes is nonsense. There is considerably more are on a set of 2.5" duals than a 3" single. Third, a 3" pipe is hardly "too small" for a 351.

 

I wasnt calling you single minded.

 

Sorry, I did not think you were, I just think some people are very single minded about what works and what does not as far as exhaust and many other things. Granted there are some things that flat-out do not work well, the quality of design is the number one factor in determining whether or not an exhaust is going to work.

 

3.5 inch single pipe
A=pi x r sq
A=3.14x3.06
A=9.6

Single 2.250 inch pipe
A=pi x r sq
A=3.14x1.36
A=3.97

Two single pipes = 3.97 + 3.97
total = 7.94

Proof enough??

If you increase the single pipes to 2.5 inch, the total area is approximately the same, so one cannot make a blanket statement to cover all scenarios.

The dual pipes have to be treated as single entities as the systems are not connected, in effect, two 4 cylinder engines. The larger pipe in the single system would absorb and dissipate more heat in this circumstance. Plus, the larger pipe will not have the velocity of the smaller pipe.

The slower moving gasses radiates into the larger surface area, and cool quicker. Cool gas is heavy gas and harder to move. When the hot gasses encounter the cooler gasses, it will slow down reducing the effect of the headers to increase velocity at slower engine speeds. For this reason,some recommend placing mufflers as far back in the system as possible.

With two separate pipes, in the smaller pipe the velocity remains high, the gasses move faster with less time to cool, and aid in the efficiency of the headers.

On a stock application, the two systems can possibly be made to equal out, but the single system takes more planning and effort to make it perform efficiently. Buying a set of Pacesetters with whatever size collector comes with the header, having the local exhaust shop weld up a Y pipe, and slapping them on pipes that you think will be the right size isn't very good planning, and may do more harm than good.

Even a good dual system with headers requires some thought to work properly, and even then, there are trade offs. jd

 

Yeah, except in your equations you choose a 3.5" system as your comparision, which is huge and considerably larger than 3". when dealing with things such as area, it doesn't take a large increase in cross section to get an enormous increase in area.

3" pipe
a=pi x r sq
a=3.14 x 2.25
a = 7.065

And your 2.25" x 2 was rounded down when it should have been rounded up

a = 7.948

while you can compare your dual exhausts to two seperate engine, in the end all formulas must be combined, since they share a common crank. No matter how you split it, you have one engine.

I agree with you on the exhaust velocity jim. Which is why if I had this same truck, here is how I would do it.

I would take 2.25" headpipes and bring them into a y-pipe, and try to make the two headpipes as close to each other in length to deal with harmonics and to ensure the exhaust pulses are never entering the y at the same time I would also have the cats in the headpipes as close to the motor as possible (this would obviously require that the vehicle has an O2 sensor for each bank, otherwise a single cat would have to be used. This minimizes the restriction they could pose to performance, since the gases are hotter and moving faster closer tot eh motor. The reaction would be more efficient, and the gases would expand as they passed through the cat, further increasing the velocity and minimizing backpressure. I would bring that y into a single 2.5" pipe and run it into a single inlet dual outlet muffler. The dual outlets would be 2.50", though what you do after the muffler is less significant. This helps project the sound outward, while minimizing the sound level in the cab.

Now thats just how I would do it, and I'm not telling anyone else to do it that way.