Its friction that resists flow. And fricton is proportional to the square of the velocity. Slow the gas down, and it loses less pressure (or looking at it the other way, develops less backpressure). Hotter gas must move faster to move the same "mass" of gas, and that means it causes more pressure drop. If keeping the gas moving fast was in any way beneficial, you would use 1" diameter tubing for the exhaust. No.... you want larger diameter tubing, to slow the gas down.

All of this applies to the design of the system AFTER the collectors. Before the collectors, in the primaries, there is benefit in keeping velocity high, which increases inertia, and allows the fast moving exhaust pulses to "scavenge" the cylinders. But once you get past the collectors, the objective is to provide the least possible pressure loss. And slow moving gas will lose less pressure in a given diameter than a fast moving gas.

And I don't think my interpretation of your anlysis came to the conclusion that a single 3 flows as well as dual 2.5's. I would say that the analysis that came up with a 3.5" equivalence is closer to correct. Using the "rules of thumb" that I've used in designing piping systems for gas and liquids, it might be more like 3.3" or 3.4". But the difference between simply squaring the diameter is not due to cooling of the gas, its due to the fact that the dual pipes have more "wetted perimeter" relative to cross-sectional area, or internal surface area if you take length into consideration. More surface means more friction.

 

I did some 3" stainless true duals about a month ago and I'll never go back. The X-pipe has like a 4 1/2 inch oval inside so I wasn't worried about loss there, and I didn't want any interior resonance because the lack of balance between the two sides dumping under the car. It's supposed to be worth some torque anyway.
The difference on my 383 was phenominal. Not worth much in town as far as mileage but I picked up a lot of throttle response, got rid of the 2300 RPM Y-pipe 'gurgle', and didn't sacrifice anything over my catback setup, and that's with dual cats. Just gotta take your time and do it yourself it you want it to fit tight. I made mine with 2 3" stainless minicats, mandrel bends, magnaflow X-pipe and 2 12" dynomax race bullets.
I can also honestly say that even though it's dumping under the car right now, the cats make it ALOT quieter and more livable than none at all. My eyes dont burn either. Compared to a buddy's car with the same setup and no cats it's a night and day difference. A little quieter than my borla with the wide open plate on it.

 

I love this guy. LOL

 

2.5" true dual here, x-pipe, no cats, pipes to the rear bumper, muffler in stock location. Quieter around town than the Borla with 1.5" plate, and no droning whatsoever - in fact my 12-bolt makes more interior noise than the exhaust around town.

I like the sound of the TD and won't go back to a single exhaust cat-back again.

 

Do you feel youre leaving much on the table with your 383 and the 2.5?

Ground clearance of the dual 3 inchs is what has been keeping me from doing it. Also the fact that I plan on adding more power down the line and don't want to have a bottle neck later.

 

Yeah. I wish I was smart like that. Im going to remember this and try to explain it to someone one day and sound like I know what Im talking about...

 

I did 3" duals over the axle with bullet mufflers. Its pretty loud, but not to bad in the car since the sound is all going out the back. The only thing I really noticed was a little bit better throttle response.

 

Not at all.

 

I agree 100% with you Fred. When I was messing around with some gas law
equations and graphing this stuff a few years ago, the equivalence was close
to the 3.4" range. The other thing to consider is exhaust piping is measured
in OD, so you really need to do calaculations based on the ID to compare
things.

For me it was about the sound and all I really wanted to prove was that a
2.5" dual system was better than a 3" single. It seems pretty clear, at least
with the calculations I did, that this is correct. To really do it right you need
to know the composition of the gas and the temperatures at different points
in the system, which I did not do, so it's definately an approximation.

I would think a 2.5" (mandrel bent) dual system would support up to 400-430
rwhp without being the source of unnecessary losses. Since 3" duals add an
additional level of fitness complexity, why go there unless you are making
serious HP. At this point you are probably at the race car status and no longer
a daily driver, so sound does not matter, and a single 4" is easiest.

 

Ya I knew the answer to the question but not the way he did. LOL

 

id just like to say, this is a very good thread. I definitely learned something reading this

 

IM GLAD I STARTED IT ! ....BUT DUE TO MY CONFUSION...DID WE COME TO A CONCLUSION ?? SINGLE 3 OR DUAL IS THE QUESTION... MY EXHAUST WILL BE GETTING DONE SOON AND WANT TO MAKE SURE IM GONNA DO THE RIGHT THING

 

Well ???

 

True dual will gain you some hp Kris...as long as you dont have a problem getting it inspected and dont mind the sound, I'd do it...

Cody

 

INSPECTION PROBLEMS ? NOT ME.....WHAT YOU THINK ABOUT 3" ?? THIS IS WHAT I HAVE... PACE SETTER LONG TUBES, 3" BULLETT CATS, MAGNAFLOW X PIPE AND 3" DYNOMAX BULLETTS....CAN GO 2 1/2 IN NEEDED BUT WILL GO 3" IF ITS NOT GONNA HARM ANYTHING........LETS GET SOME FEED BACK HERE PEOPLE,,,, 1997 TRANS AM