I was having a discussion today with my buddies dad, and it was one of those situations of math verus experience and I didn't really want to argue with him so I sort of just dropped it but I was wondering what would be right.

The conversation involved comparing a true duel 2.5" system to a 3" single pipe system.

Area = Pi * Radius^2

This means a 3" pipe has an area of about 7.0" and the total area of two 2.5" pipes is 7.9". (internal diameter may be different, but this is just a general calculation to show that 2.5"+2.5" does NOT equal the flow of a 5" pipe, that in actuality having two 2.5" pipes only flow marginally better than a single 3" pipe)

The discussion was about how a true duel 2.5" system would flow better than a compartively sized single pipe system since a Y-pipe adds an extra turn on the passenger side of the motor and that two exhaust flows combing would inhibit exhaust flow.

I've got 1.5" shorty headers into a 3" ypipe going into a 3" hooker catback system. Now the headers and ypipe are cheap flowtech, but they get the job done. He suggested that the system might not be flowing enough for my needs and that a true duel system would be better. When I tried to go into the numbers of internal area and the fact that just because you have 2.5" pipe + 2.5" that doesn't mean you can add the internal area up and say that it flows like a 5" pipe. In actually duel 2.5" would flow only marginally better than a single 3", how he brought up the fact that the single system requires an extra turn in the ypipe and stuff like that. I thought I read somewhere that due to the firing order and such the exhaust pulses don't affec that. I dunno, I'm just curious what is what.

 

With a good y pipe and straight thru muffler the single 3" system would be just as good as a dual 2.5"

 

You have not taken into account the increased surface area inside the dual pipes as compared to the single 3". This is important becuase friction comes into play in all this too... there is an equation for figuering out flow of a pipe, but you need to know the lengths... try to find an equation that takes into account friction.

what i can tell you however, that im down about 50hp in my car with a dual 3" y-ing to a single 3"....

i will be upgrading to a single 4" mufflex setup rather than fighting with dual 3" pipes under the car...

 

Those equations tend to be for steady flow of incompressible fluids. It's a considerably more difficult problem for pulses of a gas which cools as it moves through the pipe.

 

true enough, but at higher rpms (where the flow is much more important) those pulses are so close together, its almost the same as a fluid motion...

You get the general idea however, because to do it right, you would need to find the rate of flow, and to do that you also need to know the temperature differences over the length of pipe, since the exhuast gasses slow down as they cool...

 

Correct. The real thing exhaust desinginers deal with are these pulses and "tuning" them. We can assume any wall friction is negligable for any exhaust system. Flow area is where it is at. You did your math and made your assumptions correctly. Now, I would not bother making your exhaust any bigger until you get rid of your flowtechs. The exhaust closest to the engine needs to be the largest. That is where the exhaust gasses are at their greatest volume. As they go towards the back of your car they cool and in turn their volume decreases and do not require as large of a pipe.

 

Not really, you still have to account for the compressibility of the gas.

 

I know that when I decided on exhaust, I was going to shell the cash for true duals. My exhaust shop (also my trans shop, and my "everything I dont want to do speed shop" ) owner sat down with me, and took a calculator, and showed me flow through the pipes, mufflers, etc. The single 3.5 he put on my car will outflow a dual 2.5 system, any time. He offered to put the duals on, because it still wouldnt really be a restriction, but it was a hella cheaper to do the other way.

 

I have a question..

Alright, i've read people waxing on about how 3" single exhaust is better than dual because less surface area = more heat = flows faster...

How, then, does a fully ceramic coated exhaust system create better HP and better flow? Isnt a ceramic coating supposed to cool everything down and dissipate the heat more quickly? correct me if im wrong, but if i am right on the fact that metallic ceramic coatings cool the exhaust but somehow give better flow, then how does the theory that dual exhuast is worse cause it will cool work? Isn't that what ceramic coating does?(not trying to spur that dual is just as good as coating, but if dual cools exhaust down and ceramic coating cools exhaust down, and if coating creates better flow... then wouldnt dual do the same cooling effects {in comparison to a single 3"} as the coating?) just curious.. sorry if it all sounds jumbled.. damn tired and too much caffine.


Rabbitt

 

No it won't.

At least it won't using common mathematics.

A single 3.5 or 4 inch will flow just as well or better than dual 2.5 inch however.

ALSO: you have to find a Single muffler that will flow better than two (2) mufflers will if you expect it to flow better.

THAT is not easy, especially if you want to keep the SPL level of the car at a reasonable level.

If single exhausts flowed so well, everybody would be using them, there is a reason why people run true duals, and it not only the people with a gazillion hp.

 

I built a true duel 2.5 inch exaust with 2 flowmasters. I couldnt be happer with the sound and power increase (18 hp). I ran them i stock location. It had a 3 inch system that the oringinal owner had put on.

 

Ceramic coating keeps the heat inside the exahsut pipe. It decreases the heat transfter to the external area around the pipe. This keeps your engine bay and anything else cooler that is around your exhaust but not the exhaust itself. Having the whole system coated would be super expensive. Do not worry about the temperature differences between dual and single. Choose one based on what kind of power you are making along with how "custom" you are willing to go. It only matters to you in the end if the cost of duals was worth it.

 

Laminar flow. And it is by far not a negligible thing.

Area= (r*r) * 3.14159

3” A = 7.069
2.5” A = 4.909 x 2 = 9.818
5” A = 19.635

Circumference = d * 3.14159

3” C = 9.425
2.5 C = 7.854 * 2 = 15.708
5’ C = 15.708

See the difference? In all this debating back and forth you must realize that the volume of gas is increasing with the rpm. A correctly set up single exhaust will be just as good as a correctly set up dual until you start making a good amount of power as the spacing of the pulses is important and a single exhaust combines them rather quickly as compared to the dual set up. You then get into the argument of flow resistance in the dual v single muffler.

Rabbit, as Shifty pointed out, when you coat the inside you keep the energy of each pulse stronger, longer. The flow is allegedly also marginally increased but I have no data for that. Cost is about $10/ft coated inside and out last time I had it done. Again, it all depends on power output.

 

mmk, well, thanx for clearing that up for me then.

I bought the 2 1/4 true dual kit from summitt and two dynomax ultra flow mufflers about a month ago. It cost me about $275 (less than the hooker 3" catback i was thinking about by like 4 dollars) so the price wasn't bad. Im waiting for my hooker 2460 to get here from ACS (ordered them a month ago also). Unfortunately, i think i may be scrapping the idea of having the exhaust done before it gets parked this winter; my exhaust guy got in a fourwheeling accident and broke his hip.. and i like his work a lot and i know him personally, so he didn't charge me up the ***. Oh well, maybe i'll find someone else, but i have to have the tailpipes fabbed and such, and have the opposite ends of the I pipe fabbed to bend to and fit my headers... My exhaust guy was only gonna charge me like $50-75 for the fabbing work and the time it would take us to mount the whole thing... oh well, maybe i will have to find someone else.. that wont care about not putting cats on it....hehe

 

I am pretty confident 3" is more then enough. The aero chamber muffler is supposed to flow pretty well, though I have no idea since getting flow numbers that aren't manipulated is pretty tough for mufflers.

The flowtechs/ypipe were like 150$ total and I needed to get an exhaust on at the time. I'm thinking about dropping some loot to get some ceramtic coated 2055s down the road. I wonder just how much the .12" ID increase from the flowtech to the 2055 helps flow. The flowtech y-pipe is a POS, definately a bad design, I haven't seen a picture of the ypipe for the 2055 though. I can't imagine picking up too much power from the flowtech to 2055 setup, not enough to justify the 600$ it would cost me. The flowtechs flange is thin and doesn't seal up very well, but for 80$ it's hard to complain.

The thing is I'm only making 375HP at the engine max, I'm not making that much power. Does higher compression really affect the velocity coming out of the exhaust port into the header? Again, the increase in the ID of the primary is so little it seems hard to believe it would help that much.

 

cam LSA has a lot to do with it too-

110lsa cam will prefer much less back presure than a 114 lsa cam...

 

With 375 hp I bet you would see a nice jump in power with the 2055's. Larger primaries, better y-pipe, and larger collectors.

 

I watched an episode of Engine Masters where they compared a true dual exh system 2.5 inch pipe, to a single exh system 3 inch pipe. The 3 inch single did better. How can this be?

 

That was a good episode. At that horsepower level, the single 3" is enough, BUT you need a good flowing muffler to go with it, since there is only one. That episode really showcased that.

 

Don't forget 1 1/2" flow tech are 1 3/8 inside diameter
2055's are 1 5/8" inside they are 1 1/2"
Most good heads have 1 5/8" exhaust valves
1 3/4" headers are 1 5/8 inside diameter
Is it a good idea to use a smaller header with larger valves?

 

have a link or episode number? Have the MT subscription but can't see to find this episode

thanks

 

season 5 episode 7 I believe

 

Another factor...
If going single pipe, a 1 in / 1 out muffler flows better than 1 in / 2 out....

Raf

 

Doesnt look as good though..

 

Flows better... "Looks" is subjective.... all things equal, you would only see it from behind with worse flowing 1 in / 2 out muffler since they would be out in front

 

Classic car owner sees an exhaust tip. What everybody else sees is 6 foot of ugly pipe hanging below the vehicle.

Exhaust was utilitarian with our cars and it looks that way. New cars integrate with the aesthetics of the vehicle. My suggestion with a 3rd gen is to hide the exhaust and get rid of that eyesore. My 3.5-in pipes are tucked and hidden behind the bumper.

 

3 inch in dual 3 in out w a spintech custom muffler. Hidden/stock looking exit. No need for giant pipes below the bumper.

 

First, it appears that you did your math with the outside diameters of the wrong size pipes, your numbers are right for a three inch pipe but a 7.9 inch cross section is the correct number for two and a quarter-inch pipes.

Removing the average wall thickness of what people use for exhaust pipe the correct numbers are roughly:
single 3- 6.5^2in
dual 2.5- 8.9^2in
The dual 2.5 has a significantly more cross-sectional area, but since the drag of the sides of the pipe have a significant effect and two 2 1/2 inch pipes have significantly more than a single three, in the real world With a similar layout a single 3 inch will usually slightly outflow the dual 2.5. It gets significantly more complicated when you turn it into a complete exhaust system because many three inch mufflers don't flow much more than two and a half inch mufflers, many even have 2 1/2 inch insides. I don't know of any manufacturers that publish their actual information besides Dynomax, and their ultraflow welded mufflers are usually my go-to. Also the further forward you mount a restriction in the exhaust path the more effect it has on exhaust flow, because the exhaust gas cools and has less volume as it goes down the exhaust pipe. OEM 's figured this out in the last 25 or so years maybe more and you'll notice that their exhausts tend to look more restrictive as you get closer to the back of the car, and often there was no appreciable difference in performance, it was just quieter. For example, my 97WS6 trans am showed no performance difference between the full factory exhaust and an open three-inch pipe, running mid 11's on 225hp worth of N2O.

As far as ceramic coatings go, they protect the pipes and make them look better, but also hold the heat in, which may protect components around the exhaust, but as compared to an uncoated they will usually lose a few horsepower.

 

I think it is opposite of that. Reducing heat losses from exhaust system will help power output.

30-years ago I took a fascinating graduate course in compressible flow. I don't remember much, but one of my faint memories is that the energy lost by heat transfer is actually robbed from the kinetic energy of the system. In other words, heat losses cause slower velocities and that hurts engine output. It's not intuitive at all and that's why it was so fascinating.