Difference between 3" or 2.5" dual exhaust

[esinger]

I'm thinking about upgrading my exhaust system and having a problem convincing myself that a 3" dual exhaust system with separate mufflers would flow any better than a 2.5" dual exhaust with the same configuration.

Here's what I'm thinking. If you have a 3" downpipe going to a 3" cat or test pipe and then a Y that splits into two 2.5" pipes, how does an extra .5" really gain you anymore flow than if it Y's into 3" pipes?

Wouldn't the flow limit be the single 3" pipe that the exhaust is coming out of and not the two 2.5" or 3" pipes?

 

[85 ttypewh1]

Exhaust gases cool as they travel so then they condense which means they need less space. IMO if you have a 3 inch dp and test or cat pipe then a 2.5 dual would be plenty, hell you have guys running 3 inch all the way back The one thing you need to make sure is that the muff is a free flow design no back pressure or even no muffs at all. So unless you are running a combo that flows so much cfm that you need a 4 inch or bigger dp than i dont think you will outflow a 2.5 inch dual exh. JMO

 

[strikeeagle]

For the n-th time, it doesn't work that way. A 3" cat back has less pressure loss than a 2.5" cat back, regardless of what is upstream. Period.

strike

 

[85 ttypewh1]

I dont know what you mean by it does not work that way, hopefully you were not talking about gasses cooling down and condensing because if you dont think they do you have bigger problems. I agree a 3 inch will flow better than a 2.5 inch IF you are flowing enough cfm. esinger you need to decide what you want the car to do, a 2.5 will flow good enough for elevens maybe even faster. If you what to go 9-10 sec than get a 3inch. So to answer your question strikeeagle is right a 3 inch will out flow a 2.5 inch so i am going to leave it at that i have to go out in the garage and design a 4 inch cat back

 

[disco stu]

I see no difference in ET with the dump open or closed on my car with my 2.5 ATR dual exhaust. 2 2.5" pipes flow well enough for a 3 or 3.5" downpipe on my car, 10.8 @125 on a lame tune so far. I don't think a 3" inch dual exhaust would improve anything.

Will somebody explain to me why a 3" dual exhaust that flows better will be of any benefit when you have a 3" down pipe and a turbine outlet that is smaller than that?

 

[TurboTR]

Yes, because the losses are cumulative. Kinda like a string of resistors in a series curcuit. Remove one and the total loss goes down.

IME, a single 3" system flowed much better than the old 2.5" dual Hooker system. Even with dual straight through Hooker muffs installed. Uncapping ahead of the Hooker system, the car would pick up 2-2.5 mph or so, when it was in the 126-128+ mph range. Uncapping the single 3" system, it would pick up less than 1 mph. Didn't expect it, but that's what it did. Plus the single 3" system is stainless and some 25 lbs lighter to boot It recently went some 117 mph in the 1/8 through that same 3" system Didn't try it uncapped again yet though But it now has a 3.5" TH downpipe and test pipe on it too, modded to fit with the ATR headers. I have a somewhat quieter, Edelbrock muff on the exh. We'll try it again soon with a 76 GTS turbo.

TurboTR

 

[esinger]

Thanks for everyone’s response. I wasn't aware that this topic had come up before; I just hadn't seen it in my searches.

I haven't really decided yet where I'm going with my car, but I'd say that if I could get into the mid 12's then I'd be happy, anything in the 11's would be icing on the cake.
I just want to do this once and not have any regrets afterwards.

I don't really want to go for the single 3", because I like the way the car looks with the dual exhaust and I want to keep it that way.

I hadn't thought about the resistance aspect of an exhaust system, so I can see that 3" duals would have a lower resistance than 2.5.

Thanks again for the input

 

[strikeeagle]

I like the resistor analogy...

strike

 

[bsdlinux]

TurboTR,

You are right. I had a 3" single shot on Kevin's old car. It was loud but it was way better that the 2 1/2" duals I have on my current car. Thinking about a 3" single shot again but using something like the muffler you have.

Gary

 

[Mike Licht]

I can tell you this. The reason Terry Houston stopped making the single 3.5" exhaust is that the dual 2 3/4" system made more power on the dyno and fit better. A single 3" will not make the power of a dual. Noise is proportional to the case volume of the muffler (same style of muffler) which is why the ultraflow is quieter than the little ATR muffler even thought they are both straight through and of a simular design
Mike

 

[NCTURBOS]

Originally posted by Mike Licht
I can tell you this. The reason Terry Houston stopped making the single 3.5" exhaust is that the dual 2 3/4" system made more power on the dyno and fit better.

Mike is very correct here. And the reason that T.H. made the kits with 2.75" pipe as compared to 2.5" pipe was to be able to utilize a 3" muffler with the kit. Testing was done and found out that the biggest flow difference was between the muffler size (3" opposed to 2.5") rather than the pipe size (2.75" opposed to 2.5"). The 2.75" pipe made it easier to "step-up" to the 3" muffler.


K.

 

[ITSNOTAGN]

i just got a single 3 inch put on, and i used a Ravin 5800 series muff. ill tell you, it sounds really good, and isnt that loud, but just right. iamjeff has a 3" single shot and its about half as loud, and whistles just as much, (i like the whistle ). i was sceptical about a midas muffler, but i tried it, and it started and i said "weld it on!". just an opinion. i personally think a single 3 is just fine for anything down to the low 11s, even in the 10s. and it is alot lighter.
If i did it again i want to go with the ATR crossflow wich uses a single 3" to a factory style crossflow and dual 3" outlets, light, and the look is right, sounds real nasty too!

 

[strikeeagle]

Pressure loss is inversely related to the cross-sectional area:

1 X 3" = 9 square inches
2 X 2.5 " = 12.5 square inches (+39%)
2 X 2.75" = 15.125 square inches (+68%)
2 X 3" = 18 square inches (+100%)

strike

 

[Marc87GN]

Originally posted by ITSNOTAGN
i just got a single 3 inch put on, and i used a Ravin 5800 series muff. ill tell you, it sounds really good, and isnt that loud, but just right. iamjeff has a 3" single shot and its about half as loud, and whistles just as much, (i like the whistle ). i was sceptical about a midas muffler, but i tried it, and it started and i said "weld it on!". just an opinion. i personally think a single 3 is just fine for anything down to the low 11s, even in the 10s. and it is alot lighter.
If i did it again i want to go with the ATR crossflow wich uses a single 3" to a factory style crossflow and dual 3" outlets, light, and the look is right, sounds real nasty too!

I just installed the ATR Crossflow a few days ago and it does sound awesome. It comes with a new rubber piece for Cat hanger and the tailpipes bolt to the frame. It also weighs very little compared to the flowmasters and 2.75" aluminized piping I cut out. I look forward to taking the car out once all of this snow melts.