3" DP to Dual 2 1/4" catback

[AWDGreenMachine]

I just had my exhaust system re-done because I was tired of the low rpm drone on my way to work at 7:30am, from the 3" system. Well, It didn't get rid of the drone(it's not as deep now though), but the car seems to accelerate faster. Does that make sense, or am I imagining that my car seems faster now.

 

[dsm-onster]

pi (r^2) = Area of a circle

Two 2 1/4" circles have a 1 square inch larger area than a single 3 inch circle. BUT! "skin friction" (I believe is the proper term) has a HUGE impact on fluid flow. So I doubt dual 2 1/4" really flows more. I really doubt dual 2 1/2" flow more. On another turbo car, I went to from dual 2.5" to 3" then back to dual 2.5" and felt no difference either way.

 

[c4uldr0n]

I see a CAT

At least it is unique, I doubt it is going to do much for you though!

$0.02 c4

 

[PieEyedPiper]

Is it not posisble he might see better flow though from increased exhaust velocity created by the smaller tubing?
I would imagine this is a better performer than a single 3" setup. Not as good as 4" though, I would imagine.

Its hard to say one way or the other without some serious testing.
I like what you've done though. Whats the exit look like?

 

[elfroggo]

Instead of doing that.. which while interesting doesn't make much sense to me, why didn't you try adding in a resonator.

Also with that setup, what is your muffler like? dual inlet?

 

[dsm-onster]

Is it not posisble he might see better flow though from increased exhaust velocity created by the smaller tubing?
I would imagine this is a better performer than a single 3" setup. Not as good as 4" though, I would imagine.

Its hard to say one way or the other without some serious testing.
I like what you've done though. Whats the exit look like?

Testing has been done. I was wrong about the term. It is called "viscous resistance".

A single 3" exhaust flows better than dual 2.5" or dual 2.25" due to laminar fluid flow. This is best explained by Poiseuille's Law. More information about that can be found here, http://hyperphysics.phy-astr.gsu.edu/hbase/ppois.html

But to summarize, the rate of volume flow has a radius^4 dependance. The single 3" should flow around 4% more than the dual 2.5" and around 60% more than dual 2.25". BUT, that doesn't take into account the turbulence caused by the Y or flow differences of the two different sides in the dual exhaust.

Note, the radius (a function of the surface area of the tubes) plays the biggest part in determining the flow rate as it is raised to the 4th power. Much more of an impact on final results than numbers of pipes. . .

 

[PieEyedPiper]

Good link.
Thanks for corroborating the info. Never would have thought the "viscous resistance" would have been great enough to negate the ~ 56% increase in flow area from the dual setup.

 

[AWDGreenMachine]

Is it not posisble he might see better flow though from increased exhaust velocity created by the smaller tubing?
I would imagine this is a better performer than a single 3" setup. Not as good as 4" though, I would imagine.

Its hard to say one way or the other without some serious testing.
I like what you've done though. Whats the exit look like?

I'll post pics of the rear once I get home. They're a little grainy since I'm using my camera phone.

 

[T9S1i]

Yeah show us your muffler, and also show us a pic of the "Y" where it goes from the cat to split to two pipes.

Is it one muffler? or dual exhaust like a fc rx-7 or a 300zx?

 

[AWDGreenMachine]

Sorry for the poor quality of my camera phone but, here is a few pics of the muffler. Dual in/out. Actually they're two separate mufflers...sort of like th Apexi dual that you see on some 240sx's.

 

[SFRacingGST]

It looks really nice but may i ask why u did this? I mean what made you want to get rid of your 3inch for this setup??

 

[AWDGreenMachine]

I did it because I wanted to try and get rid of some of the low rpm drone of the 3" system. Ofcourse, there's still that annoying drone but, atleast now, it's not as deep. It sounds meaner, if I do say so myself! It was worth a shot.

 

[Budget90GST]

Testing has been done. I was wrong about the term. It is called "viscous resistance".

A single 3" exhaust flows better than dual 2.5" or dual 2.25" due to laminar fluid flow. This is best explained by Poiseuille's Law. More information about that can be found here, http://hyperphysics.phy-astr.gsu.edu/hbase/ppois.html

But to summarize, the rate of volume flow has a radius^4 dependance. The single 3" should flow around 4% more than the dual 2.5" and around 60% more than dual 2.25". BUT, that doesn't take into account the turbulence caused by the Y or flow differences of the two different sides in the dual exhaust.

Note, the radius (a function of the surface area of the tubes) plays the biggest part in determining the flow rate as it is raised to the 4th power. Much more of an impact on final results than numbers of pipes. . .




I don't know, but I think this only applies to incompressible fluids....

 

[Nuclear3000GT]

Radius is a large key, but dont forget pressure as well, it's the other variable in this equation becuase same gas going through a smaller radius at the same speed will have greater pressure, the question is how much greater? The 3" in theory only flows about 3.55% more assuming pressure is the same. If that pressure difference is any more than a factor of 1.0355 than I believe the dual 2.5 will flow more. Any physics, mech engineers, or aero engineers wanna chime in? I just finished my first year so im still learning.


Don't get me wrong I agree my logic tells me a single 3" exhaust should flow mor than dual 2.5" pipes from a single source because of laminar flow(gases apply too from what I remember from my intro to fluids class). However that looming part of pressure doesnt have me 100% sure only like 95%.

 

[dsm-onster]

A non-consistant viscosity characteristic in a fluid (such as exhaust gases) would exhibit MORE turbulance where bends, "Y"s, increases, and reductions are present.

It is my understanding for NON-newtonian fluids that more turbulance at a given volume of space yield a higher viscosity for the fluid in that volume. This would slow the flow down at they point of turbulance. Backing up the exuast gases raise evil backpressure. Raising the total pressure lowers the total flow for the rest of the system assuming the rest of the volume of gases has a consistant viscosity. And in turn, would cause the system w/ the "Y" to suffer from even poorer flow vs. the system w/ a single leg. As the equation shows 1/viscosity as a coefficient of the equation.

I was using this equation to show the best possible flow potential between the two. Any introduction of turbulance just makes things worse. If both fluids exhibit the same characteristics in viscosity, then the characteristics, in a sense, "cancel each other out". And, what remains is the characteristics of the tubes they are flowing through.

So it is possible for a 3" exhaust system to exhibit poorer flow properties than dual 2.25", but who do you know takes the time to put lots of extra "wiggles" in 3 inch mandrel bent piping?

Also keep in mind: air flowing through an exhaust pipe is just like making the air "stationary" and throwing the pipe through it like a spear. All the principles of drag apply here: perhaps more in the area of Stokes drag.

 

[gixrman]

How about the added weight? I'm thinking the dual setup has added a few pounds.

 

[pboglio]

Unless you guys run a CFD analysis on his exact setup, your purely theorizing. I've done CFD analysis on my exhaust setup, and biggest gains are to be had near the 02 housing where the bend is tightest, smallest diameter, and where the exhaust gas is the hottest with higher exhaust gas velocities.

Unless he's flowing more than 35-40 lb/min, the pressure losses in his setup are hardly hurting him one way or another. And I believe he did all this for noise reduction anyway.

"Burns Stainless" makes collectors (y tube style) and venturi setups that actually ADD torque to setups with existing exhaust systems. Thus the y-collector although adding pressure loss, could help "pulse" tune the exhaust system and actually broaden the torque curve, even on a turbo car.

I've personally noticed "better" bottom end power with a 2.5" high flow cat vs. testpipe. And although the cat has about 1-2 psi of pressure loss, my datalogger showed ZERO difference in airflow thru the motor between them, thus I prefer running a cat.

 

[dsm-onster]

Of course, his set up doesn't merit 3" piping. I'm discussing potential flow.

I've personally noticed "better" bottom end power with a 2.5" high flow cat vs. testpipe. And although the cat has about 1-2 psi of pressure loss, my datalogger showed ZERO difference in airflow thru the motor between them, thus I prefer running a cat.

Interesting. I wish you could get a dyno session of this. Even a log of the hertz curves comparing the two situations.

MY butt fools me. When I hear a better sounding exhaust, I tend to feel like the car is faster even if there is no difference in power. Like when I put a exhaust tip resonator on my 3" exhaust w/ glasspack. . .

All one has is theory when building something custom like an exhaust system. Trial an error gets expensive.

 

[pboglio]

Of course, his set up doesn't merit 3" piping. I'm discussing potential flow.


Interesting. I wish you could get a dyno session of this. Even a log of the hertz curves comparing the two situations.

MY butt fools me. When I hear a better sounding exhaust, I tend to feel like the car is faster even if there is no difference in power. Like when I put a exhaust tip resonator on my 3" exhaust w/ glasspack. . .

All one has is theory when building something custom like an exhaust system. Trial an error gets expensive.

That y-collector is probably equal to a 90* bend in terms of pressure loss. I wouldn't lose sleep over it. And I would prefer to see a CFD analysis of the entire setup before anybody claimed a difference one way or another, or better yet a backpressure reading somewhere upstream of the cat. By my math, twin 2.25" have a larger flow area than a single 3", skin friction aside.

 

[dsm-onster]

That y-collector is probably equal to a 90* bend in terms of pressure loss. I wouldn't lose sleep over it. And I would prefer to see a CFD analysis of the entire setup before anybody claimed a difference one way or another, or better yet a backpressure reading somewhere upstream of the cat. By my math, twin 2.25" have a larger flow area than a single 3", skin friction aside.

The thread has already alluded to the flow area. But, skin friction is not 'aside'. And, we all do not have ready access to a means to a CFD analysis. Overbuilding is a means to prevent underbuilding. The best defense is a good offense when you're unsure.

Considering how the particles of exhaust exit the turbine wheel, I an in strong disbelief that any sort of pulse tuning is relevant to flow in a turbo-back. And likely will only hurt your flow later in the upgrade path, if it is not already.

That "Y" MAY only pose the same resistance at his current flow. He already has noted no difference in "feeling". Thus, this would suggest that his set up does not merit a larger/different exhaust. Yet, we rarely keep our "builds" the same. Overtime, 99% of us upgrade. Why rebuild your exhaust system again?

 

[pboglio]

The thread has already alluded to the flow area. But, skin friction is not 'aside'. And, we all do not have ready access to a means to a CFD analysis. Overbuilding is a means to prevent underbuilding. The best defense is a good offense when you're unsure.

Considering how the particles of exhaust exit the turbine wheel, I an in strong disbelief that any sort of pulse tuning is relevant to flow in a turbo-back. And likely will only hurt your flow later in the upgrade path, if it is not already.

That "Y" MAY only pose the same resistance at his current flow. He already has noted no difference in "feeling". Thus, this would suggest that his set up does not merit a larger/different exhaust. Yet, we rarely keep our "builds" the same. Overtime, 99% of us upgrade. Why rebuild your exhaust system again?

I'm all for overdesigning to a point, do it for a living. But exhaust systems are the exception due to the noise levels that come with bigger exhausts. Dual 2.25" is a slick way to do it and eat your cake too. I just think his muffler volumes are too small and he needs some resonators too, but otherwise its a good idea, thumbs up.

I'm not trying to be a smart ass, but when you have the chance, drop your downpipe off the car and run open at the 02 housing. Then post your impressions of your power band on the low end.

 

[dsm-onster]

I have run straight from the o2 housing on this car and other platforms entirely. I have done it w/ a 14b, small 16g, TD05H 18g, RS60t, garrett t3 60trim, full garret 50 trim. I've noticed a significant positive difference w/ the onset of boost (low rpms response), transient response, and peak airflows. This is compaired to 3 inch exhaust.

If you're discussing "pulse tuning" non-boost engine response, take a look at his turbo. When is he not in boost and concerned about throttle response?

The whole premise that a quiet exhaust can flow more *constant flow* (uninterupted) than a loud exhaust is not logical or reasonable. What is noise? What makes a noise loud or soft?

Again, I am not debating whether or not his system warrants 3" exhaust, but strap on a 47-trim. Isn't that on his parts wish list? Single 3 inch can't hurt him. He said that he felt no difference (positive or negative) going to the dual 2.25".

Some comparison results of running my small 16g untuned, stock longblock, running straight off of the wastegate (done on the same day):

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Here's the spreadsheet compiling the two logs.

 

[tkelly27]

DUDE! where did you get CFD software?! Hook me up!

To the OP, you're exhaust is unique, and if you like it, hooray for you. I know there are SRT-4's that have that setup, and I think a lot of them sound really cool.

 

[underradar92]

First off let me say to the OP, I like your idea, regardless of the potential or future performance implications. I found that adding Dynomax Ultra Flow Bullets lessened the drone with no/minimal power loss, in fact it "feels" faster being a bit quieter, even though I know that adding ANY flow restriction post-turbo equals power loss.

My question may be related. What is the formula (Matt) with regards to oval tubing?

 

[JOEY A]

BMW does this for there cars, and exhaust setup like this


I always thought it added a little more TQ? My freind had the apex exhaust on his 240 and it sounded really nice....


Sound clips PlEASE?!