3.0" to 2.5" Exhaust - Bad flow characteristics?

yashart_mp3 · Oct 18, 2006

Just wanted to get people's thoughts about exhaust flow characteristics for this setup.

I am picking up a 3" GHL downpipe, and plan on getting the 2.5" flange. This will allow for me to use the JIC, Greddy, apexi, and maybe a few other catbacks. I would go with a full GHL turboback, but im not crazy about the look of the muffler, and really dont want all that noise.

My question, is if it is ok to go from 3.0" to 2.5"? Will it decrease performance (compared to a straight 3" exhaust)? I have never heard of reducing exhaust size, only increasing from 2.5" to 3.0", like the Vibrant does. Will it have an adverse affect and create a bottleneck/backpressure at higher RPMS?

I am guessing that since the "bottleneck" is so far away from the turbo, I will be ok. I know that you want a clean, seamless exhaust flow right off the turbo. I am hoping that since my 2.5" section is so far away from it, it should have little affect on flow.

Thoughts/ opinions. Im hoping Brian will have some insight on this

03orgspd · Oct 18, 2006

Your setup that you're talking about would be the same concept as the vibrant. The piping starts off at 2.5 behind the muffler and goes into a 3" downpipe, which I believe you are describing. It should be fine unless you're pushing 300hp or something, but my vibrant has been great!

Earful · Oct 18, 2006

This is the setup I have....getting a Megan catback. With HP goals around 200...this should be ideal.

03orgspd said:
Your setup that you're talking about would be the same concept as the vibrant. The piping starts off at 2.5 behind the muffler and goes into a 3" downpipe, which I believe you are describing. It should be fine unless you're pushing 300hp or something, but my vibrant has been great!

low_psi · Oct 18, 2006

I had a discussion with some friends about stepping down exhaust piping. We decided stepping it down a fair distance from the turbo is acceptable as a means of keeping exhaust velocity up as the gas cools and loses pressure on its path through the piping.

With that said, I run the GHL w/ a JIC catback and don't feel its much worse than a full 3" exhaust.

yashart@work · Oct 18, 2006

03orgspd said:
Your setup that you're talking about would be the same concept as the vibrant. The piping starts off at 2.5 behind the muffler and goes into a 3" downpipe, which I believe you are describing. It should be fine unless you're pushing 300hp or something, but my vibrant has been great!

Its kind of like the vibrant, just reverse. Mine will start off at 3" off the turbo down to the cat-back, where it becomes 2.5". ( 3in downpipe -> 2.5" catback)

yashart@work · Oct 18, 2006

low_psi said:
I had a discussion with some friends about stepping down exhaust piping. We decided stepping it down a fair distance from the turbo is acceptable as a means of keeping exhaust velocity up as the gas cools and loses pressure on its path through the piping.

With that said, I run the GHL w/ a JIC catback and don't feel its much worse than a full 3" exhaust.

That makes sense. I never thought the exhaust gas cooling and losing pressure. In a sense, wouldn't it increase air velocity, compared to a straight 3" exhaust?

That setup sounds nice. Did you go with the Titanium series? I think going this way is the best of both worlds. Great performance increase without overwhelming noise.

Notorious_V.I.C · Oct 18, 2006

You don't need 3inch in my opinion unless your trying to make large amount of power. You will lose low end torque. The reduction shouldn't have much of back pressure, the car isn't pushing a lot of boost is it?

I have a straight 3 inch from the turbo. Going to go down to 2.50" and have the axle portion go under the suspension like Apexr1's exhaust but with a more stock looking muffler.

jmv · Oct 18, 2006

low_psi said:
With that said, I run the GHL w/ a JIC catback and don't feel its much worse than a full 3" exhaust.

thats what deadalus was/is running too, and that guy has made big numbers.

msp35 · Oct 19, 2006

GHL into stock catback and no complaints as of yet.

yashart@work · Oct 19, 2006

msp35 said:
GHL into stock catback and no complaints as of yet.

How does it sound? Noticably louder?

540James · Oct 19, 2006

i have ghl full system, its a lot quieter than my buddies msp with a catback, i do have the high flow cat and the resontor though.

PlatinumMSP · Oct 20, 2006

dude the GHL is relatively quite because it has a cat, resonator, and muffler when compared to other exhaust systems like the Perrin or the Vibrant, it is better to go from smaller piping to larger piping, the small piping at the end is going to cost you a ton of power, you would be better off getting a muffler with a silencer so that you can keep it quiet and bottled up when you want and then take out the silencer when you want all the power out of it

low_psi · Oct 20, 2006

PlatinumMSP said:
it is better to go from smaller piping to larger piping, the small piping at the end is going to cost you a ton of power

can you explain why?

PlatinumMSP · Oct 20, 2006

low_psi said:
can you explain why?

Its fluid mechanics man (I got a D in that class) i'll try to explain to the best of my knowledge...

Basically a large volume of flow from the 3" will have to be crammed into the 2.5" which will create backpressure and a slowing of the flow because there is less space for it to move. The velocity and amount of flow will be greatly disrupted from this. If you go from smaller to bigger piping the velocity has no restriction at the end so the flow does not slow down and additional back pressure does not build up.

terbow · Oct 20, 2006

it is correct you will lose low end tq, but the reason its there on the stock system is cause its SO restrictive its ramming the exhaust pulses together (remember exhaust isnt just a stream, its pulses). the engine has to work harder. when you have a larger exhaust the engine doenst work as hard cause your not slamming the pulses into each other.

if you step down you will create back pressure and create a bottleneck even if its at the muffler. so it wont be the most efficient system (pulses backing up into each other) . i cant say what kind of losses you would see but im sure anything larger than stock is acceptable. id say if u step down youl prolly lose some up top with the backpressure, esp with turbo. how much? who knows.

Notorious_V.I.C said:
You don't need 3inch in my opinion unless your trying to make large amount of power. You will lose low end torque. The reduction shouldn't have much of back pressure, the car isn't pushing a lot of boost is it?

I have a straight 3 inch from the turbo. Going to go down to 2.50" and have the axle portion go under the suspension like Apexr1's exhaust but with a more stock looking muffler.

msp35 · Oct 20, 2006

yashart@work said:
How does it sound? Noticably louder?

Not so much. a nice low tone @ speed, @ WOT it sounds damn good. I think that there is power to be freed up by removing the stock Catback. But that will have to wait. If I was really motivated I guess I could find a dyno for before and after removal of the stock catback.

Notorious_V.I.C · Oct 20, 2006

What size diameter is the stock MSP exhaust 03.5 piping? And the axleback part? Do you know if the stock muffler has a 2.50" inlet? I just asking because this would save me quite a bit of money on re-doing my exhaust. I don't plan to and can't afford to go, forged and plus AI not going to boost any higher then 10psi anything lower especally with a 3" turbo back won't help much
because its not producing much power...right?

terbow said:
it is correct you will lose low end tq, but the reason its there on the stock system is cause its SO restrictive its ramming the exhaust pulses together (remember exhaust isnt just a stream, its pulses). the engine has to work harder. when you have a larger exhaust the engine doenst work as hard cause your not slamming the pulses into each other.

if you step down you will create back pressure and create a bottleneck even if its at the muffler. so it wont be the most efficient system (pulses backing up into each other) . i cant say what kind of losses you would see but im sure anything larger than stock is acceptable. id say if u step down youl prolly lose some up top with the backpressure, esp with turbo. how much? who knows.

Striker187 · Oct 20, 2006

info is in here: http://www.mazdas247.com/forum/showthread.php?t=110722&highlight=exhaust+faq

incidentally, it's best to keep the same size piping throughout the system so as not to disrupt exhaust flow. if you go 3", stay with 3". if you go 2.5", stay with 2.5". if you really wanna learn more, get a copy of corky bell's book on turbocharging.

terbow · Oct 20, 2006

i can measure it when i get home.

Notorious_V.I.C said:
What size diameter is the stock MSP exhaust 03.5 piping? And the axleback part? Do you know if the stock muffler has a 2.50" inlet? I just asking because this would save me quite a bit of money on re-doing my exhaust. I don't plan to and can't afford to go, forged and plus AI not going to boost any higher then 10psi anything lower especally with a 3" turbo back won't help much
because its not producing much power...right?

yashooa · Oct 20, 2006

I love my full 3'' GHL. Some mad Hebrew sold it to me. I wonder who that was? (glare)

HEBREWBIAN!

3.0" to 2.5" Exhaust - Bad flow characteristics?

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