Running a 3inch downpipe need help

[eclipseism]

Can i run a 3inch down pipe and still have stock exhuast? will i loose performance or would i gain?

 

[wvturbo2]

What is the point of running a larger downpipe back into a smaller exhaust. i think that defeats the purpose and would probably leak very bad. You should get a turbo back or do a catback, then downpipe later on.

 

[wret]

Posts with bad grammar, punctuation, spelling or capitalization do not get rep points. They get warnings. Also, it is poor etiquette to ask for rep points.

 

[Shane2GSX]

Yes, you can. But I would advise buying a full exhaust. I have a full Megan Racing exhaust and LOVE it. MR downpipes actually come with 2 adapters, one for stock exhaust and for a 3".

 

[ALLBoostAWD]

Can i run a 3inch down pipe and still have stock exhuast? will i loose performance or would i gain?

Doesn't make sense? Think about it... WTF.

Put a coffee straw in your mouth and blow as hard as you can through it. Not a lot of air flow is there?

Now put a straw in your mouth from McDonalds or something similar in size and blow through it, much more air flow.

The point is, the more restrictive your exhaust is the less air that will flow through it. being restrictive includes the diameter of the exhaust piping, which is what the staws represtented. Putting a 3" DP on a stock exhaust defeats the purpose of having attempting to imporve exhaust flow by opening one end up but restriciting the other wont help. The best way to open up your exhaust is with a full system. Downpipe, High Flow Cat, Muffler, and a ported O2 housing, and preferibly using mandrel bent piping.

 

[donmagicjuan]

Can i run a 3inch down pipe and still have stock exhuast? will i loose performance or would i gain?

I tried this for a couple days when my downpipe came in the mail before my exhaust, out of boredom I guess. Unless your downpipe has a neckdown to 2.5", which I highly doubt, it will leak, and the small performance gain won't outweigh your new unique 'rusty dump truck' exhaust note. Just save yourself the time and wait until you have a catback system. Then you can throw everything on at once and actually feel good about yourself.

 

[eclipseism]

Im going to be running a 2.5 inch exhuast, would that be enought for a 3inch down pipe?

 

[wvturbo2]

It would be much nicer to have 3" all the way through, but hey 2.5" is better than stock.

 

[JayRolla]

A lot of people like to run a 3" downpipe with a 2.5" exhaust. It still performs very well and will help with boost creep if you running a mitsu turbo. Also is a little quieter. You will be fine.

 

[Lordpaxin]

a full 3" system will flow better.

 

[nightspeed87]

personally I would go 3 in all the way back, but one of my friends has 3in dp into 2.5 catback and he loves it and the exhaust note, and ( he said something about you keep torque doing it that way ) but dont believe that, its better 3 in all the way...
And yea its pointless to put 3in downpipe on stock pipe, only way to put 3in dp without doing the rest was if you were running open dp then yea youd get a gain, if anything you always start backwards... Hit the muffler and cat back first, then highflow cat, and then dp and then o2 and manifold, you cant go wrong that way...

 

[xveganxcowboyx]

You should be able to mate the DP a smaller cat back no problem. Just get the stock size (2.5") down pipe flange from RRE and use it on the end of your DP.

 

[98gst14b]

Na it wont help much, do full exhaust or if your low on cash start from the tailpipe and work your way forward. Muffler, catback, testpipe, dp. Thats what I did!

 

[VicBoost]

I wouldn't get 3" all the way through if you are running a stock turbo because you will get mad boost creep, 2.5" will be plenty enough space for you. Lucky for me i can have a straight through pipe because here in winnipeg we don't care hehe.. so exhaust is much cheaper for me just need a straight 3" pipe and 3" DP

 

[jumpfroggy]

A lot of people like to run a 3" downpipe with a 2.5" exhaust. It still performs very well and will help with boost creep if you running a mitsu turbo. Also is a little quieter. You will be fine.

3in into 2.5in would be quieter? You mean quieter than full 3in? Also, how would a 3in downpipe help with boost creep? Someone else just posted that it would increase boost creep on the stock turbo. Please elaborate, I'm curious about this too.

I know what lots of posters are thinking, if you're going to get a 3in downpipe, why not get a full 3in? That's normal. Obviously a 3in downpipe into a 3in exhaust is going to flow more. But how does a 3in downpipe/2.5in catback compare to a 2.5in downpipe/catback? I'm curious about this too.

Especially since if you look at the turbo outlet size, o2 outlet size, and RRE's downpipe, they start out at 2.5, then go to 3in after the flex section. According to them, hot exhaust is ok for the 2.5 until you hit flex, and when it cools down you can use the 3in catback. But what about the other way around?

 

[xveganxcowboyx]

Necking down later in the exhaust adds turbulance and thus restriction.

Back pressure helps reduce boost creep. I think the reasoning is simply that the turbo has to work a little harder and thus can spin so wildly. I'm sure the back pressure also helps direct flow to the wastegate when it's open as well.

 

[donmagicjuan]

Necking down later in the exhaust adds turbulance and thus restriction.

Back pressure helps reduce boost creep. I think the reasoning is simply that the turbo has to work a little harder and thus can spin so wildly. I'm sure the back pressure also helps direct flow to the wastegate when it's open as well.

The driving force for any turbo is the temperature and pressure difference applied across the turbine wheel. Adding restrictions in your exhaust does two things: it raises the pressure as well as the temperature of the exhaust gases on the downstream side of the turbine wheel. As a result, you have less driving force to spin the turbo. Because forced induction engines are a positive feedback system, this result allows the wastegate to better regulate the speed of the turbine wheels, thereby mitigating the common problem known as boost creep.

 

[Siral3x]

As long as your work your way along , if you start with a DP it's not bad..it can't get worse then what you already have on
If you got BIG plans go BIG, a 3 inch full system is great.
Don't forget to swap that restrictive O2 housing.
I got a 2g manifold(for the moment), evo3 O2 housing, 3 inch Apexi N1 dp then continued with 3 inch setup and welded on straigh pipe( 4 mm thick steel) all they way to the back ending with a new SVT Cobra Flowmaster mufler
With a nice pair of cams the GSX at idle sounds like a V8 , at higher rmps like a bumbble bee..really nicee

 

[Siral3x]

PS: Final cost $300! not including O2 housing! Can't beat that

 

[JayRolla]

Running the 2.5" off of 3" downpipe will help boost creep compared to a full 3" exhuast

 

[white93pgtze]

waste of money if u run a 3 inch doun pipe and smaller stock exhaust piping

 

[donmagicjuan]

waste of money if u run a 3 inch doun pipe and smaller stock exhaust piping

While a 3" downpipe might cost a little more than a 2.5", this idea doesn't make much sense thermodynamically, either. Exhaust gases give up energy in the form of heat as they travel through the exhaust system. Thus, they are hottest upon exiting the turbine housing and coolest by the time they reach the tailpipe. Hotter exhaust gases will flow more easily than cooler ones, meaning that they require a smaller pipe diameter for the same flowrate at a given difference in pressure. With that said, it is counterproductive to run a larger pipe in the beginning of the system, followed by a smaller pipe toward the end. A solution with much better flow characteristics would be the reverse, or just a constant-diameter system.

 

[PieEyedPiper]

While a 3" downpipe might cost a little more than a 2.5", this idea doesn't make much sense thermodynamically, either. Exhaust gases give up energy in the form of heat as they travel through the exhaust system. Thus, they are hottest upon exiting the turbine housing and coolest by the time they reach the tailpipe. Hotter exhaust gases will flow more easily than cooler ones, meaning that they require a smaller pipe diameter for the same flowrate at a given difference in pressure. With that said, it is counterproductive to run a larger pipe in the beginning of the system, followed by a smaller pipe toward the end. A solution with much better flow characteristics would be the reverse, or just a constant-diameter system.

I would have thought the opposite, considering hot gases expand and cool gases contract..
Can you explain why expansion and contraction has no bearing on the issue?
And what about hot gases make them flow so much better?

Not that I advocate going 3" > 2.5 or less. I'd rather go 2.5" > 3" .

 

[Lordpaxin]

basically, going full 3" wont make your t25 any faster, you need to ask yourself how big you wanna go, If atall.