# 2.5" Downpipe vs. 3" Downpipe



## BrandonF (Mar 16, 2010)

I have been doing some research on downpipes. I'm confused on how A.W.E.'s R&D comes up with a 2.5" downpipe and it seems that everybody else uses a 3". 



> AWE Tuning 2.0T Exhaust Dyno Tests
> Below are comparision dyno tests as performed on our Mustang MD-500-SE AWD dyno.
> 
> All tests were performed on the same car (an 06 GTI 6speed), with controlled ambient conditions. Each test is average of 3 runs performed.
> ...


 Here is the real link. 

http://www.awe-tuning.com/pages/info/content_window.cfm?content=41 

NOTE: I have an A.W.E. catback exhaust. Would it pointless to add a 3" downpipe? If i understand this correctly, and i'm pretty sure i don't, the back pressure will be moved farther away from the turbo, thus resulting in an HP increase... Would it be worth it?


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## crew219 (Oct 18, 2000)

Bad "test" . . . more marketing than anything. 

3" works and is better all the way. The problem is that GIAC didn't recalibrate for the increased spool due to reduced backpressure in the downpipe, so the GIAC programming did not mesh well with a 3" pipe. Later on, GIAC added a 3" exhaust file to accommodate for the earlier onset and faster buildup of boost. 

FYI, I used to have the full AWE TBE with metal cat and switched to APR 3" RSC TBE. 

Dave


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## BrandonF (Mar 16, 2010)

crew219 said:


> Bad "test" . . . more marketing than anything.
> 
> 3" works and is better all the way. The problem is that GIAC didn't recalibrate for the increased spool due to reduced backpressure in the downpipe, so the GIAC programming did not mesh well with a 3" pipe. Later on, GIAC added a 3" exhaust file to accommodate for the earlier onset and faster buildup of boost.
> 
> ...


 Are you running the APR 3" pipe with the 2.5" A.W.E. catback exhaust? 

Would there be any difference running a 3" DP with a 2.5" exhaust versus a 3" Dp with a 3" exhaust?


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## [email protected] (Jul 29, 2008)

Dyno runs will easily vary between two identical runs. This is because the ECU will run different air fuel ratios and different ignition timing based on many factors such as ambient conditions, temperature of the engine, fuel temperature, intake air temperature, exhaust gas temperature and so on. 

We can dyno a car back to back to back, even letting it cool to the exact same temperature as the previous run and the runs will still show differences. 


The way we test products is to dyno several times, such as 6 to 8 times and then average the runs and throw out the highest and lowest runs. This gives a more accurate picture of what power output looks like. 

We used this method on the TSI and the FSI and found the 3.0" Exhaust preformed better than the 2.5" exhaust in every way.


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## RedLineRob (Feb 8, 2009)

APR all the way i have there 3"inch down pipe it fits excellent and performs excellent ! 


i think the 2.5 **** is only good for a stock turbo if you ever plan on upgrading it wont be happy


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## FullP (Apr 28, 2007)

Get the APR Turbo Back. 3" all the way :thumbup:


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## BrandonF (Mar 16, 2010)

RedLineRob said:


> APR all the way i have there 3"inch down pipe it fits excellent and performs excellent !
> 
> 
> i think the 2.5 **** is only good for a stock turbo if you ever plan on upgrading it wont be happy


 So i think its decided that the 3" downpipe is best, but running the 3" dp with a 2.5" exhaust seems almost counterproductive. Won't the pressure increase when the piping gets bottle necked?


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## FullP (Apr 28, 2007)

Good question?


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## asrautox (Sep 9, 2005)

BrandonF said:


> So i think its decided that the 3" downpipe is best, but running the 3" dp with a 2.5" exhaust seems almost counterproductive. Won't the pressure increase when the piping gets bottle necked?


 It's better to go 3" all the way through, but necking down to 2.5" for the muffler section isn't horrible. By the time the exhaust gasses reach the neck, they have cooled substantially, which means that the actual volume of gas that has to move through the pipe is reduced for a given pressure. The lower the volume of gas, the smalley the pipe can be..


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## audiqtr (Mar 1, 2007)

FullP said:


> Get the APR Turbo Back. 3" all the way :thumbup:


 PersonalSideNote: I'd love to get it, only if APR made it for the 2.0TQ


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## [email protected] (Jul 29, 2008)

audiqtr said:


> PersonalSideNote: I'd love to get it, only if APR made it for the 2.0TQ


 B8 A4? We do have a 3 inch system.


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## Uber-A3 (Feb 23, 2002)

BrandonF said:


> So i think its decided that the 3" downpipe is best, but running the 3" dp with a 2.5" exhaust seems almost counterproductive. Won't the pressure increase when the piping gets bottle necked?


 No the stock CB doesn't really affect performance that much. Most of the gain in the exhaust is noticed when changing the DP because you lose the cat right off the turbo flange. Especially if you stay with a K03, you probably wont notice any diff between stock and aftermarket CB's except for sound.


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