3” vs 2.5”exhaust on Q5 TDI

Hi all, this is my first post here and I’m hoping to get some input regarding exhaust piping. I know this could stir up some debate on what is better but keep opinions clean without attacking other people’s opinions or views. If you have data or videos to support your claim, please include them.


In the past I’ve had two modded STi’s that I’ve done myself so I know my way around cars very well...but this is my first time owning a diesel, let alone an Audi Diesel.


I got a Malone tune a few weeks ago and I love it! I’m getting the dpf/egr delete done and a retune soon. I’ve seen there are two reputable companies that sell the kits. One company sells theirs with 2.5” exhaust and the other company a 3” exhaust.. is the 3.0 liter diesel big enough/push enough air to benefit from a larger diameter exhaust, say 3”? Thanks for your input!
-Dee

The only thing a 3 inch exhaust will do is sound bigger and louder. Until you're getting into larger engines there's really no benefit, and even then I found they're very annoying to listen to personally.
Sounds preference though really.

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If you're just planning for software tuning, the 2.5" is plenty big enough. If you think you may want to do significant hardware mods (turbo, injectors, etc.)at some point, then 3" wouldn't hurt.

Bigger the pipe helps lower EGTs. It is a v6 and won’t hurt to put larger exhaust on right off the bat. Once the tdi bug bites you...you might not ever quit modding it.

Yourbuddysatin said:

Bigger the pipe helps lower EGTs. It is a v6 and won’t hurt to put larger exhaust on right off the bat. Once the tdi bug bites you...you might not ever quit modding it.

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This^

I'm already making my 4th exhaust to my 1.9. This time 4" because 3.5" produced too much backpressure.

Making 360+hp at 2.4 bar boost on my Q5 3.0 with stock exhaust (just downpipe) and its perfect. I would not spend money on exhaust, useless.
What i love is going fast and silent.

Stock exhaust pipes are huge already.

What is the stock exhaust size then?

Yourbuddysatin said:

What is the stock exhaust size then?

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Metric, just under 2.25".

I disagree about the exhaust, a 2.5" straight pipe will increase performance more than intake modifications other than a turbo and tune together.


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So if I go with a 2.5”, that will still be increasing the exhaust diameter compared to stock?

You're right actually, I just double checked mine. It's just under 2.5"
My straight pipe is the difference, and a high flow muffler is comparable
I remember reading a size chart somewhere stating 2.25, which is still better than most.

The Q5 3.0 TDI has a stock exhaust pipe O.D. of 63mm/2.5".

DieselDeeds said:

So if I go with a 2.5”, that will still be increasing the exhaust diameter compared to stock?

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The improvement comes with a cat back straight pipe, or high flow, muffler.
I remember reading due to my search for my own upgrade and here's a link to the difference between the logs of a stock muffler and straight pipe, which I intend to add to when I get my injectors in, and again when I get my high flow muffler installed.

http://forums.tdiclub.com/showthread.php?t=493570

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Unless you increase the fuel and air, Turbo and injection, modifying the exhaust is pointless.
IMO the exhuast is the coolest part of the overall aggression a car can have at the track but it comes LAST when it comes down to mods.
Put an EGT probe in and start logging EGT's unless your burning up, there is no point, maybe a nice tip to look hipster!

There are many myths and tested and disproved things about exhaust systems, 99.9% of the time on a ECU controlled engine powered by gasoline or e85, if you mess with any dynamic of an exhaust system, you are decreasing not only fuel economy but much worse, power. Unless a PROPER tune is done, modifying gasoline engines exhaust systems is bad and a waste of money in every regard.
Now with diesel, it’s a slightly different story.
Larger size pipe is not necessarily a good thing, you want to have a ballpark exhaust gas velocity is critical to low EGT's. If you have a 2.0L TDI stock and you put a large exhaust, it basically does nothing, it’s when you have LOTS of power and EGT issues, 1 4" exhaust is bad because it just adds more mass to the car, making it slower, and slows down the gasses, making the pipes and the gasses coming out of the head, slower and hotter, a 1.5" pipe is to the other extreme, a nice medium is a 2.5 or mild to moderate mods. I have seen EGTS go UP from larger pipes. The best thing for lower EGT's is LONG (for a diesel turbo) pipe runners going into an even longer collector, like most performance exhausts, the trade of is lower EGT's better scavenging of exhaust gases but less performance on the turbo as the combustion proses of the unburnt fuel is less in the turbo housing.

TLDR vs. don’t waste money on an overkill exhaust when you have not even spend that kind of money on a better tune or clutch etc.

I understand a catback by itself can be pretty minimal- but I do have a stage 2 tune, and will have a stage 2+ tune when my dpf and egr get removed. I’m not looking to make a loud Q5, just something with better free flowing exhaust to lower EGTs and potentiallly get a few more hp + tq. From my understanding, free flowing straight pipe on a turbo car is muuuch better for power and lower compared to a baffled muffler. If it’s too loud, I can stop by an exhaust shop and have them cut out some pipe and throw in an 18” resonator without significantly disrupting flow.

Yeah, the 3" won't hurt anything if you go that route. I think you'll be happy with either setup.

3” it up!

Thanks for the input everyone. I’ll keep you all posted on the end result!

Dee

DieselDeeds said:

I understand a catback by itself can be pretty minimal- but I do have a stage 2 tune, and will have a stage 2+ tune when my dpf and egr get removed. I’m not looking to make a loud Q5, just something with better free flowing exhaust to lower EGTs and potentiallly get a few more hp + tq. From my understanding, free flowing straight pipe on a turbo car is muuuch better for power and lower compared to a baffled muffler. If it’s too loud, I can stop by an exhaust shop and have them cut out some pipe and throw in an 18” resonator without significantly disrupting flow.

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Search a bit more, if you install a muffler in the stock location a 3" won't be too loud, however, any other arrangements are generally obnoxious and loud. Especially a straight pipe.
I have a cat back 2.5" straight pipe and that's what I posted the link to for log difference, I'm running a Malone stage 4 tune with a vnt17.
I searched and read a lot before choosing mine and intend to install a magnaflow dinner than later as I like silent and mine is a little throaty for my preference.
This subject is one you really need to view other's results as we all have our own preference, but, if you don't want loud 45 the trail piece, to prevent droning, and put a high flow muffler in the stock location.

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http://forums.tdiclub.com/showthread.php?t=394089



Dieseleux

I think i have the fastest Q5 around here , beating MTM and ABT SQ5 cars rated at 400hp they say, i run stock Q5 exhaust and low EGT
for me exhaust is last on the list.
from turbo i have 2.5" DP and this ..flows pretty well i say

When you have dual exhaust like that it decreases the restriction and you can use 2" having less resistance

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Yeah, the stock 2.5" size flows a lot after removing the restrictions. Two 2" pipes flow almost as much as one 3" pipe.

I appreciate all the input! Also, keep in mind that I’m not buying a catback yet.. just a buzzken downpipe dpf/egr delete & 2.5” or a rawtek downpipe dpf/egr delete @ 3”. I’m most likely going to keep the stock catback for 6months - 1year and later throw on a 3” magnaflow catback. Maybe later on down the road go with a bigger turbo to really capitalize on the exhaust mods. Checkout this post I found after some searching I did today.

https://forums.vwvortex.com/showthr...s-3-quot-back-to-back.#/topics/4969653?page=1

turbobrick240 said:

Yeah, the stock 2.5" size flows a lot after removing the restrictions. Two 2" pipes flow almost as much as one 3" pipe.

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Much closer to a 4.25 inch

If you're not doing the cat back then stick with 2.5" it's much easier to tie in twice.


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One 3" pipe actually outflows two 2" pipes. It's proportionate to the cross sectional area not diameter.

https://www.engineeringtoolbox.com/steel-pipes-flow-capacities-d_640.html

turbobrick240 said:

One 3" pipe actually outflows two 2" pipes. It's proportionate to the cross sectional area not diameter.
https://www.engineeringtoolbox.com/steel-pipes-flow-capacities-d_640.html

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Cross sectional area and skin friction drag. You need more cross sectional area when using dual pipes. That's why I like to use single big diameter pipe. It's also much lighter. https://www.engineeringtoolbox.com/gas-pipes-capacity-d_480.html

Markus L said:

Cross sectional area and skin friction drag. You need more cross sectional area when using dual pipes. That's why I like to use single big diameter pipe. It's also much lighter. https://www.engineeringtoolbox.com/gas-pipes-capacity-d_480.html

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Good point. I was oversimplifying the relationship. The gas chart is also more appropriate than the water chart I linked. I learned about the relationship of pipe size to flow at a young age setting up irrigation lines on the family farm.

When looking those charts keep in mind that 3" is 88.9mm and 2" is 60.3mm OD. Pipe measures are non-sense old units that used to referring ID size on mild steel pipes.

Yeah. At least exhaust tubing just uses a normal O.D. measurement. Doesn't make much difference to the numbers with relatively thin wall tubing.

one cool thing about my 3" set up is i made a reducer from the 2" output of the turbo to 3". its all strait pipe but because of the rapid upsized pipe, i can blow like 100 smoke rings a seconds all the way up to 30MPH before the oncoming air breaks them up.
smoke rings are awesome