Custom Exhaust Pipe Sizes

[supamannn99]

So I've done some research and I've looked at many threads. This issue hasn't really been addressed or discussed with directly so I want to start one. Some people say that a custom system should have a 3" downpipe, then narrower pipes at the split is better. Anybody know why this is a good idea? Why can't we just do 3" all around? And I noticed off-the-shelf exhaust systems such as APR don't use all 3" pipes for a reason. If this isn't good, anybody know what's good pipe sizing?

[[email protected]]

I have 3" downpipe to a Y-split, then into 2.5" ID muffler (acting as resonators) then out to 3" ID mufflers.

This setup is supposed to get rid of any backpressure I had when I ran a Milltek rear exhaust section in order to help the turbo spool faster.

So unless you are running a big turbo setup a 3" exhaust is overkill and from what I remember a K03/K04 actually needs some backpressure to function better.

[supamannn99]

Is there any drone?

Thanks for the quick reply

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There is quite a bit of drone between 2800-3200rpm.

If you do a slow acceleration if sounds like you stuck your head in a tuba.

When you do a fast acceleration the sound is intoxicating.

At 4000rpm and above the exhaust is almost silent.

This is as loud as the exhaust will be and I am hoping when the rear mufflers start breaking in, if I ever get the exhaust hot enough to get them to break in, that it will quiet up a bit.

[A4addict]

So unless you are running a big turbo setup a 3" exhaust is overkill and from what I remember a K03/K04 actually needs some backpressure to function better.

I was pondering on 3" pipes also when I had my catback customized. My tuner advised me of a quick, ghetto-physics lecture of incompressible flow equation for Bernoulli's flow of gases.

Using this, he explained that 3" pipes might actually make you lose some power/torque since, like what you exactly said, the K03's actually need some back pressure to function properly.

So I went with the 2.5" pipes.

Any actual incompressible flow equation on pressure comparisons on a 3" pipe and a 2.5" pipe, maybe design engineers of APR/AWE/etc. who make these exhaust systems already figured it out and decided that using 2.5" pipes should make the most performance you can get out of these K03/K04 turbos.

[supamannn99]

Wow that's weird. What's the science behind this?

[tomasmdiego]

x2

[supamannn99]

and do you have a 3" downpipe and 2.5" the rest?

[AlexK]

I dont know how backpressure helps turbo engines but on an NA engine, backpressure helps the exhaust valves open more easily = less force required to turn the cam = better efficiency.

[supamannn99]

Anybody wanna shed some light on this?

[A4addict]

There's a reason why I selectively, purposely kicked Physics out of my life.

[DarkW!sh]

id like a dual 3 inch one on each side burnt tip but i dont know if they make them for the A4 B6's FWD TIP if that car could get any lower on the audi foodchain it would

[Vorsprung]

Using this, he explained that 3" pipes might actually make you lose some power/torque since, like what you exactly said, the K03's actually need some back pressure to function properly.

Then why does the APR system for the 2.0T in the A3 use 3" pipes
http://goapr.com/Audi/products/apr_exhaust_8pa3.html

Does the transverse mounting make a difference?

[D0lphinGrey]

I dont know how backpressure helps turbo engines but on an NA engine, backpressure helps the exhaust valves open more easily = less force required to turn the cam = better efficiency.

That sounds right. Turbos need larger exhaust pipes to get rid of backpressure whereas N/A cars want backpressure therefore need smaller exhaust pipes. Correct me if I'm wrong.

[arbaz]

That sounds right. Turbos need larger exhaust pipes to get rid of backpressure whereas N/A cars want backpressure therefore need smaller exhaust pipes. Correct me if I'm wrong.

it's not really backpressure. backpressure is undesirable in both cases. from what i've learned, in both n/a and turbo engines, you want to achieve a high exhaust velocity which is not the same as backpressure, but the methods of doing so in either engine is different.

for a turbo'd engine, since the turbo works using a pressure difference, so the goal should be to minimize the pressure behind the turbine. if that's the case, then wider should be better to the point where it's counter-intuitive. i was told that for a turbo'd engine barely pushing 200hp, this point is a 2.5"-diameter pipe. it's pointless to have a 3" pipe.

[A4B7Quattro]

I've heard for k03 or k04 the best setup is, 3 inch downpipe to a 2.5" Y pipe because the exhaust gasses are traveling faster into the 3 inch downpipe then the 2.5 inch Y pipe funnels them out of the exhaust efficiently, so basically what everyone has been saying this entire thread :P

[D0lphinGrey]

it's not really backpressure. backpressure is undesirable in both cases. from what i've learned, in both n/a and turbo engines, you want to achieve a high exhaust velocity which is not the same as backpressure, but the methods of doing so in either engine is different.

for a turbo'd engine, since the turbo works using a pressure difference, so the goal should be to minimize the pressure behind the turbine. if that's the case, then wider should be better to the point where it's counter-intuitive. i was told that for a turbo'd engine barely pushing 200hp, this point is a 2.5"-diameter pipe. it's pointless to have a 3" pipe.

Well yes, backpressure was probably not the right term to describe the theory behind this but, in general, a turbo engine wants larger size exhaust (obviously depending on the size of turbo too) to minimize pressure and increase flow whereas for a N/A engine, larger exhaust does not always mean better.

[JRMSLINEA4]

There is quite a bit of drone between 2800-3200rpm.

If you do a slow acceleration if sounds like you stuck your head in a tuba.

I enjoyed this.

[D0lphinGrey]

What size exhaust would be most efficient for a Gt2871? The car currently has a single 2.5inch pipe from turbo back then splits into 2x 2.25inch pipes after a center muffler. Will this be restrictive for a GT2871R? Should I go 3inch single turbo back into 2.5inch splits?

[Insane_Audi]

What size exhaust would be most efficient for a Gt2871? The car currently has a single 2.5inch pipe from turbo back then splits into 2x 2.25inch pipes after a center muffler. Will this be restrictive for a GT2871R? Should I go 3inch single turbo back into 2.5inch splits?

should be fine with a 28, however if your going 3076 or 35, you might be better with 3"

[D0lphinGrey]

should be fine with a 28, however if your going 3076 or 35, you might be better with 3"

My tuner thinks I want at least an 3inch dump pipe off the turbo but can keep the current 2.25inch splits. I think the APR stage 3 (gt28) kit also comes with a 3inch test pipe.

[jimrobbington]

The only audi I have ever seen a full 3" exhaust on is my shop's twin turbo b5 s4. It has rediculous turbos, and sees the track only, no street. I am pretty sure you will never need a full 3" exhaust unless you are seeing at least 600 crank HP, which our cars will never see. I looked into this when getting my own exhaust, knowing someday I hope to go BAT and not have to replace anything again. It is not necessary.

[Charles]

it's not really backpressure. backpressure is undesirable in both cases. from what i've learned, in both n/a and turbo engines, you want to achieve a high exhaust velocity which is not the same as backpressure, but the methods of doing so in either engine is different.

for a turbo'd engine, since the turbo works using a pressure difference, so the goal should be to minimize the pressure behind the turbine. if that's the case, then wider should be better to the point where it's counter-intuitive. i was told that for a turbo'd engine barely pushing 200hp, this point is a 2.5"-diameter pipe. it's pointless to have a 3" pipe.

Right..in a N/A car, the combustion process produces a certain "frequency". The key is to tune the exhaust to that specific frequency in order to maximize power. For some cars that might mean making a 2" dia exhaust, for some it might mean making a 4" dia. Length of pipe determines that as well.

And yes, in a turbo car, the ability for an exhaust to expel gas faster, enabling the turbo to spool faster will outweigh frequency tuning.

Now, where it gets sticky is prior to the turbo spooling. On a large turbo car, let's say the turbo won't spool until 4K RPMs. Prior to that, exhaust tuning for frequency is the best power maximizer, cause you don't have the turbo aiding you in power production yet. Once you hit 4K RPMs, then you want the exhaust to override frequency tuning and get the exhaust gas out ASAP as that will produce more power. Ideally, you'd have a dual exhaust that would switch at a certain point. Prior to turbo spool you'd have your frequency tuned exhaust in use, then when the turbo kicks in, you'd switch to the turbo exhaust. Obviously that's not too practical, so most large turbo owners stick with bigger exhaust as that produces the most gains.
For our cars, however, such a small turbo will spool much earlier in the RPM band so the best bet would be to stick to exhaust expelling first and not worry so much about frequency tuning.
And finally, the size of the turbo will dictate exhaust dia too. A larger turbo would need a larger exhaust cause the more exhaust you clear will enable to turbo to spool better. A K03/04..not so much...the law of diminishing returns would come into use. At some point, the bigger exhaust won't matter cause the turbo is already running at full potential. Typically, for a K03/04 that dia is 2.5" to 3". I doubt you'd see a huge difference between the two.

[Insane_Audi]

so now the question is where to find a 3" hfc..

[D0lphinGrey]

Right..in a N/A car, the combustion process produces a certain "frequency". The key is to tune the exhaust to that specific frequency in order to maximize power. For some cars that might mean making a 2" dia exhaust, for some it might mean making a 4" dia. Length of pipe determines that as well.

And yes, in a turbo car, the ability for an exhaust to expel gas faster, enabling the turbo to spool faster will outweigh frequency tuning.

Now, where it gets sticky is prior to the turbo spooling. On a large turbo car, let's say the turbo won't spool until 4K RPMs. Prior to that, exhaust tuning for frequency is the best power maximizer, cause you don't have the turbo aiding you in power production yet. Once you hit 4K RPMs, then you want the exhaust to override frequency tuning and get the exhaust gas out ASAP as that will produce more power. Ideally, you'd have a dual exhaust that would switch at a certain point. Prior to turbo spool you'd have your frequency tuned exhaust in use, then when the turbo kicks in, you'd switch to the turbo exhaust. Obviously that's not too practical, so most large turbo owners stick with bigger exhaust as that produces the most gains.
For our cars, however, such a small turbo will spool much earlier in the RPM band so the best bet would be to stick to exhaust expelling first and not worry so much about frequency tuning.
And finally, the size of the turbo will dictate exhaust dia too. A larger turbo would need a larger exhaust cause the more exhaust you clear will enable to turbo to spool better. A K03/04..not so much...the law of diminishing returns would come into use. At some point, the bigger exhaust won't matter cause the turbo is already running at full potential. Typically, for a K03/04 that dia is 2.5" to 3". I doubt you'd see a huge difference between the two.

Great post.

So what would be an ideal exhaust dia for a GT28? Its not a BIG turbo but still much bigger than a K04.

[Insane_Audi]

Great post.

So what would be an ideal exhaust dia for a GT28? Its not a BIG turbo but still much bigger than a K04.

apr sells their 2.5 inch to run with the stage 3 kit perfectly, as for test pipe you would have to ask them..