Discussion on exhaust sizing, let er rip!

[RAudi Driver]

Well since the B6 picture thread isn't the right forum for the discussion that started to take place, I decided to move it over here. Enjoy, flame on and lets not insult anyone's mother this time.

It is my belief that a 2.25" system is the optimum size for our cars. For the record, my reasoning isn't because APR, AWE and Milltek do it like another member insinuated. It's because I have spent many years (13) tuning 5.0 Mustangs and know how exhaust systems work on V8's and on cars in general, as I'm sure many of my fellow ziner's know also.

http://www.audizine.com/forum/showth...=200766&page=8

[MOFSTEEL]

Im willing to put my 2.5" to the test.
In the near future I plan to throw it on the dyno and see what numbers it puts down. Thing is I dont have a base run so I dont know what good that'll do me.
Not every s4 will put down the exact same hp from the factory.
Anyway, it will be interesting to see how to 2.5 compares with the 2.25 and 2.75.
I too come from American muscle or from the pony cars as most people call them. I dont think you could say you're going to lose hp/torque with a larger diameter exhaust just because its a v8. It really depends on exhaust configuration/mufflers/resonators. Lots of factors.
No wonder you and I butt heads Raudi.
You're a Ford guy and im a Chevy guy.

[tadhgbrosnan]

Autospeed split the difference with their system and made it 2.38".

[D.K]

Though I haven't changed exhausts on my S4, my belief is that by going to a 2.5" diameter exhaust, you will lose some HP/TQ in the lower RPMs, and gain more in the mid to upper range. Being that we are dealing with a V8 engine, I don't believe that the down low losses will be noticible to me, however, those that curb most of their driving to stay within the 2K RPM range and under would most likely want to stay with the factory sized piping.

IMHO, 2.75" piping would be to large without major modifications, especially since the S4 is set up with a true dual system.

[RAudi Driver]

Im willing to put my 2.5" to the test.
In the near future I plan to throw it on the dyno and see what numbers it puts down. Thing is I dont have a base run so I dont know what good that'll do me.
Not every s4 will put down the exact same hp from the factory.
Anyway, it will be interesting to see how to 2.5 compares with the 2.25 and 2.75.
I too come from American muscle or from the pony cars as most people call them. I dont think you could say you're going to lose hp/torque with a larger diameter exhaust just because its a v8. It really depends on exhaust configuration/mufflers/resonators. Lots of factors.
No wonder you and I butt heads Raudi.
You're a Ford guy and im a Chevy guy.

lol

[RAudi Driver]

Though I haven't changed exhausts on my S4, my belief is that by going to a 2.5" diameter exhaust, you will lose some HP/TQ in the lower RPMs, and gain more in the mid to upper range. Being that we are dealing with a V8 engine, I don't believe that the down low losses will be noticible to me, however, those that curb most of their driving to stay within the 2K RPM range and under would most likely want to stay with the factory sized piping.

IMHO, 2.75" piping would be to large without major modifications, especially since the S4 is set up with a true dual system.

Factory id on an S4 is 2.0". I agree with you in the fact that 2.75 is way too big and a 2.5" system wil lose you some torque. We use to do so much testing on 5.0's with stock, ported stock heads, aftermarket stock heads and supercharged applications and really came to the conclusion that one should never bump up their exhaust id more than .25", if for the most part they remain stock. Aftermarket heads and superchargers are another story but we are not delaing with these on our cars. At least not yet.

Mof it would be great to see some real world numbers on a 2.5" system on an S4. Hope you get a dyno done in the future. BTW, would you ever trade your S4 for the new Camaro?

[D.K]

Factory id on an S4 is 2.0".

Wow! It's only 2" in diameter? Is that all the way through (excluding exhaust manifold primaries), or are the DPs larger?

Sorry for the questions...I'm trying to learn as much as possible about this car.

[ZONAWILDCATS4]

Something I found... of course this isn't specific to our engines but you get the idea.

Exhaust pipe size:
As a rule of thumb, a 2 inch dual exhaust is fine for a mild 215 engine, a 2 1/4 inch diameter is good for up to about 300 cubic inches, while a 2 ½ inch exhaust is good for a highly tuned 350. A 3 inch exhaust is better for larger performance engines. In theory, using too large of an exhaust can decrease low end torque because the increased gas velocity of a smaller pipe helps scavenging.



Our engines are 254 cubic inches.

[channad]

^^^ did that just end the discussion?

[Justincredible]

Food for thought The RS4 has a 2.5 inch exhust on it

[Stormin]

^Which is what started this whole thread...

[channad]

that is true...it has more power but the same size engine...

[MOFSTEEL]

Pulled this from Ecs Tuning. Corsa is a 2.5" exhaust.

[gneboardin]

hey look what I started haha

[gneboardin]

that is true...it has more power but the same size engine...

Since this all came up because of me putting the 2.5'' Miltek RS4 exhaust S4 on my car I feel I should chime in. This was kind of my point. The RS4 exhaust is 2.5 stock from the factory for the same size engine. Also we're only talking .25'' difference. I definitely agree going from a 2.25'' to a 3'' would be stupid, but I'm hardly destroying my peformance by going + .25''

I haven't had a chance to drive it at length yet becuase it's in the garage and I'm travelling for work but I'm happy with it in the driving I have had a chance to do. I'd be happy to use mine as a bench mark and get some numbers at some point also.

[Justincredible]

^Which is what started this whole thread...

Sorry I didn't know that.

From all the Dyno info from both the 4.2 and the 4.6 (ford) both cars seem to pickup power and not loose Tq when stepped up to 2.5. With the advent of the X pipe this may..MAY be some of the reason for not loosing Tq.

Although math tells us that 2.25 should be good I don't know the parameters in witch testing showed 2.25 was good enough. We have a red line of 7k this might skew things also the possibility that new smaller motors are more powerful then the possible test info may suport. I am only speculating the above info as a reason for the 2.5 to still make a power improvement over the 2.25

For me Im going to go with the 2.5 and a X pipe. The RS4 has a X pipe in it as well. I guess I would just rather have a little more than what I need than a little less.

[Stormin]

LOL... Which is why I am going to modify my B5 S4 and leave the B7 25th completely stock.

At least with the FI I know that 3" is a great plan.

[MOFSTEEL]

^^Corsa has an H-pipe design and still has an increase in HP/torque.
Here is a dyno of an AWE 2.5" exhaust also with H-pipe design on a B6 s4 and its numbers are very similar to those of the Corsa on the B7.


CONCLUSION: You will not lose HP/Torque with a 2.5" exhaust on a B6/B7 s4.
So the hypothesis that this size tubing would decrease HP/Torque is false.

[channad]

^^^^ the new A.W.E. exhaust is 2.25" - the website is incorrect...that dyno sheet is for a 2.25" system

Here's what Todd at A.W.E had to say from another thread:

All the tubing in our system is 2.25". Dual 2.50" would be inappropriate for the power level.

Heck, our Twin1/Twin2 exhaust for the B5 S4 is dual 2.25" and we can do over 500 crank hp through them.

http://audizine.com/forum/showthread...ht=awe+exhaust

[Tek4ever]

I asked Danny at F.I. prior to buying and wish I understood everything he said. What I understood was he was able to get better performance out of 2.5...and that the 2.5" pipe is tougher to make. I'm going to send him and email regarding this post and hopefully he chimes in.

[MOFSTEEL]

^^^^ the new A.W.E. exhaust is 2.25" - the website is incorrect...that dyno sheet is for a 2.25" system

Here's what Todd at A.W.E had to say from another thread:

Yeah, the website said 2.25" initially but then it showed 2.5" under the dyno results.

I don't think he could say 2.5" is "inappropriate" when the Corsa exhaust shows almost exactly the same gains as the 2.25".

[Justincredible]

Danny At FI They use 2.5 that is all I need to hear for me that is the nail in the coffin

[channad]

I don't think he could say 2.5" is "inappropriate" when the Corsa exhaust shows almost exactly the same gains as the 2.25".

well, you do have a point there...

[RAudi Driver]

Since this all came up because of me putting the 2.5'' Miltek RS4 exhaust S4 on my car I feel I should chime in. This was kind of my point. The RS4 exhaust is 2.5 stock from the factory for the same size engine. Also we're only talking .25'' difference. I definitely agree going from a 2.25'' to a 3'' would be stupid, but I'm hardly destroying my peformance by going + .25''

I haven't had a chance to drive it at length yet becuase it's in the garage and I'm travelling for work but I'm happy with it in the driving I have had a chance to do. I'd be happy to use mine as a bench mark and get some numbers at some point also.

You're not going up just 1/4". You are going up 1/2" which is a bit much in my opinion. The stock system is 2.0".

Danny At FI They use 2.5 that is all I need to hear for me that is the nail in the coffin

Danny is not God. He's got great craftsmanship and is talented, but I think that companies like Magnaflow, Milltek, APR, AWE, and Flowmaster have more money for R&D than Fast intentions and they all suggest not going to large, a 1/4" maximum. If you've seen some of the cars at Fast Intentions, you would realize why they go big. Their cars are mega horsepower. You don't need the 2.5" on your S4.

Now some might say that the RS4 has a 2.5" system. That's true but it makes it's power up top in the high rev range and does not feel very torquey down low. I imagine that it loses a little torque to be able to make extra up top when it's doing 8k rpms.

Let the debate live on. It's all good guys.

[ChronicTeutonic]

The RS4 has a X pipe in it as well.

Just to set the record straight Audi publications state that even though the RS4 system has a common center muffler the exhaust gases still flow through separate pipes. So a stock RS4 has no X pipe.

[ChronicTeutonic]

Here is an interesting article discussing the pros and cons of the x-pipe vs h-pipe thing from Car Craft magazine. Sorry about the Ford content Mof http://www.carcraft.com/techarticles...ion/index.html

[WinterRunner]

Here is an interesting article discussing the pros and cons of the x-pipe vs h-pipe thing from Car Craft magazine. Sorry about the Ford content Mof http://www.carcraft.com/techarticles...ion/index.html

Very interesting indeed.

[Justincredible]

Just to set the record straight Audi publications state that even though the RS4 system has a common center muffler the exhaust gases still flow through separate pipes. So a stock RS4 has no X pipe.

The RS4 dose have a X pipe so do almost all other Audi's such as even the B5 S4.Our cars have the X and I saw it on the RS4. The X is part of the exhaust pipping it is not over exaggerated such as the aftermarket X would be. If you look at a RS4 exhaust it just looks like a section where the 2 pipes are welded together

Danny is not God.

I know that. But fact that they do it is just one more reason why I am doing it. thus the nail in the coffin statement. Them along with the other info form the bigger companies with real testing put me in place to feel 2.5 was not a bad way to go

[ChronicTeutonic]

From Audi Self Study Publication 921603

Audi RS4
To enhance the sporty character of the RS4 engine, a fantype
manifold is used in the RS4.

Good separation of the exhaust pulses is ensured by
keeping the individual exhaust pipes apart until they
merge into one. These fan-type manifolds are also
attached by means of a clamping flange system.

The primary and main catalytic converters are metal-type
catalytic converters. Their advantage is that they have a
lower flow resistance than ceramic catalytic converters,
which is good for engine performance.

The front silencer for each of the exhaust pipes is housed
in a common casing. However, the exhaust pipes are kept
separate. The front and rear silencers are absorption-type
silencers. They are notable for their low flow resistance.

[Justincredible]

Well I must stand corrected. Thanks for the proper correction. I need to look closer next time. now I kinda feel like a A$$. Not only that but It makes me question the use of the X or H pipe need now.

This is what the X Pipe looks like on our cars this pipe is from a 2.7 tho. If you cut the section shown you will see they both merge into one section and then separate

[ChronicTeutonic]

. now I kinda feel like a A$$.

It's cool. At least you didn't say you need to pull the S4/RS4 V8 engine every 100k miles to change the muffler bearings and replace the timing belt

Just trying to keep AZ a reliable place for information

[MOFSTEEL]

Well Corsa is not exactly a small company and I think the proof is in the numbers. I don't see how you can argue with a dyno graph.

Again I state piping size alone is not the only factor to consider.

One must not look at what exhaust piping size is optimal but what exhaust system optimal for your application.

Clearly whomever said you'll lose power with 2.5" piping is wrong.

[Justincredible]

It's cool. At least you didn't say you need to pull the S4/RS4 V8 engine every 100k miles to change the muffler bearings and replace the timing belt

LOL no its Every 12000 miles you do that everyone knows that pluss the blinker fluid

I was able to go for a ride in my friends RS4 crazy fast. I jacked up his car (it totally felt wrong using a jack on that car.) But I jacked it up to look underneath (that is how I knew about the 2.5 exhaust) but I didn't really inspect the car more kinda marveled at it. I guess in my excitement I thought I saw the X pipe



Is there a like to the study guide you quited. I would love to look threw it.

[VVG]

It's because I have spent many years (13) tuning 5.0 Mustangs and know how exhaust systems work on V8's and on cars in general, as I'm sure many of my fellow ziner's know also.

Then you also know that whether or not DP and/or exhaust produces power has a lot to do with exhaust port as well as header design and geometry. An equal length tubular header, for instance, will allow the exhaust gas pulses to flow sequentially in the order of cylinder firing. As the exhaust gas pulses won't get stacked, they will not interfere with one another. Gas flow will be much more laminar and can be well handled by even a less than optimal DP/cat-back design. In this circumstance, there probably won't be a big difference in power by exhaust pipe diameters of 2, 2.25, or even 2.5". Getting bigger than that will probably reduce exhaust gas velocity and allow flow within the pipe to become more turbulent.

The optimal exhaust, then, would have the following design:

1. Tubular equal length headers with each pipe the same diameter as the exhaust port. A ceramic coating for proper heat retention.

2. The header collector should be no larger than the sum of the cross-sectional area of the individual pipes. In theory a collector the size of even one pipe would be sufficient if the gas pulses never stacked, but such a collector is physically impossible to construct properly.

3. The downpipe should be the same diameter as the header collector

4. The cat back should be the same diameter as the downpipe

The problem that occurs is that with a less than optimal header design, the whole system falls apart. Exhaust gases never leave the engine the way they should to begin with. There is exhaust gas pulse stacking within the header and it isn't until the gases reach a freer aftermarket DP and/or cat back that the gases have room to spread out and reconfigure into a better flow pattern.

It really surprises me that there aren't more aftermarket headers for the normally aspirated Audi vehicles. It is a commonly known fact that optimizing header design is the most power producing exhaust mod that you can do.

[ChronicTeutonic]

It really surprises me that there aren't more aftermarket headers for the normally aspirated Audi vehicles. It is a commonly known fact that optimizing header design is the most power producing exhaust mod that you can do.

I would venture to guess that there are some serious space limitations and the cost to install would be rather high due to the amount of labor involved at getting to the existing header/collector. But who knows...people seem to be ready to drop a packet for an S/C setup that could be further enhanced by a good header system....

[Justincredible]

OK well after all the talk I decided to get my caliper out and dust it off. I measured the B6 S4 exhaust in 3 spots

1. as high up on the Dp as I could
2 right before the cats..both sets
3 back 2 feet after the cats

results were....


1. The Dp's had Od pipe measurements of 2.5 (it was actually just a hair more)
2 Right before the cats the Pipe shrunk to 2.25
3 and then after all the the pipe whent down to 2 inches

So in conclusion

The exhaust starts off at 2.5 then goes to 2.25 right before the cats. Then shortly after the cats the exhaust steps down one more time to 2"

My .2 if it starts off at 2.5 it should stay that way all the way back and as we have said the RS4 keeps the 2.5 profile so 4.2 might not need 2.5 on paper but it seems on Audi's paper it doesnt hurt. I already decided before this 2.5 was the way for me to go. This is just helping eliminate any doubt

[Fast Intentions]

There are a lot more factors that come into play when designing an exhaust system other than the size of the tubing. At first glance the difference between 2.25” and 2.5” may only seem small, however when you take area of flow in to consideration it’s a decent size jump, especially when referring to a complete dual system. Some aspects that are rarely touched on and often over looked are efficiency of flow and scavenging. Old school of thought is that if you want more flow you increase size, not always true! There needs to be a sense of balance to create velocity and efficiency. On many of the older 5.0 mustangs 2.25” worked for several reasons. One is when most would install a system they often eliminated the tail pipes. This not only shortens the exhaust traveled it also subtracts over 290 degrees of bends per side. (The more degree a bend has the flow restriction it carries). This all also takes an effect on what the ECU can handle, as well as valve timing. The 5.0s less advanced ECU was less able to self correct for huge swings in flow. This is why you see such huge gains with after market cams and software on them.
The Audi S4 4.2 makes 115 more horsepower with fewer cubic inches then the Ford 5.0, this would lead to it being a much more efficient engine. One of the many reasons why we started to use an x-pipe on the S4 is that of efficiency. If built correctly the X is a much more efficient design of balancing the left and right bank, and in all instances other than turbo applications this is never a bad thing. The Exhaust manifold of the RS4 a more advanced design than the ones on the S4 and Audi factory exhaust systems are nothing like efficient. This is why I believe they make no attempt to balance the system down stream. (fear of disrupting flow)
When putting an exhaust system into production a whole other side of factors are introduced to an already complicated situation. It needs to be efficient to sell. There is no doubt that a 2.25” exhaust system is easer to produce for the market place. It cost less in material, it is easier to bend, easier to fit within space constraint, easier to box and cheaper to ship. All of these things come into play in the R&D/ Cost Analysis side of the business. Contrary to belief we do spend a lot of time and money on the R&D side. Our B7 system has been in the real world testing phase for about a year now just to get it exactly where we want it with no comprise.
We have found that for the 4.2 S4 a well built system comprised of dual 2.5” is more beneficial for torque and horse power than a 2.25” system. I say well built system because we have seen plenty of lethargic 2.5” systems that due to poor construction an 1.75” system would out flow them.

I hope that this helps
Thank you,
Danny

[RAudi Driver]

There are a lot more factors that come into play when designing an exhaust system other than the size of the tubing. At first glance the difference between 2.25” and 2.5” may only seem small, however when you take area of flow in to consideration it’s a decent size jump, especially when referring to a complete dual system. Some aspects that are rarely touched on and often over looked are efficiency of flow and scavenging. Old school of thought is that if you want more flow you increase size, not always true! There needs to be a sense of balance to create velocity and efficiency. On many of the older 5.0 mustangs 2.25” worked for several reasons. One is when most would install a system they often eliminated the tail pipes. This not only shortens the exhaust traveled it also subtracts over 290 degrees of bends per side. (The more degree a bend has the flow restriction it carries). This all also takes an effect on what the ECU can handle, as well as valve timing. The 5.0s less advanced ECU was less able to self correct for huge swings in flow. This is why you see such huge gains with after market cams and software on them.
The Audi S4 4.2 makes 115 more horsepower with fewer cubic inches then the Ford 5.0, this would lead to it being a much more efficient engine. One of the many reasons why we started to use an x-pipe on the S4 is that of efficiency. If built correctly the X is a much more efficient design of balancing the left and right bank, and in all instances other than turbo applications this is never a bad thing. The Exhaust manifold of the RS4 a more advanced design than the ones on the S4 and Audi factory exhaust systems are nothing like efficient. This is why I believe they make no attempt to balance the system down stream. (fear of disrupting flow)
When putting an exhaust system into production a whole other side of factors are introduced to an already complicated situation. It needs to be efficient to sell. There is no doubt that a 2.25” exhaust system is easer to produce for the market place. It cost less in material, it is easier to bend, easier to fit within space constraint, easier to box and cheaper to ship. All of these things come into play in the R&D/ Cost Analysis side of the business. Contrary to belief we do spend a lot of time and money on the R&D side. Our B7 system has been in the real world testing phase for about a year now just to get it exactly where we want it with no comprise.
We have found that for the 4.2 S4 a well built system comprised of dual 2.5” is more beneficial for torque and horse power than a 2.25” system. I say well built system because we have seen plenty of lethargic 2.5” systems that due to poor construction an 1.75” system would out flow them.

I hope that this helps
Thank you,
Danny

Haha, I knew it was only a matter of time before you chimed in. What about a well built 2.25" system vs. a well built 2.5" system?

On another note, what size system are you running for the RS4's?

TIA

[VVG]

Haha, I knew it was only a matter of time before you chimed in. What about a well built 2.25" system vs. a well built 2.5" system?

On another note, what size system are you running for the RS4's?

TIA

I don't think Fast Intentions makes an RS4 system. Not sure that at this point it is even worth the effort. There are so few cars, and between MTM, ABT, Milltek, GMG, Magnaflow and Tubi, most of the people who want to mod their exhaust have already done so.

[RAudi Driver]

I don't think Fast Intentions makes an RS4 system. Not sure that at this point it is even worth the effort. There are so few cars, and between MTM, ABT, Milltek, GMG, Magnaflow and Tubi, most of the people who want to mod their exhaust have already done so.

What exhaust are you running on your RS?