Why does 3" exhaust neck down to 2.5"?
08-11-2011, 10:50 AM
#1
Burning Brakes
Thread Starter
Why does 3" exhaust neck down to 2.5"?
This question could be directed at OE or aftermarket exhaust setups.
My understanding is that a factory Z06 has a 3" exhaust until the muffler inlet and then it necks down to 2.5". Why? And what would be the difference if it necked down to 2.5" after the cats and then included a base style 2.5" midpipe?
If I wanted to add 1 7/8 headsers to my LS2. The collector size would be 3", right? I have seen midpipes that are 3 x 2.5, I am guessing this means it reduces at the midpipe so it will match the factory 2.5 pipes? But if I went with a 3" midpipe Z06 style muffler, it would still neck down to 2.5 at the muffler, right?
I guess my main questions are:
What is the point of having 3" exhaust anywhere in the system if it necks down to 2.5?
From a performance standpoint, does it matter where the reduction to 2.5 occurs in the system? Closer to the engine or muffler?
Sorry for all these questions but my experience with exhaust is, if you have 3" then you have it all the way back. Same with 2.5.
Thanks
My understanding is that a factory Z06 has a 3" exhaust until the muffler inlet and then it necks down to 2.5". Why? And what would be the difference if it necked down to 2.5" after the cats and then included a base style 2.5" midpipe?
If I wanted to add 1 7/8 headsers to my LS2. The collector size would be 3", right? I have seen midpipes that are 3 x 2.5, I am guessing this means it reduces at the midpipe so it will match the factory 2.5 pipes? But if I went with a 3" midpipe Z06 style muffler, it would still neck down to 2.5 at the muffler, right?
I guess my main questions are:
What is the point of having 3" exhaust anywhere in the system if it necks down to 2.5?
From a performance standpoint, does it matter where the reduction to 2.5 occurs in the system? Closer to the engine or muffler?
Sorry for all these questions but my experience with exhaust is, if you have 3" then you have it all the way back. Same with 2.5.
Thanks
08-11-2011, 12:15 PM
#2
Premium Supporting Vendor
This question could be directed at OE or aftermarket exhaust setups.
My understanding is that a factory Z06 has a 3" exhaust until the muffler inlet and then it necks down to 2.5". Why? And what would be the difference if it necked down to 2.5" after the cats and then included a base style 2.5" midpipe?
If I wanted to add 1 7/8 headsers to my LS2. The collector size would be 3", right? I have seen midpipes that are 3 x 2.5, I am guessing this means it reduces at the midpipe so it will match the factory 2.5 pipes? But if I went with a 3" midpipe Z06 style muffler, it would still neck down to 2.5 at the muffler, right?
I guess my main questions are:
What is the point of having 3" exhaust anywhere in the system if it necks down to 2.5?
From a performance standpoint, does it matter where the reduction to 2.5 occurs in the system? Closer to the engine or muffler?
Sorry for all these questions but my experience with exhaust is, if you have 3" then you have it all the way back. Same with 2.5.
Thanks
My understanding is that a factory Z06 has a 3" exhaust until the muffler inlet and then it necks down to 2.5". Why? And what would be the difference if it necked down to 2.5" after the cats and then included a base style 2.5" midpipe?
If I wanted to add 1 7/8 headsers to my LS2. The collector size would be 3", right? I have seen midpipes that are 3 x 2.5, I am guessing this means it reduces at the midpipe so it will match the factory 2.5 pipes? But if I went with a 3" midpipe Z06 style muffler, it would still neck down to 2.5 at the muffler, right?
I guess my main questions are:
What is the point of having 3" exhaust anywhere in the system if it necks down to 2.5?
From a performance standpoint, does it matter where the reduction to 2.5 occurs in the system? Closer to the engine or muffler?
Sorry for all these questions but my experience with exhaust is, if you have 3" then you have it all the way back. Same with 2.5.
Thanks
With that said, from a performance standpoint there can be good reason to decrease the diameter of an exhaust system as it gets closer to its exit point. The simple concept is that as exhaust gasses move further away from the motor they cool dramatically and as the temperature of these gasses decrease, so does their velocity. Furthermore, as the exhaust gasses cool, density increases. Basically, the gasses become slower and more dense. Decreasing the diameter of the exhaust at the correct point in the system can help maintain the velocity of exhaust gasses to ensure maximum flow. Think of it like putting your thumb over the outlet of a garden hose, you decrease the diameter and increase the velocity. The goal is to get the exhaust gasses out of the tailpipe as quickly as possible. And yes, it does matter where in the system this reduction occurs. If it occurs too close to the motor, where exhaust gasses are hot and less dense, it will cause back pressure. If it occurs too far from the motor, the gasses will have already cooled and slowed way down meaning back pressure will have already been created.
It's important to note that this is not appropriate in all circumstances and may not be the reason why it exists relative to your specific question.
I hope this helps answer your question. Cheers.
08-11-2011, 12:15 PM
#3
Safety Car
Z06 mufflers are 2.5" on the inside, so they need to neck down for that.
I have not researched why the piping is bigger, but the closer you are to the engine, the hotter the exhaust. Since PV=NrT, I assume that piping near the mufflers can be smaller diameter because the pressure will be lower due to lower exhaust gas temps.
I would run 3" back to the mufflers if I were doing the exhaust.
I have not researched why the piping is bigger, but the closer you are to the engine, the hotter the exhaust. Since PV=NrT, I assume that piping near the mufflers can be smaller diameter because the pressure will be lower due to lower exhaust gas temps.
I would run 3" back to the mufflers if I were doing the exhaust.
08-11-2011, 01:11 PM
#4
Z06 mufflers are 2.5" on the inside, so they need to neck down for that.
I have not researched why the piping is bigger, but the closer you are to the engine, the hotter the exhaust. Since PV=NrT, I assume that piping near the mufflers can be smaller diameter because the pressure will be lower due to lower exhaust gas temps.
I would run 3" back to the mufflers if I were doing the exhaust.
I have not researched why the piping is bigger, but the closer you are to the engine, the hotter the exhaust. Since PV=NrT, I assume that piping near the mufflers can be smaller diameter because the pressure will be lower due to lower exhaust gas temps.
I would run 3" back to the mufflers if I were doing the exhaust.
08-11-2011, 01:38 PM
#5
Drifting
Depending on the application and combination of exhaust parts there can potentially be many reasons for why this would occur. It could simply be a packaging issue, or (as you mention) a way to ensure an aftermarket part fits properly with OE equipment.
With that said, from a performance standpoint there can be good reason to decrease the diameter of an exhaust system as it gets closer to its exit point. The simple concept is that as exhaust gasses move further away from the motor they cool dramatically and as the temperature of these gasses decrease, so does their velocity. Furthermore, as the exhaust gasses cool, density increases. Basically, the gasses become slower and more dense. Decreasing the diameter of the exhaust at the correct point in the system can help maintain the velocity of exhaust gasses to ensure maximum flow. Think of it like putting your thumb over the outlet of a garden hose, you decrease the diameter and increase the velocity. The goal is to get the exhaust gasses out of the tailpipe as quickly as possible. And yes, it does matter where in the system this reduction occurs. If it occurs too close to the motor, where exhaust gasses are hot and less dense, it will cause back pressure. If it occurs too far from the motor, the gasses will have already cooled and slowed way down meaning back pressure will have already been created.
It's important to note that this is not appropriate in all circumstances and may not be the reason why it exists relative to your specific question.
I hope this helps answer your question. Cheers.
With that said, from a performance standpoint there can be good reason to decrease the diameter of an exhaust system as it gets closer to its exit point. The simple concept is that as exhaust gasses move further away from the motor they cool dramatically and as the temperature of these gasses decrease, so does their velocity. Furthermore, as the exhaust gasses cool, density increases. Basically, the gasses become slower and more dense. Decreasing the diameter of the exhaust at the correct point in the system can help maintain the velocity of exhaust gasses to ensure maximum flow. Think of it like putting your thumb over the outlet of a garden hose, you decrease the diameter and increase the velocity. The goal is to get the exhaust gasses out of the tailpipe as quickly as possible. And yes, it does matter where in the system this reduction occurs. If it occurs too close to the motor, where exhaust gasses are hot and less dense, it will cause back pressure. If it occurs too far from the motor, the gasses will have already cooled and slowed way down meaning back pressure will have already been created.
It's important to note that this is not appropriate in all circumstances and may not be the reason why it exists relative to your specific question.
I hope this helps answer your question. Cheers.
08-11-2011, 01:56 PM
#6
Burning Brakes
Thread Starter
Depending on the application and combination of exhaust parts there can potentially be many reasons for why this would occur. It could simply be a packaging issue, or (as you mention) a way to ensure an aftermarket part fits properly with OE equipment.
With that said, from a performance standpoint there can be good reason to decrease the diameter of an exhaust system as it gets closer to its exit point. The simple concept is that as exhaust gasses move further away from the motor they cool dramatically and as the temperature of these gasses decrease, so does their velocity. Furthermore, as the exhaust gasses cool, density increases. Basically, the gasses become slower and more dense. Decreasing the diameter of the exhaust at the correct point in the system can help maintain the velocity of exhaust gasses to ensure maximum flow. Think of it like putting your thumb over the outlet of a garden hose, you decrease the diameter and increase the velocity. The goal is to get the exhaust gasses out of the tailpipe as quickly as possible. And yes, it does matter where in the system this reduction occurs. If it occurs too close to the motor, where exhaust gasses are hot and less dense, it will cause back pressure. If it occurs too far from the motor, the gasses will have already cooled and slowed way down meaning back pressure will have already been created.
It's important to note that this is not appropriate in all circumstances and may not be the reason why it exists relative to your specific question.
I hope this helps answer your question. Cheers.
With that said, from a performance standpoint there can be good reason to decrease the diameter of an exhaust system as it gets closer to its exit point. The simple concept is that as exhaust gasses move further away from the motor they cool dramatically and as the temperature of these gasses decrease, so does their velocity. Furthermore, as the exhaust gasses cool, density increases. Basically, the gasses become slower and more dense. Decreasing the diameter of the exhaust at the correct point in the system can help maintain the velocity of exhaust gasses to ensure maximum flow. Think of it like putting your thumb over the outlet of a garden hose, you decrease the diameter and increase the velocity. The goal is to get the exhaust gasses out of the tailpipe as quickly as possible. And yes, it does matter where in the system this reduction occurs. If it occurs too close to the motor, where exhaust gasses are hot and less dense, it will cause back pressure. If it occurs too far from the motor, the gasses will have already cooled and slowed way down meaning back pressure will have already been created.
It's important to note that this is not appropriate in all circumstances and may not be the reason why it exists relative to your specific question.
I hope this helps answer your question. Cheers.
With reagrds to your hose analogy, my recollection of fluid dynamics says that when pressure goes up then flow goes down. Is this the same for gasses? If so then once you get past the scavaging effect of the headers, wouldn't bigger always be better for performace? Please keep in mind I am not challenging you, I just want to understand this better.
I think what you are saying is for any pratical pupose the reduction in diameter far away from the heat source has little negative impact on power.
08-11-2011, 02:32 PM
#7
Premium Supporting Vendor
It's common to think that a larger pipe will reduce backpressure because everyone knows that a larger pipe flows more air. However, in many cases this is a false assumption.
When we speak about exhaust pipe diameters (for bikes, cars trucks etc.) I would say 90% of the discussions involve diameters between 1.25" and 3" and it's very hard to imagine how fluid dynamics are affected by such small diameter changes. However, if we use an extreme example it becomes much easier to visualize and understand. Imagine a constant and consistent stream of water flowing through a garden hose. The water flows through the hose, moving away from the source and out the other end of the hose. Now imagine taking that same constant and consistent flow of water but this time move it through one of those 5' concrete sewage pipes. What will happen? The water will pool up and build upon itself and take much longer to make it out the end of the pipe. As you can clearly see from this example, a larger pipe will not always outflow a smaller pipe, it depends on the amount of air (or in this case water) being moved through the pipe. You can actually create backpressure with a larger pipe because you slow down the exhaust gas velocity, making it take longer to exit the tailpipe which inhibits the efficiency of the engine.
Another contributing factor to this phenomenon is friction. Larger diameter pipes have more surface area which creates more friction and friction slows down the exhaust gasses even further.
Pipes that are the wrong size, too big or too small, inhibit the efficiency of the engine. Bigger is not always better.
The one big caveat to this is that at high RPM, when the engine is pushing a lot of air, larger pipes can (not always) create more high end horsepower. However, it's not worth losing all that torque down low to pick up a few HP up top.
There's so much more but let's start with this for now.....
08-11-2011, 02:56 PM
#8
Le Mans Master
When I added TT's I researched and found no-one at the time had a true thru 3" system right out of the box other than APS. (I listened to 07montereyred's TT car and was sold 
). I had to have thier exhaust fab'd to fit around my A6 tranny and it wasn't a cheap exhaust upgrade by any means. But it is of very high quality, sounds mean as he!! and looks bad a$$ with the big quad tips. Not to mention it flows like a **** for my cammed/TT set up. 
). I had to have thier exhaust fab'd to fit around my A6 tranny and it wasn't a cheap exhaust upgrade by any means. But it is of very high quality, sounds mean as he!! and looks bad a$$ with the big quad tips. Not to mention it flows like a **** for my cammed/TT set up.
08-11-2011, 03:05 PM
#9
Burning Brakes
Thread Starter
You can actually create backpressure with a larger pipe because you slow down the exhaust gas velocity, making it take longer to exit the tailpipe which inhibits the efficiency of the engine.
Another contributing factor to this phenomenon is friction. Larger diameter pipes have more surface area which creates more friction and friction slows down the exhaust gasses even further.
Pipes that are the wrong size, too big or too small, inhibit the efficiency of the engine. Bigger is not always better.
There's so much more but let's start with this for now.....
Another contributing factor to this phenomenon is friction. Larger diameter pipes have more surface area which creates more friction and friction slows down the exhaust gasses even further.
Pipes that are the wrong size, too big or too small, inhibit the efficiency of the engine. Bigger is not always better.
There's so much more but let's start with this for now.....
So if I buy a full LT exhaust for my car (header-tips), it will neck down to 2.5 somewhere, correct?
What options do I have for where it necks down? My understanding is, right after the collector as with a 3x2.5 midpipe, at the end of the midipipe before it connects to the muffler pipe or the muffler pipe itself necks down going in the muffler.
08-11-2011, 04:40 PM
#10
Not an answer to the post, but there is a solution (-:
You guys can run a true three inch if you do Z06 headers all the way back/Z06 catback. Julio is doing it on my A6.... It's a pain to install and isn't a perfect fit, but can be done...
Kooks Headers Z06
3 inch offroad
Z06 Corsa 3 "
You guys can run a true three inch if you do Z06 headers all the way back/Z06 catback. Julio is doing it on my A6.... It's a pain to install and isn't a perfect fit, but can be done...
Kooks Headers Z06
3 inch offroad
Z06 Corsa 3 "
Last edited by xstang; 08-11-2011 at 04:45 PM.
08-12-2011, 01:42 AM
#12
Race Director
Member Since: Sep 2007
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C6 of Year Finalist (performance mods) 2019
If you look at the Z06 muffler, not only does it have a 2.5" inlet, the outlet is even a smaller 2.25".
When I was running a nearly full Z06 system on my LS3, I had the H-pipe reduced to 2.5" about 18" from the back. GM reduced it at the muffler because the LS7 created more volume due to both large displacement and higher RPM. The LS3 needed the reduction closer to the motor to maintain velocity.
I'm currently running LG Street headers (1.75 pipe/3" collector/2.5" X-pipe) to Z06 mufflers. Too many people think bigger is better, but that's not true when dealing with fluid dynamics where velocity is king.
Ideally, I'd like the 1.75" primary pipes to gradually taper to 1.5" at the collector and the Z-pipe to taper from 3" to 2.5".
When I was running a nearly full Z06 system on my LS3, I had the H-pipe reduced to 2.5" about 18" from the back. GM reduced it at the muffler because the LS7 created more volume due to both large displacement and higher RPM. The LS3 needed the reduction closer to the motor to maintain velocity.
I'm currently running LG Street headers (1.75 pipe/3" collector/2.5" X-pipe) to Z06 mufflers. Too many people think bigger is better, but that's not true when dealing with fluid dynamics where velocity is king.
Ideally, I'd like the 1.75" primary pipes to gradually taper to 1.5" at the collector and the Z-pipe to taper from 3" to 2.5".
02-26-2019, 11:16 AM
#13
I have been considering a LS7/z06 exhaust for my 2008 A6 LS3. Since the 2008 LS3 exhaust system is different than 2009/ and on ...where to reduce the 3 inch to 2.5 inch exhaust can not only effect hp but cost. I posit that most of the effect on the Z06 exhaust is derived at the exhaust manifold + converter head pipes. ....what if the reduction occurred right after the OEM cats versus just after the H pipe? I also think it would be really neat if someone like Magnaflow made an X pipe that had 3 inch inlets and 2.5 inch outlets.
