Potential negative factor with third muffler bypass systems
03-19-2015, 03:59 PM
#1
Instructor
Thread Starter
Potential negative factor with third muffler bypass systems
I wanted more sound from the exhaust, so I recently purchased an aftermarket third muffler bypass. I tried to install it, but my curiosity made me hesitated.
I am not a technician nor an engineer by career, but I have a MEng in mechanical engineering in aerospace and am affiliated with IMechE. I am not very handy, but I work on cars at least once a month. I am not a professional, and please forgive and correct me if I am wrong. I have spent almost half an hour to reorganize my thoughts before making this thread, so please be patient with me.
Topic: Does replacing the factory third muffler with an aftermarket bypass hurt the aerodynamics originally designed by Porsche MS on the 991 GT3?
More specifically, I am believing the factory third muffler is part of the diffuser system on the 991 GT3. As quoted from Porsche, “The aerodynamics have also been enhanced by the new underbody paneling with Diffuser function at the rear.” If my memory serves me right, the 997 GT3 doesn’t have the metal underbody paneling like the 991 GT3 underneath the rear axle.
Notes:
What is likely to happen when we replace the factory third muffler with an aftermarket bypass?
When an aftermarket bypass is installed, dimensions and shape of the bypass tubing will not match the line and flow with the factory designed inclining underbody paneling. As a result, a flow separation will occur because of the drag caused by the hollowed area with the aftermarket bypass. Note number 4 is violated, and the factory designed diffuser function will no longer work.
Although the 997 GT3 and 991 GT3 third mufflers are pretty much interchangeable, Porsche MS has used the third muffler as a part of their new underbody paneling system to achieve diffuser function on the 991 model. The improved aerodynamics and “Considerable downforce for considerable driving stability and a drag coefficient of 0.33” are not achieved by magic but physical design.
Would installing an aftermarket bypass creates unnecessary drag without beneficial downforce and ruining the factory drag coefficient?
I hope the answer in my mind is not correct.
I am not a technician nor an engineer by career, but I have a MEng in mechanical engineering in aerospace and am affiliated with IMechE. I am not very handy, but I work on cars at least once a month. I am not a professional, and please forgive and correct me if I am wrong. I have spent almost half an hour to reorganize my thoughts before making this thread, so please be patient with me.
Topic: Does replacing the factory third muffler with an aftermarket bypass hurt the aerodynamics originally designed by Porsche MS on the 991 GT3?
More specifically, I am believing the factory third muffler is part of the diffuser system on the 991 GT3. As quoted from Porsche, “The aerodynamics have also been enhanced by the new underbody paneling with Diffuser function at the rear.” If my memory serves me right, the 997 GT3 doesn’t have the metal underbody paneling like the 991 GT3 underneath the rear axle.
Notes:
- The underbody paneling of the 991 GT3 under the rear axle is angled and inclines upward to flow and match with the bottom part of the factory third muffler.
- The third muffler sits lower than the rear valance.
- Diffuser helps to create downforce by accelerating the flow in the front of its location.
- Diffuser is the section that aids high velocity airflow transiting into slower freestream.
- Diffuser provides pressure recovery by allowing the underbody airflow to decelerate and expand with a constant density.
- Diffuser cannot provide pressure recovery if there is a flow separation or drag.
What is likely to happen when we replace the factory third muffler with an aftermarket bypass?
When an aftermarket bypass is installed, dimensions and shape of the bypass tubing will not match the line and flow with the factory designed inclining underbody paneling. As a result, a flow separation will occur because of the drag caused by the hollowed area with the aftermarket bypass. Note number 4 is violated, and the factory designed diffuser function will no longer work.
Although the 997 GT3 and 991 GT3 third mufflers are pretty much interchangeable, Porsche MS has used the third muffler as a part of their new underbody paneling system to achieve diffuser function on the 991 model. The improved aerodynamics and “Considerable downforce for considerable driving stability and a drag coefficient of 0.33” are not achieved by magic but physical design.
Would installing an aftermarket bypass creates unnecessary drag without beneficial downforce and ruining the factory drag coefficient?
I hope the answer in my mind is not correct.
03-19-2015, 04:12 PM
#2
Race Director
That's a very interesting question regarding aerodynamics; I'd not thought about that before. I have been told by my contact in the GT3 development team that changes to the exhaust flow and layout would very likely require changes to the ECU to insure performance is maintained given the interrelationship of all the engine systems, but I never asked about aero.
The manufacturer spends thousands of development hours on a car to achieve a result and then people make random changes to "improve" or personalize it. That's fine, as long as they recognize that there is no free lunch.
The manufacturer spends thousands of development hours on a car to achieve a result and then people make random changes to "improve" or personalize it. That's fine, as long as they recognize that there is no free lunch.
03-19-2015, 04:19 PM
#3
Instructor
OP, you are probably right but most buyers of aftermarket mufflers won't be deterred. They just want better sound. The lower weight probably causes the rear springs to be less compressed causing a fractional increase in rear height too but the changes will be too small to be discernible. I removed the Sharkwerks I fitted to my 991 GT3 because it caused an unpleasant resonance vibration at 3,000rpm. Pity - it sounded great...
03-19-2015, 04:20 PM
#4
Burning Brakes
I have had similar thoughts to you on this but am not qualified to render an opinion. That being said, I would hope that the effects would be minimal as the under tray is still in place and is the larger piece of the diffuser where the majority of effects are occurring ??
Look forward to hearing from our more learned friends on this...
Look forward to hearing from our more learned friends on this...
03-19-2015, 04:39 PM
#7
Race Director
Good points but not important for me. There are pretty good Drivers out there already tearing up the tracks with the 991 GT3 and a rear bypass. I also think Porsche is very aware that most would delete the third muffler so if it has an affect it would be very minimal
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03-19-2015, 04:40 PM
#8
Burning Brakes
Agree and Vision Motorsports will fabricate a cover for the center bypass. Looks factory. Well, actually it is a factory part, cut and welded to look like stock. I will be doing this to my RS. I'll post a pic shortly.
03-19-2015, 06:01 PM
#9
That's a very interesting question regarding aerodynamics; I'd not thought about that before. I have been told by my contact in the GT3 development team that changes to the exhaust flow and layout would very likely require changes to the ECU to insure performance is maintained given the interrelationship of all the engine systems, but I never asked about aero.
The manufacturer spends thousands of development hours on a car to achieve a result and then people make random changes to "improve" or personalize it. That's fine, as long as they recognize that there is no free lunch.
The manufacturer spends thousands of development hours on a car to achieve a result and then people make random changes to "improve" or personalize it. That's fine, as long as they recognize that there is no free lunch.
as usual, extremely well put...
03-19-2015, 07:17 PM
#11
Instructor
Thread Starter
akra cf rear valance doesn't really look too different but the material they use - carbon fiber has better heat resistance against plastic - factory. another function of the valance is to accommodate the 100mm exhaust tips better. however, the akra's ti third muffler has a similar box style as the factory, so i am guessing akra exhaust won't have an issue in this case.
03-19-2015, 09:09 PM
#12
I wanted more sound from the exhaust, so I recently purchased an aftermarket third muffler bypass. I tried to install it, but my curiosity made me hesitated.
I am not a technician nor an engineer by career, but I have a MEng in mechanical engineering in aerospace and am affiliated with IMechE. I am not very handy, but I work on cars at least once a month. I am not a professional, and please forgive and correct me if I am wrong. I have spent almost half an hour to reorganize my thoughts before making this thread, so please be patient with me.
Topic: Does replacing the factory third muffler with an aftermarket bypass hurt the aerodynamics originally designed by Porsche MS on the 991 GT3?
More specifically, I am believing the factory third muffler is part of the diffuser system on the 991 GT3. As quoted from Porsche, “The aerodynamics have also been enhanced by the new underbody paneling with Diffuser function at the rear.” If my memory serves me right, the 997 GT3 doesn’t have the metal underbody paneling like the 991 GT3 underneath the rear axle.
Notes:
What is likely to happen when we replace the factory third muffler with an aftermarket bypass?
When an aftermarket bypass is installed, dimensions and shape of the bypass tubing will not match the line and flow with the factory designed inclining underbody paneling. As a result, a flow separation will occur because of the drag caused by the hollowed area with the aftermarket bypass. Note number 4 is violated, and the factory designed diffuser function will no longer work.
Although the 997 GT3 and 991 GT3 third mufflers are pretty much interchangeable, Porsche MS has used the third muffler as a part of their new underbody paneling system to achieve diffuser function on the 991 model. The improved aerodynamics and “Considerable downforce for considerable driving stability and a drag coefficient of 0.33” are not achieved by magic but physical design.
Would installing an aftermarket bypass creates unnecessary drag without beneficial downforce and ruining the factory drag coefficient?
I hope the answer in my mind is not correct.
I am not a technician nor an engineer by career, but I have a MEng in mechanical engineering in aerospace and am affiliated with IMechE. I am not very handy, but I work on cars at least once a month. I am not a professional, and please forgive and correct me if I am wrong. I have spent almost half an hour to reorganize my thoughts before making this thread, so please be patient with me.
Topic: Does replacing the factory third muffler with an aftermarket bypass hurt the aerodynamics originally designed by Porsche MS on the 991 GT3?
More specifically, I am believing the factory third muffler is part of the diffuser system on the 991 GT3. As quoted from Porsche, “The aerodynamics have also been enhanced by the new underbody paneling with Diffuser function at the rear.” If my memory serves me right, the 997 GT3 doesn’t have the metal underbody paneling like the 991 GT3 underneath the rear axle.
Notes:
- The underbody paneling of the 991 GT3 under the rear axle is angled and inclines upward to flow and match with the bottom part of the factory third muffler.
- The third muffler sits lower than the rear valance.
- Diffuser helps to create downforce by accelerating the flow in the front of its location.
- Diffuser is the section that aids high velocity airflow transiting into slower freestream.
- Diffuser provides pressure recovery by allowing the underbody airflow to decelerate and expand with a constant density.
- Diffuser cannot provide pressure recovery if there is a flow separation or drag.
What is likely to happen when we replace the factory third muffler with an aftermarket bypass?
When an aftermarket bypass is installed, dimensions and shape of the bypass tubing will not match the line and flow with the factory designed inclining underbody paneling. As a result, a flow separation will occur because of the drag caused by the hollowed area with the aftermarket bypass. Note number 4 is violated, and the factory designed diffuser function will no longer work.
Although the 997 GT3 and 991 GT3 third mufflers are pretty much interchangeable, Porsche MS has used the third muffler as a part of their new underbody paneling system to achieve diffuser function on the 991 model. The improved aerodynamics and “Considerable downforce for considerable driving stability and a drag coefficient of 0.33” are not achieved by magic but physical design.
Would installing an aftermarket bypass creates unnecessary drag without beneficial downforce and ruining the factory drag coefficient?
I hope the answer in my mind is not correct.
03-19-2015, 09:34 PM
#13
Intermediate
Join Date: Dec 2014
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Suggested as part for SW
I agree that this will have downforce effect. Most downfoece come from the underbody design versus from wing etc. Disturbing the airflow right where it matters the most (as it exits the underbody) will definitely have impact....
I suggested this as opportunity for a new part to sell along with bypass to SW .... Would love to get one!
I suggested this as opportunity for a new part to sell along with bypass to SW .... Would love to get one!
03-19-2015, 09:43 PM
#14
Three Wheelin'
I am not an aerodynamics expert but at 150+ mph my a** says that the car feels very planted. I'm certain that removing the center exhaust probably makes a small change in aero, but I believe it's very minimal. We aren't talking about a full blown ground effect system in place with large venturi tunnel on bottom of car exiting rear skirt? Maybe at 195mph my car will shoot off straightaway like a F-18 Hornet but I won't ever see that speed in real world track stints so to me it's a non issue.
I'll take the minimal weight savings and the associated aero risk of the bypass.
I'll take the minimal weight savings and the associated aero risk of the bypass.
Last edited by sccchiii; 03-19-2015 at 10:04 PM.
03-19-2015, 09:47 PM
#15
Three Wheelin'
I agree that this will have downforce effect. Most downfoece come from the underbody design versus from wing etc. Disturbing the airflow right where it matters the most (as it exits the underbody) will definitely have impact....
I suggested this as opportunity for a new part to sell along with bypass to SW .... Would love to get one!
I suggested this as opportunity for a new part to sell along with bypass to SW .... Would love to get one!