The Exhaust Gas Flow Noise Looks - A Technical Reference/FAQ
21 May 2003, 11:14 AM
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There always seems to be plenty of threads in the technical sections asking for advice about different exhaust systems. I thought it would be good if we could compile a sort of technical reference/FAQ for this, its probably the most popular upgrade/modification carried out by us Scooby fans. Please note I am not trying to find which is the best as they all have their own place depending on application.
As a source of information I thought the companies that offer products in this range that post on Scoobynet could supply the technical data for their products, in addition all the techies that post here your input and opinions please.
Please dont turn this into a I have a 6 pipe that sounds well loud mate sort of thread. Im hoping we can get some real data from the exhaust manufacturers/supplier and from those who have tried and tested many different exhaust components, so at the end we can see what is best for our purpose.
At the moment Im not sure how best to go about compiling all the data, maybe best to see what response there is and make that decision later but if anyone has an idea then lets here it.
On the other hand maybe trying to compile this sort of information isnt possible without the facilities to test each part under controlled conditions? What do people think? There are just so many parts to consider.
Headers
Down pipe
Centre pipe
Back box
Then for each of the parts the various properties like
Gas Flow
Noise
Looks (only applicable to the parts we can see)
Cost
Material
Quality
Value for money
Connectivity between exhaust components
Guarantee
There are just loads of things to consider, but just some things that spring to mind are gas flow speeds and volumes. I imagine details like these need a more technical explanation so Im looking for someone to start us off. Noise is a big factor especially with noise restrictions on some circuits. Can someone say what the legal aspect is for noise? To lag the down pipe? If so, why? Twin dump or baffled open neck down pipe? To remove the cat or not? Should the exhaust have a resonator box?
Basically you can see where Im trying to go with so if you think its a good idea or you feel you have something technical to offer, lets here it!
As a source of information I thought the companies that offer products in this range that post on Scoobynet could supply the technical data for their products, in addition all the techies that post here your input and opinions please.
Please dont turn this into a I have a 6 pipe that sounds well loud mate sort of thread. Im hoping we can get some real data from the exhaust manufacturers/supplier and from those who have tried and tested many different exhaust components, so at the end we can see what is best for our purpose.
At the moment Im not sure how best to go about compiling all the data, maybe best to see what response there is and make that decision later but if anyone has an idea then lets here it.
On the other hand maybe trying to compile this sort of information isnt possible without the facilities to test each part under controlled conditions? What do people think? There are just so many parts to consider.
Headers
Down pipe
Centre pipe
Back box
Then for each of the parts the various properties like
Gas Flow
Noise
Looks (only applicable to the parts we can see)
Cost
Material
Quality
Value for money
Connectivity between exhaust components
Guarantee
There are just loads of things to consider, but just some things that spring to mind are gas flow speeds and volumes. I imagine details like these need a more technical explanation so Im looking for someone to start us off. Noise is a big factor especially with noise restrictions on some circuits. Can someone say what the legal aspect is for noise? To lag the down pipe? If so, why? Twin dump or baffled open neck down pipe? To remove the cat or not? Should the exhaust have a resonator box?
Basically you can see where Im trying to go with so if you think its a good idea or you feel you have something technical to offer, lets here it!
21 May 2003, 04:53 PM
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Complete chapters in performance books have been written on this!
The "facts" dictate particularly on turbocharged cars that pretty much without exception, bigger is better.
pban
I've mailed you a sample extract that may make interesting reading as a starter, then I can recommend a few good books if you want to look further into the "mechanics" of it.
As for some of the other stuff, there is no set answer, cosmetics are a personal thing as are a preference for a particular exhaust note.
Mike.
Scooby Mania
The "facts" dictate particularly on turbocharged cars that pretty much without exception, bigger is better.
pban
I've mailed you a sample extract that may make interesting reading as a starter, then I can recommend a few good books if you want to look further into the "mechanics" of it.
As for some of the other stuff, there is no set answer, cosmetics are a personal thing as are a preference for a particular exhaust note.
Mike.
Scooby Mania
21 May 2003, 08:12 PM
#3
I think there is deffo a swing towards quieter exhausts that still yield big power. SO that's one headache (pardon the pun).
The other is headers, people seem to have views every which way on that too, std, tubular, tuned length, equal length, 4-2-1, 4-1, big upipe, small uppipe.
Paul
The other is headers, people seem to have views every which way on that too, std, tubular, tuned length, equal length, 4-2-1, 4-1, big upipe, small uppipe.
Paul
22 May 2003, 02:59 PM
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I think it could be an interesting thread.
But....you will get the well mines better than this one cos of xyz brigade rather than hard 'facts' methinks....
Plus as Mike has put in his original post, it's all down to personal preference as well, rather than what / who gives the best power increases.
[Edited by TBMeech - 5/22/2003 3:00:33 PM]
But....you will get the well mines better than this one cos of xyz brigade rather than hard 'facts' methinks....
Plus as Mike has put in his original post, it's all down to personal preference as well, rather than what / who gives the best power increases.
[Edited by TBMeech - 5/22/2003 3:00:33 PM]
22 May 2003, 03:06 PM
#6
I think there is some good info about, but some of it is closely guarded. And for every person that thinks exhaust component X is great, there is one that thinks it's pants (big and fierce).
Paul
Paul
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22 May 2003, 04:21 PM
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So saying that - what is the difference between say the OE STI Back box and the Scoobysport (old style ala H&S) back box that makes the the vast difference in noise levels?
22 May 2003, 04:28 PM
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H&S will be less restrictive.
The STI box has a big bend in the pipework just before it meets the bb.
Also, the STI box will probably / almost certainly have some sort of sound wadding inside, wheras the H&S box will have a far less amount or maybe none at all.
The STI box has a big bend in the pipework just before it meets the bb.
Also, the STI box will probably / almost certainly have some sort of sound wadding inside, wheras the H&S box will have a far less amount or maybe none at all.
22 May 2003, 06:16 PM
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Paul
I've just cut open an STi7 backbox to show what happens to the gas inside.
Here's what happens:
The pipe reduces from 2.25" internal diameter to 1.75" internal diameter just after the flange and then proceeds to the silencer box.
[img]http://images.fotopic.net/?id=532448&outx=600&oq=0[img]
It enters the box through the middle level pipe and exits this through the open end and through the perforations in the centre of the tube.
It then enters the upper perforated tube and the direction of travel is reversed. The end of this pipe, between the front end of the box and the baffle, is also perforated and the gas goes through these perforations and down to the lower pipe, again changing direction 180 degrees.
From here it leaves the silencer to a pipe that has a big shiny jacket around it.
You will see that there is no wadding or packing at all. The noise is reduced by the Helmholst priciple. This is achieved by the spacing of the pipes and the location of the two baffle plates causing the sound waves to cancel each other out.
To compare this with a Scoobysport (and most other) silencers, these send the gas through a straight perforated section that is surrounded by wire wool. Around the wire wool is fibreglass quilt. The quilt absorbs the energy of the sound wave without reflecting the sound back (ever notice how there is no echo in your loft?). The pipe on a Scoobysport silencer maintains 2.5" internal diameter all the way through to the end of the box, where it tapers out to a 3.5" tailpipe.
[edit for formatting]
[Edited by Pete Croney - 5/22/2003 6:22:13 PM]
I've just cut open an STi7 backbox to show what happens to the gas inside.
Here's what happens:
The pipe reduces from 2.25" internal diameter to 1.75" internal diameter just after the flange and then proceeds to the silencer box.
[img]http://images.fotopic.net/?id=532448&outx=600&oq=0[img]
It enters the box through the middle level pipe and exits this through the open end and through the perforations in the centre of the tube.
It then enters the upper perforated tube and the direction of travel is reversed. The end of this pipe, between the front end of the box and the baffle, is also perforated and the gas goes through these perforations and down to the lower pipe, again changing direction 180 degrees.
From here it leaves the silencer to a pipe that has a big shiny jacket around it.
You will see that there is no wadding or packing at all. The noise is reduced by the Helmholst priciple. This is achieved by the spacing of the pipes and the location of the two baffle plates causing the sound waves to cancel each other out.
To compare this with a Scoobysport (and most other) silencers, these send the gas through a straight perforated section that is surrounded by wire wool. Around the wire wool is fibreglass quilt. The quilt absorbs the energy of the sound wave without reflecting the sound back (ever notice how there is no echo in your loft?). The pipe on a Scoobysport silencer maintains 2.5" internal diameter all the way through to the end of the box, where it tapers out to a 3.5" tailpipe.
[edit for formatting]
[Edited by Pete Croney - 5/22/2003 6:22:13 PM]
23 May 2003, 09:14 AM
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LOL at Mega Stream 
I don't mind cutting open an Sti one, but not a decent one Here's a CAD pic of what's inside.
[Edited by Pete Croney - 5/23/2003 9:19:34 AM]
I don't mind cutting open an Sti one, but not a decent one
Here's a CAD pic of what's inside.[Edited by Pete Croney - 5/23/2003 9:19:34 AM]
23 May 2003, 10:20 AM
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Pete
Thanks for the description and pictures, finally the thread seems to be getting off the ground. Are you sure you dont want to cut open a brand new back box? lol
Ok some more questions then to try and move on.
Would it be fair to say that most down pipes of the same design i.e. Open neck that are 3" to 2.5" will flow approximately the same amount of exhaust gas allowing for slight variations in flow characteristics? Would it also be fair to say the same about the centre pipe, both without cats or resonators? Maybe I am stating the obvious here! Do the headers have a bigger influence than the back box? I guess not as would all change these first!
Assuming the above is true then the back box is the main restriction in the system now. How much extra flow is achieved from fitting a performance back box over the OE one? I know that most people quote a BHP gain, but surely this is proportional to other modifications (induction, turbine, etc.) Could the gain be expressed as percentage?
Thanks for the description and pictures, finally the thread seems to be getting off the ground. Are you sure you dont want to cut open a brand new back box? lol
Ok some more questions then to try and move on.
Would it be fair to say that most down pipes of the same design i.e. Open neck that are 3" to 2.5" will flow approximately the same amount of exhaust gas allowing for slight variations in flow characteristics? Would it also be fair to say the same about the centre pipe, both without cats or resonators? Maybe I am stating the obvious here!
Do the headers have a bigger influence than the back box? I guess not as would all change these first!Assuming the above is true then the back box is the main restriction in the system now. How much extra flow is achieved from fitting a performance back box over the OE one? I know that most people quote a BHP gain, but surely this is proportional to other modifications (induction, turbine, etc.) Could the gain be expressed as percentage?
23 May 2003, 11:58 AM
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Downpipes... yes for open neck designs the flow rates will be close between different manufacturers. Most flow so much gas that the few percentage difference will make no difference. Go for the one thats teh best quality and fit. On twin dump versions, they could vary quite a bit. Some are 2.5" mains, some are 3", some have seperaters, some have small wastegate pipes and the location of the wastegate pipe in the flange will be very important. The flow from the wastegate is at least as important as the flow from the impeller as this will detrmine the accuracy of the boost control and so the accuracy of the high boost mapping.
Centre sections will flow very similar rates, if they are the same diameter. The flexy sections on some Jap ones will affect flow as the inside of these is normally a concertina shaped pipe.
Headers... people write books on these. 4-2, 4-2-1, 4-1, pipe diameters, pipe lengths, pipe convergence joint shapes, bend radii, cross over pipes, pulse smoothing, pulse addition... the variables are endless.
Next time I'm at Janspeed, I'll get some standard back boxes flow tested but as an example, just on pipe diameter, a 2.5" ID backbox will have the capacity to flow 43% more gas (for the same back pressure) than the STi7 one in the pictures, if assuming that there was no restriction from the gas going through the internals of the silencer box. And this is rather unlikely
Centre sections will flow very similar rates, if they are the same diameter. The flexy sections on some Jap ones will affect flow as the inside of these is normally a concertina shaped pipe.
Headers... people write books on these. 4-2, 4-2-1, 4-1, pipe diameters, pipe lengths, pipe convergence joint shapes, bend radii, cross over pipes, pulse smoothing, pulse addition... the variables are endless.
Next time I'm at Janspeed, I'll get some standard back boxes flow tested but as an example, just on pipe diameter, a 2.5" ID backbox will have the capacity to flow 43% more gas (for the same back pressure) than the STi7 one in the pictures, if assuming that there was no restriction from the gas going through the internals of the silencer box. And this is rather unlikely
23 May 2003, 01:13 PM
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Pete,
That was excellent mate, it's interesting to see what happens when a std bb is cut open to view the contents.
Theres always been a lot of talk like this for motorcycle exhausts, where MCN used to always cut exhausts open to see the way the gas can escape.
That was excellent mate, it's interesting to see what happens when a std bb is cut open to view the contents.
Theres always been a lot of talk like this for motorcycle exhausts, where MCN used to always cut exhausts open to see the way the gas can escape.
23 May 2003, 02:34 PM
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Can I have an exhaust that flows better than the standard one but sound not much louder?
Most performance back boxes are much louder than standard, is this because they can not be made/designed to flow better than standard(increase performance)and do it quietly or is it because we just want to have really noisy exhausts?
Most performance back boxes are much louder than standard, is this because they can not be made/designed to flow better than standard(increase performance)and do it quietly or is it because we just want to have really noisy exhausts?
23 May 2003, 03:15 PM
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Straight through rather than baffled I think is the answer? Straight through flows far better but doesn't do the resonance stuff. Baffled bounces around all over the place & so it dissapates.
I've been looking at all this for some time (I've always rather hoped for a silent free-flowing exhaust - so far, nothing has come up
) and it really does seem that noise level & flow pretty much go hand-in-hand. Pat's exhaust is meant to be quiet for the flow by using a much longer muffler - sound reduction is meant to be strongly related to the length of the muffler rather than width (oo-er), although results vary rather a lot here... what doesn't help with the comparison is of course that it's 3.25" diameter, which is wider than anything else so not a good comparison!
[Edited by nom - 5/23/2003 3:16:27 PM]
I've been looking at all this for some time (I've always rather hoped for a silent free-flowing exhaust - so far, nothing has come up
) and it really does seem that noise level & flow pretty much go hand-in-hand. Pat's exhaust is meant to be quiet for the flow by using a much longer muffler - sound reduction is meant to be strongly related to the length of the muffler rather than width (oo-er), although results vary rather a lot here... what doesn't help with the comparison is of course that it's 3.25" diameter, which is wider than anything else so not a good comparison![Edited by nom - 5/23/2003 3:16:27 PM]
23 May 2003, 05:15 PM
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Nom
Length is indeed what matters most to noise reduction, but so does diameter. The smaller you make the bore, the more of the sound's waveform goes through the perforated tubing and is dissipated.
Length is indeed what matters most to noise reduction, but so does diameter. The smaller you make the bore, the more of the sound's waveform goes through the perforated tubing and is dissipated.
23 May 2003, 05:30 PM
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Thatnks for that, Pete.
So, is it possible (ignoring the 'would anyone actually buy it' possibility) to make a very free-flowing yet very quiet exhaust for the sccob, or is this wishful thinking?
Lastly on the 'noise creation' front, I think it should be noted how much the headers can change the noise level as I don't think they have been mentioned? They might not reduce the volume, but they do appear to be able to not create the volume in the first place that the BB then has to remove!
So, is it possible (ignoring the 'would anyone actually buy it' possibility
) to make a very free-flowing yet very quiet exhaust for the sccob, or is this wishful thinking?Lastly on the 'noise creation' front, I think it should be noted how much the headers can change the noise level as I don't think they have been mentioned? They might not reduce the volume, but they do appear to be able to not create the volume in the first place that the BB then has to remove!
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