bi22le
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| posted on 31/5/23 at 11:18 AM |
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Other ways to get noise down
All,
My car seems quite loud measuring 103db and 105db at recent track days. I need to get it down to <101db.
I have a slide in db killer which I didn't try and may or may not make a difference. I also have an additional silencer I am going to try and
bolt on in series in some kind of fashion (may not work as I don't have hard brackets to mount to).
I was thinking of other ways as well though. In particular I was thinking of adding baffles into the central perforated tube within the main exhaust
muffler. Doing so will slow down the gases earlier and force gasses to take other routes and therefore disturb pulses. I thought it was a smart idea
to add 2 or 3 of these in. Taking a look at muffler designs in google, nobody does it. The closest is to have 2 perforated tubes side by side at
angles so the gasses have to go down one and then find their way to the other tube to exit, another option which may work.
Anyone done something better and more dramatic then just slide in a db killer? I probably have a clever acoustician somewhere at my work, I might ask
them what they think is the best option.
Ill keep you all posted with my solution.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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gremlin1234
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| posted on 31/5/23 at 11:25 AM |
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could you reduce noise on the induction side?
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bi22le
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| posted on 31/5/23 at 12:25 PM |
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It's super charged and has no induction noise to note.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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nick205
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| posted on 31/5/23 at 01:27 PM |
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Does your current exhaust exhit out to the side - like mine below?
If so, would rotating the exit down towards the ground help "re-direct" some of the noise?
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Slimy38
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| posted on 1/6/23 at 07:41 AM |
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Is it worth looking at what production cars use to reduce noise? When I opened my MX5 exhaust I found the parallel tubes that you mention, but I also
found a huge amount of fibreglass wadding. What do you currently have for yours? The problem with that design is that it's physically bulky, an
MX5 exhaust runs the entire width of the car and is a big old beast compared to what you probably have now.
Also think about where the microphone is placed to do the test, I believe it's something like half a metre away at 45 degrees? As Nick points out
that means for a Seven the exhaust is pointing directly at the mic? I have seen a couple of Seven's with rear exit exhausts that also give other
options such as larger (or multiple) mufflers. But obviously that's quite major car surgery.
The test is also done at 75% of peak revs, do you have the option of reducing your rev limiter for your track time?
You mention no induction noise but doesn't the charger itself has the characteristic 'whine'? Could you add anything under the bonnet
to reduce that?
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bi22le
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| posted on 1/6/23 at 12:38 PM |
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I am taking inspiration from production car and aftermarket muffler designs. Its quite interesting reading actually!!
I currently have the standard style se7en exhaust, perforated tube with wadding wrapped around it down the middle of an outer can. Known as an
absorption muffler.
I could turn the end cap down a little, that may help.
There is no whine from a rotrex, its all overpowered by the exhaust noise.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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coyoteboy
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| posted on 1/6/23 at 01:15 PM |
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Use an app on your phone to run an FFT to get the noisiest frequency and add a resonator to cancel it. If it's just broadband noise, just another
absorber to drag it all down inefficiently.
[Edited on 1/6/2023 by coyoteboy]
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nick205
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| posted on 6/6/23 at 09:43 AM |
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MOT "Advisories" for my MK Indy included
"loud exhaust note,in keeping with type of vehicle."
Never a fail though, just an advisory. The MOT guys seemed to enjoy having a different vehicle to MOT (not their bread and butter tin tops).
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coyoteboy
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| posted on 7/6/23 at 09:41 AM |
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Unfortunately there's no scope for leniency on track day volume tests!
Plenty of ghost MOTs happening even on tin-tops that are a bit more unusual, but none of that helps when you roll round a track and get pulled off!
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bi22le
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| posted on 7/6/23 at 10:53 AM |
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quote:
Originally posted by coyoteboy
Use an app on your phone to run an FFT to get the noisiest frequency and add a resonator to cancel it. If it's just broadband noise, just another
absorber to drag it all down inefficiently.
[Edited on 1/6/2023 by coyoteboy]
This point was not ignored and was spot on with my thinking. The problem being is static testing is obviously at a different frequency to that on
track. So really adding another absorber is the only wholesale way of getting static and drive by noise down.
I am considering to modify the perforation route see if I can run parallel tubes for example. I'll see how it can all get packaged up.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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obfripper
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| posted on 7/6/23 at 07:29 PM |
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There's this secondary silencer that Mcmillian motorsport supply for the 420r caterham, is supposed to be good enough to pass brands hatch drive
by noise limits, reducing drive by noise by about 6db.
Looks like the sort of thing you're thinking of.
 
Description
I was looking to do something similar as with a repacked silencer I was below the 105db static limit at Llandow, but was told the drive-by at wot was
up around 100db and would need something done to get through next time, they were good enough to let me finish the day before mentioning it.
I was also going to try a spiral baffle within the perforated pipe first, which should break up the pulses but not increase the backpressure
significantly.
Something like the following picture:
https://www.grantuk.com/media/2414/vbs05.jpg
I don't know how significantly this might affect the power but is worth a try if I can find a source of the correct size spirals, or a way of
making them. I guess they are drawn through a die, but I might be able to do similar with the lathe and some parallel rollers.
I have previously used a universal dB killer insert (also at llandow at a previous visit) which worked for the noise, but it absolutely sapped engine
power, i was loosing about 10-15mph on the straight.
Dave
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coyoteboy
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| posted on 7/6/23 at 07:48 PM |
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Add a spiral baffle on a bearing, connected by a shaft to another spiral baffle. Put them in two close fitting housings and feed the output of the
cold spiral baffle into the intake.
[Edited on 7/6/2023 by coyoteboy]
[Edited on 8/6/2023 by coyoteboy]
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bi22le
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| posted on 7/6/23 at 08:09 PM |
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quote:
Originally posted by obfripper
There's this secondary silencer that Mcmillian motorsport supply for the 420r caterham, is supposed to be good enough to pass brands hatch drive
by noise limits, reducing drive by noise by about 6db.
Looks like the sort of thing you're thinking of.
Description
I was looking to do something similar as with a repacked silencer I was below the 105db static limit at Llandow, but was told the drive-by at wot was
up around 100db and would need something done to get through next time, they were good enough to let me finish the day before mentioning it.
I was also going to try a spiral baffle within the perforated pipe first, which should break up the pulses but not increase the backpressure
significantly.
Something like the following picture:
https://www.grantuk.com/media/2414/vbs05.jpg
I don't know how significantly this might affect the power but is worth a try if I can find a source of the correct size spirals, or a way of
making them. I guess they are drawn through a die, but I might be able to do similar with the lathe and some parallel rollers.
I have previously used a universal dB killer insert (also at llandow at a previous visit) which worked for the noise, but it absolutely sapped engine
power, i was loosing about 10-15mph on the straight.
Dave
I actually have something like this already but I have never used it. I'm going to see a fabricator tomorrow to see what they can do to mount it
as it's my best bet
The spiral, albeit fixed may also do something.
I also have a dB killer but don't want to use it as it will kill power. Mind you the bend I have to bring the side exit back up to the second
absorber muffler is very tight.
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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