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Discussion Starter #1
I want a lighter muffler than my RB. I did a flushed and bottom-relieved bumper cover recently and for the moment I have the RB off and a Straight Pipe in place. Pretty loud, but I did some measuring to get a sense of what muffling needs to be done. I want light, no-drone, and event legal along with the usual.

So far my lightest concept is: thin wall SS or alloy tubing combined with a Low-Q branch resonator followed onward with large can but short length/height resonator going straight back (left of center) or perhaps out the center.

Here is my truly temporary pipe with my instaflange™ made from an old tool...the exhaust bolts go thru the wrench. This end of the pipe mates with the RB mid-pipe into which it slightly inserts after some tweaks.



Installed it looks like this with the relieved and flushed bumper:



And here is a spectrogram of the sound...1st outside the car parked, and then some in-car. This shows about 3 seconds of data at a time, scrolling. Up and down is the frequency and the color is how loud with red/yellow being louder than say blue. See L/R labels if this is unfamiliar to you. I can see the the normal drone issues mostly around 100-200 Hz along with the rasp and SPL woes waiting near 1000 hz (for dBA tests).

[VIMEO]180611465[/VIMEO]
 

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Discussion Starter #3
how did you get your bumper to sit so flush?
I shortened it along with doing an underside cutout preliminary to a diffuser project. Which is also part of why I am redesigning the muffler system accordingly. Free up the space....and a more efficient arrangement gives me more room along with the F/R instead of side-to-side stock muffler layout.
 

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I looked into making an Helmholtz resonator a bit ago. Just don't have the cash right now to pay someone to build a chamber, and test it.
Most of the drone on my car is around 65-70mph, and occurs between 125-130hz. The spectral analyzer app I got on my iPhone showed a peak at 129hz. It's not the most high tech equipment for testing, but it gives me a general idea of what to tune the chamber for.
 

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Discussion Starter #6
Killer, please keep this thread updated. There is pretty much zero helmholtz resonator info out there for the NA/NB.
On branch resonators, those are pretty generic tech-wise. But I'm doing it in a simpler way I think will be more effective (several extra details I know from acoustics). We'll see...the theory and sims show merit. So mainly at this point I want to make it all small and light and to layout well for diffuser use.

I'll update as time permits.
 

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^^^
Good deall!

I looked into making an Helmholtz resonator a bit ago. Just don't have the cash right now to pay someone to build a chamber, and test it.
Most of the drone on my car is around 65-70mph, and occurs between 125-130hz. The spectral analyzer app I got on my iPhone showed a peak at 129hz. It's not the most high tech equipment for testing, but it gives me a general idea of what to tune the chamber for.
Bit pricey for what you get, but these are an OTS solution. Just cut and weld

http://synapseengineering.pinnaclecart.com/index.php?p=catalog&parent=23&pg=1
 

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Those have several glaring issues. I don't think they understand the tech.
They absolutely don't, watching the video in which they explain the product is cringe-worthy. It's not an ideal solution but it should work to an extent.
 

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Discussion Starter #10
They absolutely don't, watching the video in which they explain the product is cringe-worthy. It's not an ideal solution but it should work to an extent.
Yikes, stay away from junk imo. Then again much of the aftermarket is like that. Irking to me as a technical person is that some become fads based on their "merit". If you try to say something about it you can get banned. Years later the view shifts and you turn out to have been right lol. Not for me and my car hobby. With the baselines I measured I can clearly compare my exhaust solutions to other strategies.
 

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Discussion Starter #11
Sample branch resonator try out....before (red) and after (blue) Y-axis is dB, X-axis is frequency...near the arrow is the drone-zone.

 

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Discussion Starter #12 (Edited)
The above graph was at idle. Here is the sound of a straight pipe with resonators in the mid pipe...versus the same with an early branch resonator tuned for drone reduction. I don't have a loaded on the move sample yet...this was just branch-rezzed and then straight pipe by hand:

http://picosong.com/zE2r

PS - play the clip back listening thru decent speakers or headphones. Ones that have some low end...and you'll hear the droning.
 

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Discussion Starter #13
One of the versions I made. View of tailpipe exit (closest). Branch Rez goes upward and then rides along beneath the trunk floor. This one is length and Q adjustable.

 

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I looked into making an Helmholtz resonator a bit ago. Just don't have the cash right now to pay someone to build a chamber, and test it.
Most of the drone on my car is around 65-70mph, and occurs between 125-130hz. The spectral analyzer app I got on my iPhone showed a peak at 129hz. It's not the most high tech equipment for testing, but it gives me a general idea of what to tune the chamber for.
What app are you using? I'm interested to see where my exhaust site now.
 

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One of the versions I made. View of tailpipe exit (closest). Branch Rez goes upward and then rides along beneath the trunk floor. This one is length and Q adjustable.

I can't read the graph very well, but that looks like a good reduction in noise!
 

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Discussion Starter #17 (Edited)
I can't read the graph very well, but that looks like a good reduction in noise!
Yes classic branch resonator behavior. Since the graph was made, I broadened the drone zone attenuation, and gained some benefit above 600-700 hz. I can further increase attenuation in the 200-600 hz region with additional bits. The benefit in the above-drone zones will come in handy on dBA spl readings which concentrate on results near 1000 hz and up. I'm going to play around with it for a bit before seeing how much additional reduction is needed in the mid range frequencies so that I can use a more conventional / smaller / lighter straight-thru resonator in the tailpipe to deal with the higher frequencies. Or none at all! I have about 22 inches of space for that since I have the branch resonator living just past the mid-pipe flange. You have to be careful there since the middle frequencies can fail you on an SPL test however they also provide much of the character of a nice sounding exhaust system. You want the right tones, but not too loud. Some of the harmonics such as the even orders are euphonic (nicer sounding) than some of the odd orders - so we can target rather than shotgun.

So far the complete piece is at 5.5# using thick mild steel for the tailpipe and branch. I can lighten that up quite a bit using thin wall stainless or possibly aluminum for all or part. Add a light straight-thru resonator and I'll be quite light for a no-drone SCCA compliant piece. Add a bit more for proper hangers once things stabilize.

 

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Very nice!

My idea was to run a bottle style chamber, and depending on how loud it is, a canister style exhaust. They would be stacked next to each other, but oriented like the factory exhaust. That way the hangers will be in the right place.

I need to find someone with a welder and some scrap material I can borrow for a day or two, and experiment.
I wouldn't mind a little more volume at WOT, but less drone at cruise on the highway.
 

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Discussion Starter #19 (Edited)
Branch resonators done right, *end* drone. Gone. Used for generations. Many are built into mufflers, however they tend to be less effective than could be the case since they have to use small diameter passages.

I'm still fiddling with the middle frequencies a bit. Rasp, higher frequency sounds when ''on it'. Possible SPL issues at events although idle, cruise, and low rpm use is fine. So I made a resonator by modifying the outside of the existing straight pipe. It's a bit *too* effective as it is also removing some of the nice low tones. So..I will make the length shorter and the diameter larger. This will tune the attenuation to where I need it more, while reducing the effect the the deep tone ranges (which came out great so I want to retain that result). A nice thing about integrating the resonator like this is the weight reduction. Before the straight pipe and branch was a 5.5# using mild steel. Now it is at 5# total with the additional feature! Once I finalize things further I should be able to hit 3-4 pounds with better material and fewer joints.
 

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Discussion Starter #20
This is at mid/upper RPMs WOT. Nice reduction in the mids / treble ranges. But the nice sounding lower notes are getting cut too much...my DIY resonator / absorption device is too effective (not talking about the branch resonator which is in use in both cases and measured separately above). Solution - make the straight -thru resonator smaller (but in a specific manner) and even lighter.

 
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