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PostPosted: July 28, 2015, 10:56 pm 
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Hey guys,

In order to run my car at the local autoX pad, I need to be under 92dB at 50ft. That is not much. I also don't like the sound of my V6 exhaust, so quieter is better. I'm thinking I will have to toss my long-tube headers and make some shorter ones, then ????, ending with a tip aimed at the ground (or supertrapp if all else fails). header collector needs to be 2.5", but tailpipe could be 2.25" if required.

For the ???? part, I was thinking maybe 2x turbo mufflers on each side. Or a resonator, then turbo muffler.

Does anyone know of some good muffler/exhaust design resources? The 90° V6 is pretty tough to quiet down.

Thanks for any help.


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PostPosted: July 29, 2015, 2:03 am 
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Is there the length to slip an auger muffler in between the collector and the existing muffler? IIRC 2 1/2 " is the smallest diameter of those.
I don't remember wave guide theory that well (and I was only interested in public address systems when I looked at it) but. .... What about pointing the outlet down maybe 30-35 degrees and slash cutting it so the outlet is parallel to the ground (which gives it a greater area than the pipe itself) then welding a small strip (1/4" wide?) across the outlet the long way?

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PostPosted: July 29, 2015, 2:07 am 
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C10CoryM wrote:
I was thinking maybe 2x turbo mufflers on each side. Or a resonator, then turbo muffler.
Yes, I do think one of these "turbo mufflers" on either side would do the trick...Although, I would put it upstream of the resonator. :mrgreen:

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Louder exhaust is one of the down sides to supercharging. A good place to start might be looking for how much muffler people are having to run with other similarly engined cars, and maybe even try to find videos that provide some idea of what it sounds like. The general rule of thumb with muffling is that you want to maximize muffler cross sectional area vs exhaust tube cross sectional area, as well as generally maximizing muffler volume. From there it tends to be a lot of trial and error, especially since sound quality is such a subjective thing. Before throwing away the long tubes, I'd probably start with the longest, largest bodied, single muffler that will fit on each side and see if that gets quiet enough. Possibly 6 or 7 inch diameter, if not a 5x8 or 4x9. If you think straight through designs will be too loud, I know that Magnaflow (among others I'm sure) makes their XL "turbo" mufflers, including both 4x9 and 6 or 7 inch round. If that's not enough, then I'd move to shorter headers, keep the big mufflers, and add another muffler of some sort behind each as necessary.

When facing strict sound limits, albeit measured at a mere 0.5m, we found the greatest success in regards to directing the sound by pointing it skyward and extending it as much as possible above components that would reflect the sound. Goofy looking, but functional...Bonus points for installing a tractor flapper valve. Probably not very practical for your current exhaust location, but thought I'd mention it.

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Last edited by Driven5 on July 29, 2015, 9:57 am, edited 1 time in total.

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PostPosted: July 29, 2015, 8:07 am 
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Seach Jones exhaust, They have 30" and 36" long muffler core blanks, and they even have 4" Dia. Vs the typical 3.5" Dia. cores.
You want the muffler tip turned at a 45* angle down ward and angled back. [compound angle] So the sound is directed at the ground at an angle to break up the sound wave. Do not cut the out let tip straight across the tube but cut it at angle.
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PostPosted: July 29, 2015, 8:37 pm 
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Hi Cory,

I think that we need to hear a sound clip of the existing exhaust in order to make an informed decision. :D

Your car is really coming together and I'm sure that it is going to be a monster on the track with that motor!

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PostPosted: July 29, 2015, 9:18 pm 
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I think you're not looking for flow here but silencing. First thing is the noise doesn't really help power, listen to a modern motorcycle with their high power units and then look how small the outlet is on them. So a small outlet and a 90 degree bend will help by providing a marginal pressure for the internals of the muffler to work against. If you don't have a little resistance at the outlet, all the upstream stuff is sort of short circuited.

When you have a small resistance it doesn't mean the same thing as reducing flow. THe cylinders nly breath exhaust for a short period but the outlet of the pipe is available all the time - so you want the pulses to even out and turn into a more steady flow...

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PostPosted: July 29, 2015, 9:34 pm 
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Check out www.spintechmufflers.com

JMR

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PostPosted: July 29, 2015, 9:43 pm 
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I've got a Jones resonator in front of an 18" long x 5" dia Jones turbo muffler. It is loud. But in my defense, I am running a rotary engine. I did some testing a few years back comparing it to 2- parallel Harley touring mufflers. The result was less than stellar. At the end of my testing, I found one of my mufflers was shot. 5dB difference is a bunch. If there was some way to redirect the exhaust, and I had 2 good mufflers, it might have worked.

here is my testing:

viewtopic.php?f=27&t=15159&p=165441&hilit=+chuck+harley#p165441

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PostPosted: July 30, 2015, 8:31 am 
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If you can find a copy of Scientific Design of Exhaust and Intake Systems , ISBN 0-8376-0309-9. There are several methods explained on reducing the sound levels, but to be honest they would be VERY difficult to package in a Seven. The only good solution, if you are not worried about looks, is a big ass muffler. Dave W


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PostPosted: July 30, 2015, 8:52 am 
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Look at the size of motorcycle exhausts, they are not usually huge. Some bikes are made loud on purpose but look around at large twins used for cruising.

I remember some of the big 4 cylinder road race style bikes had a single exhaust outlet that was very small, you almost couldn't hear them next to your car at a traffic light.

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PostPosted: July 30, 2015, 7:09 pm 
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rx7locost wrote:
I've got a Jones resonator in front of an 18" long x 5" dia Jones turbo muffler. It is loud. But in my defense, I am running a rotary engine. I did some testing a few years back comparing it to 2- parallel Harley touring mufflers. The result was less than stellar. At the end of my testing, I found one of my mufflers was shot. 5dB difference is a bunch. If there was some way to redirect the exhaust, and I had 2 good mufflers, it might have worked.

here is my testing:

viewtopic.php?f=27&t=15159&p=165441&hilit=+chuck+harley#p165441


I also originally used Jones mufflers on mine as well, I thought with the reverse flow design they would be quiet, but not in the least, quite loud.
I ended up taking them off and selling them for a few bucks. I am currently using two Walker 1962 Chev truck mufflers, for my application that do work quite well and are reasonably quiet, still a bit loud but no louder than the Rapters on my AMX.
Personally Cory I would skip the Jones for these applications.
I was at a car show and the guy parked next to me was running a street rod with an old style hemi and 2x4's on it, his headers went into a elongated collector, on the end he had a tube with some plates that he had drilled a series of holes in, he said it could slip it on the end of the collector or off depending on what he wanted to use it for, said he made up a few tubes as it was simple and cheap, tuning was no more difficult than switching the collector cans.
Can't say what a V6 would sound like but his sounded sweet. 8)

Al

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PostPosted: July 30, 2015, 10:44 pm 
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Thanks for all the replies so far guys.

I think what I will do for now is just stuff the biggest turbo or OE type muffler where the current one is and drive it. Realistically I probably won't make it to autoX this year with it (and I shouldn't with a stupid hernia anyhow). Over winter what I will try is: cut longtubes shorter, weld a new collector on, and fit a second muffler in the gained space. After that, aim the tailpipe correctly and see where I am at. Fortunately I have the same sound meter that autoX uses so can test. The biggest problem is going to be higher rpm noise. Engine gets very loud around 4000-4500+. I'm sure I am well over 100dB. I'd love to get the tailpipes out the back but don't think it's viable. Interestingly though the Buick Regal this engine comes from only has one fairly small 2" muffler with about a 30" resonator (and cat/manifold). They are really quiet.

Comments:
-Auger insert probably won't help much, but if I am close I will try that. Thanks for reminding me of them.
-I have a perfectly good s/c, I don't need to ruin its nice stable powerband by putting silly turbochargers on :þ Pretty sure straight through mufflers of any type are going to be too loud.
-45° to ground is better than straight at ground? I figured it would be better to hit the ground and bounce up, away from the sound meter.
-Mark, my little camera has a bad mic so not much luck there. This is the only video so far: https://www.youtube.com/watch?v=30DD-Y8pBi0 At 4 seconds the sound is painfully loud. Car is now registered (hooray) so you'll have to see it in person :þ

Thanks again for the help.

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PostPosted: July 30, 2015, 11:04 pm 
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One of the bigger issues isn't the exhaust noise, but the supercharger whine. Other than a hood there's not anyway to tame that which means your exhaust needs to be quieter than most.

I can't find a pic, but I saw one built with a very long oval muffler that basically stretched the whole length of the car. It looked really nice. Actually nicer than most where we try to stuff the shortest muffler there.

I have no idea how quiet it was, but it was bound to be quieter than most.

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PostPosted: July 30, 2015, 11:22 pm 
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I have yet to hear my supercharger as the exhaust is that loud.......

Yeah, I've seen induction noise get guys. Currently I just have a filter on a tube under the hood. I need to fab up an airbox for a better filter, but should also shut it up. Probably not going to go much past stock boost levels depending on how my knock ends up, so it won't get too whiney.

I considered making my own muffler by taking some 6" tube and squashing it oval, then putting a drilled tube in the middle with lots of packing. Make it run from collector to tip. I still don't know that it would be quieter than two turbo mufflers.

Question: If top end WOT noise is the main concern, how does one pick a muffler to kill that? Seems like lots of aftermarket mufflers try for loud at idle, quiet at cruise and low load, and loud at WOT. For aesthetics I would prefer a welded muffler over a rolled muffler, but I have a feeling to get it quiet I'll need an OE style rolled muffler.

Cheers.

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PostPosted: July 30, 2015, 11:38 pm 
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C10CoryM wrote:
I'm sure I am well over 100dB


If you've got an iPhone there is a free app that turns your phone into a decibel meter. Might be useful as you experiment.

Good luck, Bill

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