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1 1/2" primaries, there we go. ..
The exhaust gases cool as they proceed down the pipe, reducing volume of each charge. Stepped primary design uses that concept.
The exhaust pulses are just that, pulses. "puhpuhpuhpuhpuhpuh"
You can void the system at the exit as a steady flow. "pffffffffffffffffffffffffffffffffffff"
If you wanted to get really fancy you could calculate the total exhaust charge volume at redline as volume over time, subtract maybe 5% to allow for charge cooling, and then calculate exit od based on the chamber after the collector filling up 62% at a desired psi rating of backpressure. (desired psi of pressure provided by exhaust charge volume in measured volume times 62%)
Then all you (yeah riiight ALL ) you have left to figure out is diameter of the exit hole desired to maintain that pressure with the calculated charge volume.
Think of it as a pressure bleed or a metering orifice on a vacuum source, same kind of thing.
I think 62% will work well, it's a wave dynamic thing and my brain doesn't do those so easily.
Someone else will need to chime in on acceptable back pressures.

Or;

You could and use a piece of 1 1/2" pipe (to match the primary size) as an insert and capping the outer pipe except for the center hole, put 3" of pipe in the center hole with 2 3/8" on the inside.
A longer bit inside of the exhaust tube will be quieter than a short one would be so you can tune it there as well.

Or;

Just cap it and drill it at 1 3/8" then measure. If it's under the limit go bigger.
This is what I'd do.

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"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

Is the local club really that picky?

I've heard of large events having sound equipment to police competitors but I'd be impressed if a local club did the same (I-Phones don't count - they're pretty inaccurate from my experience). For how much effort you're putting into this, I'd say show up and run as you have it.

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-Andrew
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I can't imagine they would have a 92db limit if they weren't forced to be picky. That low of a noise limit just screams "pissed off litigious neighbors", and its strict enforcement is likely the only reason those folks can use the site at all.

Thanks Jack. Yeah, for some reason pulsing gasses that contract and expand in the multiple tubes are a little tough for me to figure out the flow in my head......

I guess backpressure wouldn't be a bad way to go if you knew what your max spec was. Set up a gauge, then choke the exhaust off until WOT met the spec (or sound was OK). I'm going to make a day of playing with tips/caps, and testing the sound levels. I'll start with caps with a small hole, and open up as required. Likely will do the same to a friend's V8 Camaro which is 102dB.



Bingo. We've been close to losing our pad in the past due to (questionable) noise complaints. Since the bylaws are not on our side we have to play nice. There is a sound meter going 50ft from the cars on course all day, every event. Anyone hits 92dB and they get talked to, if they can't sort it out they have to park. And, even if it's under and the bosses think it's too loud (can hear it echoing off things) you can still get pulled. Other than safety, noise levels are the most serious concern for the organizers. It sucks, but it's what it takes to keep running.

Cheers.

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Build log: viewtopic.php?t=9291

We're not really talking reducing the flow, more like evening it out. The muffler has a lot of volume and simulates the open atmosphere, the flow out of the muffler should be averaged down to a continuous flow, not a pulsed flow. You can buy reducer adapter pipes. Buy a couple of them in different reductions and put them on the end of a 90 degree elbow. That will make for a quick test setup and let you know if this is a good path to go down.

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So I stuffed my 90° tips in, and taped a 1.5" cap on the end. 102dB. Again, just in shop revving with dB meter held at shifter (which is close to air inlet). Ambient noise is 50dB if it matters. At idle it is much quieter, and it is less raspy. Starting to sound better; I am no longer embarrassed to hit the gas. I can now clearly hear my induction sucking air at idle, and probably will be loud at WOT (supercharger whine). Unfortunately this is as far as it will go for a while. Until I get surgery and heal I can't do anything (can't even drive the car). Next step is to get it (and me) driving again and do some proper 50ft testing at WOT. If it's close, I will shorten the headers and get a front muffler in place, try different tips/sizes, and make an induction muffler. If it's not close, I'll just go straight to a giant muffler bolted to the rollbar and go from there. I can always lower my rev limiter too and stay out of the nasty rpm range, but that's a last resort. It's around 93dB before hitting the bad range.

Cheers.

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Build log: viewtopic.php?t=9291

Of course, in the shop and not under load is not an accurate way to measure sound. Noise is reflected and you get higher than real numbers. It should be done in free space. For every 2x the distance, the sound level drops 6dB. So conversely, if the limit is 92dB at 50ft, the limit would be 98dB at 25 feet or 104 dB at 12.5 ft.

Recover quickly

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Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

Feel free to pull this off if you find this offensive.

But I am guessing you'll like it...

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Geek49203 aka
Tim Wohlford
Louisville, KY
Hayes front, S10 +2 rear, Lalo body.

Yeah. I agree totally. We've has a few tracks shut down here due to that. The most frustrating part is the main complainer lives directly under the active airport runway. Prop planes taking off maybe 200ft over their house, and our barely audible tire noise is too loud...... right. Basically you have to sit and listen (between aircraft) to hear our noise. As I mentioned, the bylaws are not on our side so it's a matter of keeping people happy, and making sure the city officials like us.

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Build log: viewtopic.php?t=9291

I think the issue is the pipe/passage size versus the size of the muffler and that it is a soundfx versus a quiet flow Walker. I'd cut and weld the bolt-on collector to taper down to 1.875 or 2 inch to butt weld to a muffler. To make room for a outlet turn down/out, shorten the muffler inlet and outlet also.

You might consider a Walker Quiet Flow, PN 22483 for $57, shipped. It is made to fit Toyota 4wd pickups from 88-95 with the 3.0L v6 and has 2 inch pipes. It is 7 inch round with a 21 inch body. I ordered one for my trike and will be comparing the noise levels with a meter. The system is actually shorter than a locosts and the 1300cc is all going into one muffler, so the results might be of some value to you.

Probably the most quiet would be PN 21949 for many 1970-90 volvos, but you'll need to get the reducer down to 1.875" and shell out about $70 each. It is 6 inch round by 22 long.

The muffler I am replacing was installed because it had no muffler. It is oval with a 10 inch body and a turn out.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

Also maybe consider the PN 21028 for 1962 chevy trucks, it is the one I am using on my V6, so far after 5 years I have had good luck with it, 2 inch in as well as out.

Al

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Super Seven 3.4

A possible option is to add an extension or modify the tip of the muffler outlet, to function like a Brookslands silencer. It's a large fish tail type that expand into a flat sided wedge with a long narrow slotted hole for the outlet. They are ugly but functional!!!
Dave W

Stumbled across this. Seems like a good way to quickly insert silencers:
https://www.youtube.com/watch?v=01JdY0WTyZE

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Build log: viewtopic.php?t=9291

I'd forgotten about your mufflers, though I think you posted they are not very quiet? Spec $45 each and 5 x 21 inches.

I received the pn 22483 today. I'll be working on that tomorrow.

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12

[/quote]
I'd forgotten about your mufflers, though I think you posted they are not very quiet? Spec $45 each and 5 x 21 inches.

I received the pn 22483 today. I'll be working on that tomorrow.[/quote]

I realize that I probably did say that somewhere in my early posts about thinking it was loud.
Perhaps I wasn't being quite fair, to be honest it is very difficult to really rate the noise of a muffler considering one's expectations, I think maybe we expect more from our exhaust systems, which in turn creates a little discontent or one long search, short of a db meter of course.
When I startup my AMX, the neighbors seem to be well aware of it even from several doors down from me, to me it is not that loud.
On the other hand when I ask about my 7 build, I quite often get, Oh did you go for a run lately, they are not aware of it when I start it up.
So the concern is how much of this noise issue is me, or us. I wonder some times if we are not searching too hard.
I am not sure but the 22483 looks like it could be a reverse flow design. I first tried a Jones reverse flow it was like having no muffler.
Not good at all.
Seems like mufflers make us our own worst enemy, a muffler that sounds like a million bucks on one engine can easily sound like a horror on another.
I look forward to what you think of the 22483.

Al

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Super Seven 3.4