Fluid dynamics and muffler sound

The SAE offers an engineering-oriented seminar on exhaust theory. I'd love to take it, but it costs hundreds of dollars. Screw that. Most of it would probably fly over my head.

Hopefully I can rely on the HAMB for the same kind of information. Or even better information: HAMB technical information tends to be a lot more practical than the academic world.

I'm looking for this kind of information to help me lay out an exhaust system. I want to control the different variables that effect the exhaust note. I'd like a moderately loud, very deep sound. In this case, I want to avoid any "rap". So I want something that reduces high frequency sound waves without reducing the pressure, right?

I've been searching around the internet, and I find a lot of bad information out there. A lot of people chose Flowmasters for their Camaro or Fartcans for their Honda. And these guys love their exhausts. No matter what the cars actually sound like, it's music to the ears of a guy who just shelled out good money.

I'm having a hard time of collecting the wheat and discarding the chaff (as the minister says). Internet experts have all sorts of opinions on sound pulses, tubing diameter, scavaging, resonating, backpressure, echo cans, pencil tips, crossover pipes, and so on. Some of the theories make sense to me, others don't. The ones that make the most sense are the most technical, and therefore the most difficult for me to understand!

So I start with the little bit that I do know. When a trombone player makes a deeper note, he lengthens the trombone. So longer, in general, is deeper. Does that apply to the length of a glass pack? Or the length of the entire exhaust?

What happens if, instead of extending a tube, one reduces the diameter? So that the diameter-to-length ratio is the same? Is the sound still deeper? Different? Does the restricted flow come out at a higher velocity? if so, does that translate to higher pressure when it reaches my ears? Could 1" tubing actually sound deeper and louder than 4" tubing?

What happens to a fixed length exhaust system when I move the muffler closer or further from the engine? What happens to a glass pack when you reduce (or increase) the outside diamter of the muffler? What happens when you reduce (or increase) the length of a glass pack?

I'd really appreciate it if someone with experience could explain it all to me. Or, if someone who's trained as an engineer could break it down for me so that I could understand it.

Thanks,

--Matt

 

I bet you could make some serious music if you took a couple of pipes from one of those big pipe organs and slapped them under your car.

Your neighbors would be like,"Here comes Boris Karlof again."

 

Great topic! I will be watching this one.


I am curious about crossover pipe location. I know putting it behind the collectors Increases midrange? How about BEHIND the mufflers? I have no room in front.

 

While we're on the subject, anyone ever made their own glasspacks?

 

I do know you can experiment with some different things on your own car....if you run a crossover(which helps equalize the pulse) it doesnt do much for sound but it helps performance.If you dont run one its pretty important to have both sides equzl, as you dont want the motor seeing different scavageing or backpressure issues. If you do run one the pulse will equall itself out just behind the motor and you can play with different mufflers on each side, run a glasspack on the left and turbo style muffler on the right, things like that it'll give you different options to get the tones and the frequencies to work together or to cancel each other out.

 

The distance of the muffler from the engine will affect where the exhaust "tunes"- closer will tune at high speeds, further back will tune at lower speeds, but I can't say what it does to your tone. For either power or tone, I think its position has a relatively minor effect. I read an internal Ford memo years ago that discussed a dyno study they had run with varying tailpipe diameters- it really didn't effect power at all, until they ran such a stupid long tube that it was like sucking a grape through a straw.

The velocity of a gas through a pipe is directly related to the area and flow rate: Flow (Q) = Velocity (V) * Cross-sectional area (A). So, for a fixed flow rate, the larger the pipe, the lower the velocity. Now, a smaller pipe will cause more "head loss" or "backpressure," so if your pipes are restricting your engine's exhaust, then your mass flow rate will be less, but for a difference in 2 1/4" to 2 1/2" pipes, it's not going to affect your mass flow significantly.

Much of the way exhaust systems work (sound and flow) relates to organ pipe theory, which is pretty simple stuff when you get down to it, and the formulas aren't too bad (but it's more than length-to-diameter ratios)- I'll write more on it later.

A muffler's (glasspack, flowmaster, whatever) sound is influenced by its natural frequency and attentuation, and for relatively fancy devices like Flowmasters, there's no good way for an ordinary person to figure all that shit out- the geometry is just too complicated. Just listen to other people's cars until you hear something you like, then buy what they did.

I've been meaning to do a really nerdy tech post on exhaust tuning- maybe this will make me get off my ass and do it.

 

I am certainly not a fluids flow or air flow expert, but general engineering principles apply. The volume of air and the velocity of the air have a lot to do with it. Lower volume and lower speed will be deeper and seem less loud, and high volume and high speed will be more rappy and louder to the ear.

Another factor that makes V-8 sound is that they do not have even firing pulses on a true dual system. This creates some of the sound quality. It is the principle behind split manifolds on sixes, it creates the characteristic sound.

If you want a lower rumble, then larger diam pipes will help. However, for effective scavenging you also want some velocity which means smaller pipes. So it probably means that the muffler has more to do with it than most other variables. Mufflers create disturbance that helps cancel out the noise. Doing this without excessive backpressure is the trick. Longer pipes seem to mellow the sound vs short exhaust, maybe the turns and bends in the system help the cancelling out of noise without any significant baffle or disturbance in the pipe?

Personally I like the sound of flowmasters on a real performance engine, or on a musclecar. Not on a rod, especially one built in traditional style.

Here is a food for thought idea. What would the benefit be of smaller head pipes up front to help scavenging, but then increase to larger pipes in a couple transitions as it goes out the back? Such as 2 inch off the manifold/header, then 2.25 and finally 2.5 tailpipes?

 

Nothing to add except this is a good thread and I'd like to hear more, so....
BTTT

 

I will state up front that I am not an engineer by any stretch of the imagination,but have always leaned toward reading tech articles that are a distance above my head.
There was an article many years ago in Hot Rod(I believe) by Roger Huntington titled something like,"Vortex fluid dynamics and the Coanda Effect" which explored the usage of a tapered cone installed in the megaphone of a 2-stroke motorcycle to increase exhaust extraction. I checked the 'net and there are many articles related to the Coanda Effect but none related to exhaust.
There were also articles;again I believe by Mr. Huntington,relating to exhaust tuning.Many of them covered experiments by by Ford and Stahl Engineering in relation to how to develop exhaust systems that would actually utilize the pulse created by one cylinder firing to assist in the extraction of the succeeding one.
I'm sure many on the board remember the Lotus-Ford Champ cars of the early 60's with their,"nest of snakes" exhaust systems and similar Super Stock vehicles with like styled exhausts running every which way under the car.
I am sure this also had a lot to do with what Chrysler was doing in the late 50's and early 60's with their,"Ram" intake manifolds.
It had a lot to do with not only the exhaust pulse coming out of the pipe but also the negative pulse or,"wave" returning up the pipe.Like I said way over my head but it made interesting reading.
You might search the SAE files and type in Huntington's name to see if they have a list of papers bearing his name,but be prepared for a long look as he wrote a LOT of them.He was a contributing writer to Hot Rod as far back as I can remember.Maybe Hot Rod has an index of his articles.Good luck hunting!
Ray

 

Fluids was never my stong point, but I can offer some ancedotal evidence.

Back when I lived in an apartment building, the other '70 Chevelle guy had a 454 SS (I had a 307 Malibu), mullet and Motley Cru shirt for every day of the week. He ran 2.5" exhaust to some 12" glasspacks and then some 1.75" tailpipes. You could hear the thing rap a half mile away and he was proud of the fact that he routinely got citations for it. The sound was very sharp, high pitched and loud. Perfect match to his personality.

With that experience in mind, I had 2.5" exhaust with rear-mount turbos (just in front of the rear axle) and 18" long 4" diameter megs on my 283-powered '70 C10. Quiet and mellow with a nice growl when I got on it. The only real problem with that system was a really loud resonance inside the cab at about 30mph and I think I lost a little torque due to the relatively large pipes.

C

 

I made a set out of Sanitary Stainless tubing, the stuff you see in Food Plants and Nuclear Facilitys. Two 28" pieces of 3", and two 24" pieces of 4".

I cut slits in the 3" tubing with a Plasma cutter, then used a hammer and Punch to knock one side of the slit down a little, forming rough "Louvers". I then wrapped each tube with 3/4" of Refractory Insulation(Ceramic fiber), and forced the whole thing into the 4" tubing. Cut 4 "Donuts" out of 18g SS, and TIG'd them on each end to close the ends of the muffler casing.

I cut the flange off my headers and welded the Mufflers directly to them, then had an exhaust shop run about 3' of 3" tubing with turndowns just behind the Cab. My 355 with 292 Magnum cam sounded pretty mean with that system at idle and WOT, but at highway speeds it had a terrible "Drone" to it that gave me a headache everytime I drove it.

 

Darby, post helpful information if you have it; don't try to sound like an engineer if you're not. A lot of what you did post is the kind of misinformation I'm specifically not looking for.

Thanks, safariknut, for a few solid leads.

Here's the full text of an book written in 1882 about the physics of pipe organs:
http://www.measure.demon.co.uk/docs/Theory.html

And here's a paper written on the didgeridoo by someone at the Research School of Physical Sciences at the Australian National University:
http://www.phys.unsw.edu.au/~jw/dij/dij.html

Don't bother visiting either site; they're not much help.

 

Screw the whole theory thing. Look at it this way. Your exhaust pipe is like a whistle, the smaller the pipe the higher the pitch.

What I have always done is move my muffler as far away from the collector as I can and still have room for a decent length tailpipe. Your tailpipe (after a mffler) will have a mellowing effect on your muffled exhaust. If you want a low rumble go with a large diameter exhaust, if you want to raise the pitch neck it down.
Normally you don't have near the resonance problem with a glass pack as with a chambered muffler, its like the difference between a pickalo and a guitar. The pipe the sound goes right out the end, while the guitar depends on a chamber to make its sound.

Uh, does that make any sense?

 

way over my head!

however,i have made two sets of mufflers,and am happy with both.
they were copies of two manufacturers products.
first one was "spin-tech",now called spin trap i think.
i just went by their cutaway views of their products.
this particular set sounds like an old v8 boat at the marina when idling(on a 59 ford with an FE),and none of the noisy droning and resonance of the flowmaster types when tooling around town.
also search "dr. Gas"

second set copied from Ravin mufflers(goravin.com),again copied from their cutaway views.
this pair on a 350 buick,has a good sound at idle,and an evil howl(outside)at speed.

first pair made of .040 stainless,no packing.
second set made of .080 chrome moly,(explosion proof,car has flamers),again no packing.

both vehicles have homemade X pipes.

just start playing around with some you make,make them all the same length,and put some quick disconnect flanges.

i used scrap "V band" couplings on mine,(scrap aircraft and turbine engine parts)

hope this helps

 

Probably the reason for the droning sound at highway speeds has to do with the positioning of the downturned pipe. I had a friend long ago who had a 55 Chevy 150 two door(el Cheapo;not even chrome around the windshield)with a 265 in it and steelpaks with scavenger pipes exiting just under the axle.At 60 mph on the highway,the downward pointing pipes set up a,"resonant frequency" with the floorpan(A resonant frequency in my understanding is when the positive(or outgoing)pressure wave and negative(ireturn)pressure wave are traveling at the same velocity and meet in a designated area;in this case the floor of the car)and would drive you out of the car.
A good example of this and one I'm sure we've all heard at one time is the sound the exhaust pipe on a school bus makes when it attains a certain speed and/or engine rpm.Sort of like someone blowing air through a split piece of rubber tubing like those old,"fart makers".It was especially noticeable when the bus wes decelerating under compression.Since a lot of vehicles these days are blessed(cursed)with some type of an overdrive transmission which virtually eliminates,"compression braking",very little noise is observable.
Simply relocating the exhaust on the Chevy completely eliminated the vibration.As for actually controlling the emission of the sound,I think there are way too many variables for anyone to give an exact answer.
Anyway this is a good thread.Hopefully some one with a lot more knowledge on this subject will chime in with a response.

 

Sorry that wasn't helpful to you. I only tried to answer some of your simpler questions there because I don't have any of my engine design textbooks with me here at work, and didn't want to type in wrong equations off the top of my head. If what somebody writes down is true, then it shouldn't matter if they have a specific college degree or not.

But, for what it's worth, I have two degrees in mechanical engineering, and all of my education and work experience has been in engine design and fluid dynamics. I don't know much about acoustics, so if you want a sure-fire equation for determining what a muffler will sound like, I won't be much help to you, nor will I try to be with a response like that. I don't want to get into a flame war on here about credentials or who knows what. If you are interested in how the fluid dynamics of exhaust tuning and scavenging works, let me know and I'll try to help you or other H.A.M.B'ers out with a more detailed explanation later on, as I offered to at the end of my post.

 

On my chevy I am running a 350 V8 with 10:1 compression and a heavy cam. I wanted to get away from the "standard" 50's 2 inch glasspack sound and I am going to run a single 3 inch exhaust maybe 3 1/2. Id reallyl like to have a cool cruiser with a mean sound. Any one think this idea will work? (Oh its a 54 Bel-Air.)

 

That would be great.

For what it's worth, the piece on didgeridoos was written by a guy whose day job involved reducing the resonance that ocurs in the little tunnel created when a tire groove meets the road.

From further searching, there's a fellow by the name of Helmholtz who did a lot to advance our collective understanding about sound and wind instruments in the 1880s. Automotive engineers have named an relatively common exhaust resonator after him.

 

if you have a 2D (3D prefered so i dont have to build it in 3D) representation of the Entire exhaust system i can run it thru a fluid simulator and post a movie clip of it. the more detailed the better, but render (calculation) times go up.

you will see exactly how the gases will react.

the turn around will vary as i will do it as time allows.

people pay thousands if not hundreds of thousands of bucks for this stuff depending on the amount of fluid dynamics needed

 

I don't know if this is worth anything, but here is some experience I have had on my car:

When I put the SBC in my car, I had 2" aluminized pipe run all the way out unde the rear bumper, with regular Cherry Bomb glasspacks placed approx. under the rear seat of my 52 Chevy sedan. Once the glasspacks broke in, it was pretty loud, and had a fairly hard rap.

I won some stainless Megs tips as adoor prize at a show, so I installed them. they are 3.5" dia (IIRC), and about a foot long. This deepened the tone quite a bit, and cut down a little on the rap.

When I put my Chassis Engineering rear leaf spring kit in, my exhaust interfered with the spring placement, so I cut it off at the back of the glasspacks, and welded a turndown to the tail end of each muffler. (A temporary fix.) This gave the exhaust a much "looser", and way louder sound. The harmonic drone between 30-40mph was nearly unbearable. It actually wasn't too bad at hwy speeds, but the neighbors (and my wife) sure complained about it when I pulled it out to leave every morning! There wasn't a lot of "rap", just a lot of noise.

Then I ordered up a pair of 30" Smithy's (the longest they make). I took it to the Muffler Shop, and had all new 2" aluminized pipe run with the Smithys in nearly the same location as the old Cherry Bombs. I also put the Megs tips back on, and they are located nearly the same as before. Now that the Smithys are broke in, the car has a much, much deeper tone, and nearly no rap. It is also much quieter... which is what I was after. It's still a good "performance" sound, but not "hollow" like Flowmasters.

I don't know if any of that helps, but based on my experience, it would suggest:
- Shorter Pipes = louder
- Shorter Muffler = louder
- Larger Diameter tips = deeper
- larger distance from muffler to tailpipe = "tighter"

 

Thanks, Slide. That's some helpful information.

Why did you go with the 30" Smittys? What's the difference in sound between 22", 26" and 30" Smithy's? Are longer mufflers deeper, or just quieter?

 

Thanks for the offer. I appreciate it.
At this stage, though, I'm looking for input on how to design the system in the first place.

 

Well, I was after "quieter" more than anything else, so my theory was that more muffl-er would mean mor muffl-ing. Since the exhaust had to pass over more muffler real estate, I was thinking it would make it that much quieter. I got what I was after, so I am real happy.

I know there are several HAMBers that have posted that Smithy's are "too quiet" or "don't rap enuff" compared to "Cherry Bomb" style glasspacks. If loudness or a hard rap are what you are after, then I'd agree. But for me, I often leave the house pretty early, so I try to be somewhat considerate of my neighbors.

 

I've got dual 2 inch pipes with 30 inch Smittys on my 54. They've been on there 20k miles and haven't gotten much, if any, louder at all. Very quiet with no rap at all.

Also, the deal above about increasing the diameter of the pipes as it goes back out the car is backwards. The hot exhaust gasses cool as they flow through the system, so the contract. Therefore, the pipes need to be smaller further back. Flowmaster did some dyno testing, and they recomend one size smaller pipes aft of the muffler than in front of them.

I know very little about fluid dynamics though, so I just read what others say and try to decipher the bottom line. So, my words may be worthless.

 

I'll add what I can and then a question.

My 883 Sportster came from the factory with a whisper quiet exhaust that was really really lame. Before I even had the bike for a week, I decided that I wanted it loud - really fuxking loud. Down to the local shop I went, and home I came with a set of 2" drag pipes. An hour later, I fired up the bike. POTATO POTATO POTATO, loud as shit, just like a Harley should be. I had a grin from ear to ear as I hopped on the bike and rolled out of the driveway. It sounded great until I layed into it, where it would distinctly sound like a muted wet fart. It sounded terrible going up hill or with a passenger on the back. Coasting or letting it pop between gears sounded horrid. Not good.

Back to the local shop I went. This time I came home with a set of 1-3/4 pipes. I fired up the bike, POTATO POTATO POTATO, just as loud. Rolled out of the driveway, jumped on it, and sweet jesus it sounded great. Ran it up through second, let off the throttle, and started coasting. The bike sounded terrible - kinda like a jake brake, but in a bad way.

Back to the local shop for a third time. This time I came home with a set of AR cones. The bike sounded the same at idle and almost everywhere else, except that when I coasted, it went BLAT BLAT BLAT a couple of times and then smoothed out nicely.


Anyway, the point of all that blabber was that I created a poor sounding and performing exhaust system by installing too large of a pipe. I'm going to try and paraphase the gospel of David Vizard here. There is no such thing as too little back pressure. What people mean when they say back pressure is exhaust velocity. If your pipes are too large, you'll end up with a slow exhaust velocity that will allow the gases to revert back into the cylinder head. The AR cones were a stop-gap fix for pipes that were too large to prevent reversion under high vacuum situations.


What does that have to do with the original question? Too large of a pipe for the engine's output will result in an exhaust note that sounds like ass.





OK, so here's my question. In an effort to eliminate a really irritating droning noise between 45 - 55 mph, I replaced the Flowmasters on my daily with Dynomax Super Turbos. The volume decreased drastically, but the resonance didn't change in any way.

What gives?

The engine is a '65 383 Mopar B engine, mostly stock. The exhaust system is 2-1/4 from the headers to the tailpipes, which exit straight back at the bumper. There's an H-Pipe installed directly after the headers. The resonance was made before and after the headers and H-Pipe.

Any suggestions would be greatly appreciated. I'm almost thinking about trying glasspacks at this point.



Thanks,
Ed

 

I had a 2.3L Turbo 84 tbird a couple years ago. had single 2.25" out the turbo that Y's under the trans and dualed out the back with 24" long 3" tips. no mufflers as the turbo knocked it down. the car had the lowest mellow idle I have ever heard. lots of people thought I Was running a 5.0 in the car. this was all crush bent pipe which kills flow.


now I put exhaust on my merkur over the summer (same motor) and ran 2.5" and split it behind the diff (IRS car) and ran just plain 2.5" pipe out. all welded J bends and had hardly and bends to it. was basicly a straight shot to the rear (5 degree kink in the middle) and my Y was made from 2 90's welded together. it was louder, droaned bad, sounded ricer like.


so from these to systems I would say run larger long tips (will give a lower freq for the end tone) as far as mufflers I have heard that placing them farther forward will help to expand the gases and allow it to cool so when it reaches the tips it won't rap bad.

the tbird woul dcackle a bit on downshifts.....were as the merkur would pop.

I am no expert yet (am a SR in ME but focused more on mechanical systems and not fluid/thermo stuff.....although I am in CFD right now)

 

Can you FEEL the drone, or just hear it?

It may be a structural issue. Every vehicle interior has a resonant frequency and vibrations from the engine or driveline or exhaust may be driving it through an exhaust mount or engine mount or tranny mount or...

If the floor or steering wheel or seat is vibrating during the drone, you're likely dealing with a problem that is at least partially structural and the chances of fixing it by changing the muffler are small. Unless the mass of the muffler change enough that the properties of the exhaust system change to where it works as a damper.

Or there may be a standing wave in one of the pipes. Similar to an organ pipe, there may be a certain rpm where the exhaust is at the right frequency to set off a tone in the pipe itself. This is usually more prevalent in larger diameter, longer pipes. Easiest fix is to break up the pipe length with a resonator.

 

Tuning the acoustic properties of an automotive exhaust can get rather in depth. Just about any rule of thumb can be disproved in some application, and it's not nearly as much of a science as some people would like to believe, so take all this with a grain of salt. It's my learned opinion, and keep in mind I'm talking about acoustics only, power is a whole different department. Related, yes. But not the same.

The oversimplified basics of auto acoustics:

Mufflers don't create sound, they filter it. The engine and manifold design provide the source sound, a muffler just tunes out frequencies of that sound. Different mufflers tune different frequncies better and worse than others.

In general, the more tuning volume you have (more/bigger mufflers), the quieter the exhaust will be. Trivial fact #1: the rule of thumb is, if you want a quiet car, you want the "tuning volume" to be ten times larger then the engine's displacement. So for a 5 liter engine you'd want 50 liters of "tuning elements". Often overlooked (by Amercian OEMs) fact: You don't want all the volume to be in one place. Multiple small volumes are better than one large muffler.

Low frequncies are attenuated by small diameter pipes and long lengths. I.e. if you want low sounds you use larger diameter, shorter length pipes. This doesn't mean you need to terminate the pipe after the collector for a low rumble, it means if you have a choice don't run the pipes all the way to the back of the car and then add the muffler. Long pipes are a bad idea in other ways too; they are susceptible to standing waves, or drones. The trombone player wanted that note to stand out, so he made the horn the right length to do it. A long pipe can do the same thing to your exhaust note. Hit the right note (rpm) and BOOOM... Large diameter pipes have issues too. Does anyone here like the sound of a school bus? Long pipe, large diameter...

The fibers put in mufflers (glasspacks) are for high frequency sounds. They do little for frequencies under about 400hz or so. In a glass pack, the thickness of the mat is important. Thicker matts will be more efficient. Length is good too, but if you have a 3" od glass pack with a 2.25" pipe running through it, it's not going to do nearly as much as a 4" od part of the same length would. Speaking of glass packs, the louvers do NOT have to go into the pipe to direct the flow. We're talking about sound here, not flow. Keep the two seperate in your mind. One thing that might help here is to remember that the sound will ALWAYS travel at MACH 1. The flow won't travel anywhere near that fast. If the flow exits the pipe at about .4M (.3M?) or more, you're likely going to hate the sound because it will have a lot of flow noise. Remember your buddy's mom's Vista Cruiser and how it sounded like a jet when he stomped on it? Yeah, That's what I'm talking about... Flow noise is bad.

The postion of the muffler does make a difference. Sometimes simply flopping the muffler end for end can make a noticable difference. Someone mentioned Helmholtz. (He was well known long before the automotive guys started stealing his ideas by the way) Helmholtz tuners are simple little things that attenuate specific frequencies of sound based on the geometry of the part. Simple explanation is, a tube of a certain diameter and length attached to a volume of a certain, um, volume... will attenuate a given frequency. At least in theory. In practice, the postitioning of this tuner is critical to how well it performs. So if you have a problem frequency, there is a sweet spot for the tuner in the system. Put it in the wrong spot, it won't work. Put it in the right spot and you're hero. Maybe. What you're doing is moving energy around. Great, you just killed the 120hz moan. But what's up with this 85 hz issue now?

So on your buddy's car, the muffler may work best 6" behind the collector, while on ours it might work best 6" in front of the rear axle. The kicker is, you and your buddy may even have the same car, but he likes something different than you do.

There are a ton of variables the effect the exhaust note, some of which you can do something about. But I'm afraid you wont be able to learn it here. We offer a class at work on the basics of it, it's a five day class, eight hours a day, and you're pretty much expected to have an engineering background to attend (and $850). The intention of the class isn't even to teach you how to do it, it's more of an appreciation of what we do when we spend 12 weeks or so playing around with vehicles and computer models and building parts to get to a sound someone likes.

Here I've been rambling for longer than I care to admit, and I never even talked about what makes up the sound to begin with...

 

I'm guessing it's gonna be tough to get the sound you want by utilizing flow dynamics and exhaust theory.
In the end, it will be a subjective decision as to whether you like what you've wrought.

Copying a good sounding car almost always works . . . but . . . running an engine built a little more radical - or milder - than the car you're copying will impinge on the exhaust note to a considerable degree.

Small case in point; my friend had a cherry 51 Olds coupe. Ran a stock 303" engine w/hydro, exhaust manifolds, dual exhaust setup with 22-24" length glasspacks.
It was a great sounding car.
The system rapped a little after the engine was wound up a bit, but it wasn't obtrusive for the most part and if driven reasonably it didn't bother the Gendarmes or the general public.

We built the engine, knocked it out to 324", 56 heads, Engle #116 cam - which wasn't too radical, but it did have a nice lope to it - tri-power setup with three Rochesters on it, milled heads, slighltly lower diff gears, 37 Caddy floorbox and that was about it.
A fairly typical late 50's hop up for a street runner.
We were still running the stock exhaust manifolds.
The car ran good and held it's own with most of the early 60's street-driven muscle cars.

What amazed us was the change in exhaust note after the buildup.
We felt the more radical cam was responsible.
The rap had come way down the rpm scale and in fact was present not too far above idle.
The rap was there even with a light throttle foot.
Full throttle, a whole other ball game.

Sticking on a set of the longest glasspacks we could find (36 inchers) brought the sound down nicely and it sounded a lot like it used to.
The rap was nicely attenuated and it was still there a bit, but the best part was the car wasn't as offensive as it had been.

I see a lot of guys here on the HAMB that want to run straight pipes, open headers and the like.
Aside from being a pita to their neighbors and the general populace I often wonder how far they get before they get busted by the Gendarmes.

Yeah, I know, it's all about being an outlaw. Kinda hard to ride the outlaw trail when you can't use your horse....

 

That seems to be true from the information I've gathered. It's just like Ed Murder's experience with his Sportster. Or, as DesmoDog put it: "Large diameter pipes have issues too. Does anyone here like the sound of a school bus? Long pipe, large diameter..."