FAQ: The Exhaust Thread

AA wrote:

Quote :

I am thinking the 14" resonator gets rid of the drone and lets a lot else higher and lower frequencies through - kind of like a midrange filter - and the longer (22 1/2") resonator gets rid of more harmonics but also probably emits low frequency, low amplitude sound. Think of these resonators like organ pipes. Longer tube = lower note.

This also applies to the overall proportions of pipe diameter vs. length, and the overall volume of air inside the system. I don't know for sure what the numbers are for our vehicle, but as a whole, an exhaust system itself works as a Helmholtz resonator.[useful stuff omitted but you already read it]

Conversely, decreasing tubing diameter will tend to create a lower, growling sound. The trick is, when you narrow the diameter enough, you can also decrease length because the proportional relationship is preserved. This is how you can have that deep sound coming from Harleys and other bikes. The tubes are narrower, but they're also shorter. A bonus is that exhaust flow is also improved by shortening the length of the system.

Ah-ha! Aaron, this is probably why the stock exhaust emits that nice burble at idle. The rear pipes are on the small side - and the curve lets the designer set the resonant tone. Get it "right" and it burbles at idle but is silent much of the rest of the time. There is a horsepower penalty at higher RPMs but there's plenty of grunt at lower RPMs, which is what Riviera's target drivers (folks who'd want to tool around in a personal luxury coupe with buddies and golf clubs) were assumed to be after -- not prowling around looking for riced-up poseurs to dust (that would be the target drivers' kids!).

I am guessing the result of my escapade will be a durable and quiet exhaust that will have one-of-a-kind and hopefully pleasant harmonics.

And of course I'll let folks know what we hang on it, how it sounds, and such.

Albertj

PS: if you wanted a muscle car sound from the 3800SC, Rob's got it figured out - you should hear his Grand Prix. No kidding he's got it tuned to sound like a nice big 8. It's drone-y for my highway driving but I bet it'd be the cat's pajamas for some of the folks on this edit board who take mostly short trips. Me - I drive long trips. Don't want drone or rolling thunder. Rob has nicely tuned rolling thunder.

When you finally get that exhaust built.... audio/video please!

Any pictures and videos from Rob?

Btw good luck - I'm on my 5th exhaust system now

No pictures/vid yet, we're doing the work Monday July 6.

I will drag along a camera of some sort.

I don't think I will get the 'perfect' exhaust but that's because my ability to specify is not expert. I'm getting a straight thru resonator (packed) and a pair of quiet SS mufflers. Also getting a replacement cat, probably a magnaflow pre-rigged as a 'direct fit' item. Finally, a set of 7.5" long resonated, oval tips (if they don't fit right for some reason on the finished system we are just going to bend turndowns in the tailpipes with the mandrel).

So what's not perfect? This system is not going to be everyone's cup of tea. It will breathe well and be QUIET. For many of you quiet is not perfect. What's up with this? well - here is a wavelength calculator:

http://www.sengpielaudio.com/calculator-waves.htm

If you put the frequencies in (150-225 Hz) and convert from centimeters to inches it turns out that the 22.5" resonator is an almost perfect length to trap highway drone at 65 miles per hour. What it's doing, in general, is making the noise into standing waves and at 1/4 wavelength it's reflecting them back on themselves and essentially turning them into heat.

Anyway - if I can I'll get footage of Rob's Grand Prix. No kidding it sounds *sweet* but that exhaust would drive me nuts on a long trip. The setup on his car would be pretty easy to mimic on the Riv.

Finally - I need to be told how to post videos.

Albertj

There is some info on posting videos in the Board FAQ link at top of the page. You will need to sign up for a YouTube account, upload the video, then post the assigned "embed" URL into your post.

Thank you Aaron.

By the way - those of you reading this thread may find the following of interest:

http://www.autopia.org/forum/exterior-performance/113590-so-you-want-do-performance-exhaust-read.html

Albertj

the links in this page no longer work

So the new exhaust is in. No pics/vid yet. Kinda drony up hills but not too loud to talk over. Sounds best driving around city. New Walker cat; new 14" resonator (I think I want a different one), custom T pipe to 2 1/4" exhausts; custom (baffled) mufflers, baffled 3"x5" oval tips.

Also. this may amuse you

http://www.autolounge.net/calculators/exhaust/exhaustresonance.html

Albertj

i like that one!

I feel like the old cartoon character: "We're doomed..." by a 60-degree engine trapped in a 90-degree body.

Well, it's not the 'perfect exhaust' but in many ways it is an improvement.

Downers:

it is a bit drony on the highway, I am kind of getting used to it.
the mufflers sit pretty low, not really in the stock locations, wasted space
and I am in trouble on steep driveways and if I have to have it towed

Uppers:

if this is all you have to do to get great 40-70 MPH performance, just do it. I should go up to Watkins Glen on open track day and get timeslips.
at idle it rumbles (not burbles) like a big block but without the lope
Today a technician (well, at Sears Auto Center, so it may not count for wolf tickets) asked if it's registered sports car (in NY the type declaration is detailed) - I said "no, 2 door sedan" and his buddy said, "sleeper" (they test drove it on a nearby interstate, it was in for strut mounts)
the changes did not induce popping or backfiring
it's still a Riv.

Albertj

albertj wrote:

if this is all you have to do to get great 40-70 MPH performance, just do it. I should go up to Watkins Glen on open track day and get timeslips.
Albertj

I'm intrigued that you specifically mentioned this speed range, which I usually refer to as "the bottom of 2nd gear"... Is this the first time you've modified the exhaust system?

Quote :

I should go up to Watkins Glen on open track day and get timeslips.

I agree. I would like to see the actual number gains associated with an exhaust-only mod to a 3800 engine.

Eldo wrote:

albertj wrote:

if this is all you have to do to get great 40-70 MPH performance, just do it. I should go up to Watkins Glen on open track day and get timeslips.
Albertj

I'm intrigued that you specifically mentioned this speed range, which I usually refer to as "the bottom of 2nd gear"... Is this the first time you've modified the exhaust system?

It is the range I care about - I drive a lot on rural highways and this is the speed range that counts for passing slow vehicles quickly.

Albertj

albertj, are you happy with it?
It sounds like a lot of my own sentiments as I was playing with exhaust set-ups. I'm on my 5th or 6th different exhaust now and I think I'm finally happy with it. It seems like no matter what muffler you use, if its a generic performance muffler, it will drone to some notable degree.

My take on this is that you went through a lot of trouble and research to get a painfully typical result. I haven't heard it or getten a ride so I don't know but I'm basing off of your description.

I've found that a more complex adaptable exhaust is the answer for this car. The way I saw it, there are definitely practical limits to how much you can change the diameter and length of the pipe and a muffler inevitably has an inverse relationship between flow and quietness so I'm kind of using two different mufflers via an electric cutout to switch between quiet mode or performance mode.

Are you gonna post a video?

AA wrote:

Quote :

I should go up to Watkins Glen on open track day and get timeslips.

I agree. I would like to see the actual number gains associated with an exhaust-only mod to a 3800 engine.

Yes, I suspect the real timeslips will tell a very different story than the ego-enhanced Butt Dyno that I used today. I suspect the timeslips will show there's no real difference, and anyway I'd have to rely on slips from someone else's '98 stock riv because I don't have "before" slips. . There is a difference in sound - when accelerating the exhaust tone drops after passing ~3000 RPM. I suspect it's just because it's 2 1/4" tubing from the Y and so it can move more exhaust than the stockers.

Albertj

turtleman wrote:

albertj, are you happy with it?
It sounds like a lot of my own sentiments as I was playing with exhaust set-ups. I'm on my 5th or 6th different exhaust now and I think I'm finally happy with it. It seems like no matter what muffler you use, if its a generic performance muffler, it will drone to some notable degree.

My take on this is that you went through a lot of trouble and research to get a painfully typical result. I haven't heard it or getten a ride so I don't know but I'm basing off of your description.

I've found that a more complex adaptable exhaust is the answer for this car. The way I saw it, there are definitely practical limits to how much you can change the diameter and length of the pipe and a muffler inevitably has an inverse relationship between flow and quietness so I'm kind of using two different mufflers via an electric cutout to switch between quiet mode or performance mode.

Are you gonna post a video?

Happy enough. I look at it kind of like school tuition, I learned a bunch about the trade-offs.

Also, the custom exhaust did not cost that much more than having a local Monro Muffler put on a set of Walker aluminized mufflers and resonator.

If I find some $$ under a rock I will probably put in at least one flex coupler and put some length and curves in the pipe after the Y to break up the resonance. As it is now it's kind of one big organ pipe with an unusual "Y" T304 is downright melodious...



Albertj

I would suspect the attenuation in sound above 3000 RPM means your engine's output pulses have raised above the exhaust system's resonant frequency range. It doesn't mean it's flowing any more or less exhaust. Based on your exhaust system diameter vs. length as a whole (including mufflers), the resonant frequency is the exact point at which you hear the drone. The reason our stock system has no drone is simple - its point of resonance is below the frequency range that we can hear.

AA wrote:

I would suspect the attenuation in sound above 3000 RPM means your engine's output pulses have raised above the exhaust system's resonant frequency range. It doesn't mean it's flowing any more or less exhaust. Based on your exhaust system diameter vs. length as a whole (including mufflers), the resonant frequency is the exact point at which you hear the drone. The reason our stock system has no drone is simple - its point of resonance is below the frequency range that we can hear.

I was thinking about this and it confused me here. How does that whole resonance-pipe concept not go out the window once you put in something totally irregular like a muffler.

So far I've only heard one non-original muffler that cancels virtually all detectable resonance and thats the walker muffler I'm using now that's designed for the 4.3L v6 motor. That muffler was the only thing on the exhaust that was really changed from the previous highly-resonant set-up using the dynomax superturbo.

I think of it like this. Every part of the system has an effect on the sound of the exhaust. The manifolds, the cat converter, the resonator, the mufflers, and the pipes - they all do something to either cancel or amplify the sound waves coming from the engine. But most people forget about the pipes, which imo is a mistake, because that's where the exhaust pulses spend most of their time hanging out. Even a slight change in pipe diameter though the entire system will have some impact on the sound.

Ever wonder why there is a resonator after the cat converter? It reduces noise the cat converter missed. Why are there mufflers after the resonator? They reduce noise the resonator missed. The point is, each of these are sound reducing components, but they don't cancel all noise completely. Together they silence things quite a bit. Even so, you can still hear some noise come out the tail pipes.

So while all of that sound is being pumped from cat converter to resonator to mufflers to the tips, what's going on inside the pipes? The exhaust gets amplified at certain frequencies because of acoustic resonance. This is the same principle that makes a pipe organ work. Depending on the length and width of the exhaust pipe tubing, there is a frequency of resonance that makes the exhaust VERY loud near that frequency. For a given length of exhaust, the larger you make the pipe, the higher the resonant frequency is. As this frequency gets higher, it becomes more and more audible - this is called drone. Try talking through a very long, narrow tube and you'll hear it.

All of the components used on our stock exhaust were probably designed to take into consideration what the final resonant frequency is, and then work to cancel out that frequency. When you change the pipe diameter or delete/replace noise reducing components from stock, the chances they will work with each other to cancel the resulting resonant frequency is close to zero.

Another thing the GM engineers probably did is made sure the total resonant frequency was below a certain value, so that wasn't as audible (lower pitch sounds don't seem as loud). That way, if the noise reducing parts don't get all the noise, what's left to escape the tail pipes won't be that unpleasant.

AA wrote:

I would suspect the attenuation in sound above 3000 RPM means your engine's output pulses have raised above the exhaust system's resonant frequency range. It doesn't mean it's flowing any more or less exhaust. Based on your exhaust system diameter vs. length as a whole (including mufflers), the resonant frequency is the exact point at which you hear the drone. The reason our stock system has no drone is simple - its point of resonance is below the frequency range that we can hear.

You're right of course; thank you.

I also notice the tone is changing, I suppose mostly because I'm getting used to it but also I was advised it would change a little due to thermal cycling of the organ pipe and soot deposition. Frankly, "out of the box" the system was quieter than the replaced worn-out mufflers and quieter than most sports cars (you can still talk at normal conversation volume at highway sped). If it still bugs me (and I find some $$) I may put on a longer resonator than Rob initially installed, and maybe low-VOC-cement some membrane roofing and felt in the trunk under the carpet (think Dynamat at 1/4 the price)

By the way, AA, about the resonator - most resonators are set up to change the apparent acoustic length of the pipe (change the resonant frequency thereby hence the name resonator). Most mufflers are set up to absorb sound via internal absorption or internal reflection and cancellation. By the way - some years ago, my kids named my car Eleanor. I think I'll now give her a last name... maybe Hammond (for the Hammond organ, the ones with the funky fridge-size Tall Tone speaker box that had the Leslie unit - speakers and a big acoustic drum rotated by synchronous electric motors...)

So one thing I learned here is that what 'tuners' are prone to do - put in a glasspack, say, in place of a resonator - brings the resonant frequency of an exhaust (say on a Honda Civic) up into an audible range by shortening the apparent length of the pipe. The tuner crown\d, not thinking about or even curious about the physics of acoustics, confuses noise with flow. So an exhaust that makes noise or does not is attributed to flow better or worse when the fact of the matter is it simply sounds off or not.

Another thing is, there is only so much you can do with those teeny Civic explosions versus what one gets from the 3800 or 4.3L six... this must be a reason why the tuner glasspacks are called Fart Cans by many folks... and why it's so much furn to outrun the Tuners with a quiet, stock Riv.

Albertj

Likewise, Albert, it seems you have a good understanding of what's going on with the exhaust system. At least we know in part why we get the results we do. The main reason I've never modified my exhaust beyond the cat, it's not that I don't think it can be improved, but that I am afraid of getting drone, and based on what I've seen from others' experiments, it seems all too likely. The truth is, I don't know how to calculate the exact resonant frequency of an exhaust system with any accuracy. If I did, you can bet I'd be chopping it off and designing something better. Maybe someday...

turtleman wrote:

AA wrote:

I would suspect the attenuation in sound above 3000 RPM means your engine's output pulses have raised above the exhaust system's resonant frequency range. It doesn't mean it's flowing any more or less exhaust. Based on your exhaust system diameter vs. length as a whole (including mufflers), the resonant frequency is the exact point at which you hear the drone. The reason our stock system has no drone is simple - its point of resonance is below the frequency range that we can hear.

I was thinking about this and it confused me here. How does that whole resonance-pipe concept not go out the window once you put in something totally irregular like a muffler.

So far I've only heard one non-original muffler that cancels virtually all detectable resonance and thats the walker muffler I'm using now that's designed for the 4.3L v6 motor. That muffler was the only thing on the exhaust that was really changed from the previous highly-resonant set-up using the dynomax superturbo.

just comments...

1) the whole resonance pipe concept stays in (does not go out window) with a muffler because from a physics point of view the muffler modifies the length and diameter of the pipe BUT does not make it suddenly be not a pipe.

2) I am thinking some of this 'drone' comes from how Rob hung the muffler at the stock points - changed the stock rubber hangers. I will take a closer look one day when I have time on my hands. The other thing is I think I can improve things with a layer of dynama, somethign I'd been meaning to do.

3) I also wonder if I have a wheel bearing that's dying, and so some of the noise has nothing to do with exhaust.

Albertj

You might compare the material thickness of pipe and components between new and original. Some aftermarket exhaust parts try to shed weight by using thinner walls, which lets out more of the sound before it can even be muffled. A good example is comparing cast iron manifolds to stainless tube headers.

There is wall thickness difference between 304 stainless and 403(I believe that was what the custom shop I talked to was going to use) I could be wrong about the grade of stainless, I just know he suggested something less expensive than 304.