Fluid dynamics and muffler sound

To add to C9's comments, here's a pretty much useless piece of info from me:

A while back, I changed from a Weiand Stealth intake to an Edelbrock Performer EPS. I swear the exhaust sounded different after that. I didn't change anything but the intake-- kept the same carb, dizzy, everything but intake and gaskets. I can't really explain HOW it sounded different... just that it was different.

So I guess my useless point is that everything affects everything.

 

I think you're probably right, Jay. But I'd like to figure out as much as I can in theory first. Thanks for posting. I always appreciate your insight on rodding history.

By the way, this is for a flathead, which also has the great sound created (I'm guessing) by the shared exhaust ports.

 

Thanks, guys, for all the responses. I'm beginning to digest some of this information. Please keep it coming.

Thicker matts will be more efficient at what exactly? Attenuation of the soundwave? Which means that it's going to be more quiet, but not necessarily deeper, right?

(That was a really good point, DesmoDog, about sound always travelling at MACH 1.)

All things being equal, a longer glass pack does what exactly? Reduces amplitude or frequency? Does it necessarily reduce both?

 

Not useless at all. I think you're exactly right.

I don't expect to learn about exhaust theory this week, and then fabricate the best sounding exhaust system in the world this weekend. I simply want to know what the big variables are.

 

You're right.
It was different.

I read a header tech piece a while back and the statement was made that intake tract changes affected header performance and the converse, header changes affected the intake system.

It's all just one big air pump in the end . . . cept for those little explosions somewhere in the middle.

~~~~~~~~~~~~~~~~~~~~~

To strike off on a small tangent although it pertains to the discussion and particularly to what Slide just posted.

In the early 60's I was running a 335" built Olds in a 50 Ford coupe.
Desiring to change the torque curve, a six pack of tall Oly's was purchased, emptied, the tops and bottoms cut out with a can opener, a small slit cut in the bottom side with a hacksaw and the now open at both ends tubes were hose clamped onto the Stromberg 48's sitting on a six two barrel intake.

The Oly cans weren't polished tubes like you'd find on a fuel injection system, but they had their own form of good looks and charm (can I say 'charm' on the HAMB?) with the cool design printed on the cans.

The Oly cans were two barrels short of an injector setup - among other things, but they did alter the torque curve.

What they really altered was the part and open throttle intake noise.
Especially so the part throttle bit.
The exhaust note changed as well.
What had been a pleasant sounding car with lumpy cam and single 4 bbl went to a mean sounding SOB with the swap to the six 2bbl setup and it got even meaner sounding after the Oly cans were added.

The Dragmaster collectors (about 2 1/2 x 9" oval) added their own sound to the mix.
With the caps closed and running factory T-Bird muffs on a 2" system, it was fairly quiet except for the collector rattle at idle.
Run the rpms up a touch and the collector rattle went away.

Collectors open, it was the loudest car I ever owned....

 

Someone mentioned an article in Hot Rod back in the eighties.
The smaller head pipes offer better scavenging.(Due to the pressure drop into the larger pipe....??)
I had a Dart that ran 225 Slant six with a stock (Small) head pipe and then put on the mid pipe off a Cordoba (2.5")with a California Turbo muffler. Nice deep tone and plenty of torque. Really woke that car up.

My Pick up has a 350 that runs 2.25 head pipes into a merged collector through a single three inch Flowmaster that's placed about two feet behind the collector (Y-pipe) and then another 5-6 feet of pipe over the axle and out the back. Co-workers thought it had a big block in it. Not a lot of rap with a nice mellow rumble.

Another truck with the same size headpipes (about 30" long) ran into an 'X' crossover through 2.5" glass packs (About 30-36" long) dumped in front of the tire on the passengers side Sweet sounding and still not too raspy.

As for the resonance, someone also mentioned multiple mufflers (Resonator).
GM did this and you notice a big difference when the resonator is replaced with an eliminator pipe. Rappy.

Over on the Diesel forum I frequent they have noticed the same thing. "My new exhaust really resonates". They were advised to add an additional muffler and the problem was almost always solved.

My empiracal 2c added to the theory of others.

I'm googling to see if I can find that article. I've tried searching Hot Rod a couple of times and it doesn't show up.

 

Craig,

Thanks for the reply. I feel like we're getting somewhere here.

To answer your first question, I can't feel the drone vibrating through the car. It sounds like I'm trapped inside of a kettle drum that's buzzing along at 50 mph. FWIW, the resonance comes back around 85 MPH. I have no tach in the car to verify this, but I'd be willing to wager that both spots are at the same RPM.

Your second suggestion was to install a resonator. Since I've never played with resonators, your answer only brings up more questions! If the muffler is installed just ahead of the rear axle, where would you suggest installing the resonator. What type of resonator should I get and where can I find one? Quick google research shows that they're fairly common on late model imports, but little in the way of aftermarket support.

I hope you're not suggesting those Pace Setter resonator/exhaust tips. I'd rather listen to the resonance!



Ed

 

Thicker 'glass attenuates higher frequencies better. Maybe .5" of 'glass will take the edge off of the high frequencies. A 1.5" matt might effectively kill a lot of the high frequency content. (In general, if the sound you're looking at is 10dB below the overall sound level, you won't be able to hear the sound you're looking at.)

A longer glass pack can have dual effects. More glass will attenuate more of the high frequency content. The glass won't effect the lower frequencies but the longer can that holds it might.

Any expansion can (change in area) will attenuate certain frequencies. For instance a 9" long can may attentuate 100hz, 200 hz, 300 hz, etc. A 6" long can may attenuate 150 hz, 300hz, 450 hz, etc. By changing the length you've changed what frequency the can itself is attenuating (with or without the glass).

The can itself will effect lower frequencies, independent of glass in the can. If you take a glass pack and plot out the attenuation vs frequency, and compare that to the attenuation of the same can without glass, up to about 400hz the curves will match up pretty well. Above that the can with the glass will kick ass compared to the bare can. The glass is "broad band" tuning, meanign it covers all high frequencies. The can itself, and Helmholtz tuners, are more narrow band so they're effective at one frequency but may even boost frequencies on either side of the band it's tuned to.

Also keep in mind that the glasspack will not lower the frequency of anything. It will filter out the high frequencies and leave the lower content, but it won't lower anything that wasn't already there to begin with. That may be nit picking, but it's important to remember the muffler is only filtering sounds, it's not creating any. (Ok, if you want to really get into it I can show where that isn't absolutely true, but "In general"...)

 

Your intake affects exhaust ALOT. I haven't really payed a whole lot of attention to this, but I know when we're figuring out how long to make the primaries on our header for the Formula SAE car that one of the biggest variables is intake volume. We run a restrictor, so we have a very flat horsepower curve at around 75/80 hp and then we can tune in where it "spikes" up to 85/90 and then the "sharpness" of the spikes by how long the intake runner and exhaust is. but they have to be adjusted in unison or we'll lose the "spike" which is cause just by harmonics in the engine.

 

weird question for DesmoDog... what would happen if you ran two short glass packs per side, but different lenghts? say a 12 incher and then a 16 incher. Would the second muffler attenuate the the frequencies that the first muffler leaves alone?

 

Without knowing the rpm it's hard to guess where the problem is. That said, I'm guessing you're looking for a length of pipe that's about five feet long. If that's the tailpipe, putting a resonator in the middle of that pipe should help. If it's the pipe leading to the mufflers, the resonator could work at 1/2 the length or maybe 1/4.

By resonator I mean anything that changes the cross sectional area of the pipe. More is better. A glasspack would work, I'd look for something at least 3.5" diameter, it's hard to say how small you could go and still get good results.

Of course this assumes the problem occurs in that part of the pipe...

 

Sort of... the two cans would tune different frequencies, but they'd both work on the high frequency stuff. Neither one would work too well in filtering low frequencies.

*********
Regarding other comments here,

"In general" point #342. Anything that changes the power of the engine will change the sound. When you change the intake, you change the BMEP. Change the BMEP and you change the sound.

 

This is what I did on my '50 plymouth with a hopped up 230 flathead six. My tube headers have 2 1/2" flanges on them, so I ran 2 1/2" pipe from the headers to a pair of 22" long 2" Smithy mufflers. From the mufflers back to the rear bumper I ran 2" pipe with a pair of rolled edge chrome tips. I also put a crossover pipe forward of the mufflers.



Here's the sound

The tone is deep and mellow, no raspy or cackling sound at all, just a nice growl.

Pete

 

Craig,

I'm planning to install a tach in the car this weekend. Would that data help you determine if a resonator would cure the resonance? I can also get a picture of the system. I have a set of 22" glasspacks handy that I could throw into the system... I was considering installing them in place of the Dynomax Super Turbos that are already on there. FWIW, I had Flowmaster 2-chamber mufflers on before the turbos. The resonance occured at the same spot, but was noticeably louder. I installed the turbos after enough people told me that they would cure the resonance.

Have you evaluated the Car Chemistry inserts that someone posted a link to? I've considered using those on my '31 where physical space will be a premium.



Thanks,
Ed

 

Here are two good links with lots of technical advice concerning exhaust systems.

The first is a performance oriented article from a Pontiac magazine on how to quietly make horsepower. It is where I got the recommendation for the Dynomax Super Turbos that are currently on my daily.

Pontiac Magazine Exhaust Article

Well, I can't find the second link so there is only one.



Ed

 

Good discussion.

Lots of good points as well as been-there-done-that info.

Early 60's some of the street racers and a few street runners with big CID engine muscle cars made a four Y's out of 4 U-Bends and ran two mufflers on each side.
Most of these on cars large enough to handle 4 muffs inside the chassis.

With the exhaust pipe doing a one into two split into two muffs and then a two into one split back into the single tailpipe.
Note that this setup has the muffs in parallel on each side and not in series as a muff/resonater setup would be. (Resonaters in 50's/60's era cars were behind the axle in front of the rear bumper. Like you'd think, they ran cooler and rusted out before the muffs proper.)

Choice of mufflers was at times four glasspacks or the popular at the time 2" in and out T-Bird muffs from the four seater 'bird.
Glasspacks were noisy, but not too bad. The stock style T-Bird muffs were almost as quiet as your Granny's old 440 powered Dodge with stock muffs.
(Similar to a pair of Hemi-muffs if you've ever heard those on a 390 & up CID engine.)

All the splitting etc. done to keep the street racers quiet and innocent sounding.
For the Gendarmes as well as for the victims....

 

I'm running that same set-up on my chev truck with a lower compressin motor. a cheap set of fender whell headers into a 2 1/4 y-pipe and into a 3" single pipe using a 3" in and out flowmaster. Has a distinctive sound, not too rappy but with a nice rumble to it.

go for it!


jerry

 

THE MUFFLER FELL OFF MY 88 CHEVY work truck,so I just inserted a length of pipe to replace it for a while...It actually sounds sweet! Just a Catalytic convertor and pipe!! Ill never replace it.....[helps if you can't afford one]

 

Sounds good!

Thanks for the link, Ed. That's good information.

 

I've seen the web page before, but never used the actual parts. I do have a problem with at least one of the claims though, namely "This uniquely designed system of plugs and disks (patent pending) actually help pull the exhaust gases through your exaust system".

Someone's going to need to explain to me how adding a restrictor to a pipe helps PULL flow through the pipe.

Their sound testing results are misleading too. The graph starts at 0, but their background noise was probably at least 40dB. Nit picking I suppose, but still - cover up everything below 40 and the graph looks a bit different. And the difference between their "stock" (stock = aftermarket from what I can tell) reading and their 2 disk reading is huge, roughly 60dB vs 95dB? Consider that the dB scale is logrithmic. I wouldn't be showing that graph if I wanted to show how effective those things were. Yes, I realize the 3 disk version is much quieter.

The performance claims are suspect too - not that they exist, but that they're due to any special design of their part. I'd expect most any street vehicle to do worse with an open pipe than with some sort of restriction, but that's getting into the performance side of things and I said I wasn't going to do that.

Another thing that bugs me about that page: "Many performance mufflers will increase low range power through the addition of desirable back pressure, yet lose power in the midrange as the back pressure increases as a means of controlling sound."

No one I know uses backpressure as a means to control sound when designing a "performance muffler"... unless you're talking about a system with a valve in it.

Basically they make so many lofty claims that BS alarms start going off in my head when I read their descriptions. Especially when they don't really back them up with anything. Where's the backing for their claim of the insert helping to pull flow through the pipe?

Bottom line is, I guess it depends on what you expect from the inserts as to whether you'll like them or not. Will they quiet down a car? Sure. But there's no magic going on there. Guys have been doing similar things with washers and tubes for years.

 

On a lark, I called up Dynamax on my way home from work to ask them thier opinion on why my Super Turbos and the Flowmasters they replaced both resonate. The tech that I spoke to was convinced that switching to a different, free flowing straight through muffler would cure the resonance. Her theory was that the relatively small diameter of my exhaust pipes (2-1/4") is causing too high of an exhaust velocity. The high velocity gases are being trapped in the muffler, where more exhaust gas is entering than can exit, causing sound waves to bounce back and forth in the muffler at certain RPM's. Many apoligies to her if I botched that. Her recommendation is to remove the Super Turbos and install a set of Ultra Flows.

For a second it sounded like she was trying to sell me on thier better muffler.

She then told me about Dynomax's 90-day return policy, where they would exchange mufflers and give me $50 to have them installed. That's a customer satidfaction policy that goes above and beyond expectations.


When I asked her about installing resonators, she was pretty convinced that they wouldn't solve the problem. She thought it more likely that they would slightly decrease the resonance or move it in the RPM range, or maybe if I got really really lucky eliminate it.


So anyway, should I take them up on it and install the Ultra Flows? I don't really mind that they'll be louder.. anything but the resonance would be OK at this point! The theory seemed good on the phone, but then again so did all the theories that led me to replace the Flowmasters. Am I barking up the wrong tree by replacing the mufflers?


Thanks,
Ed

 

You have dual 2 1/4 pipes on a relatively stock 383. The resonance occurs at relatively low rpm, at part throttle. What size pipes did she think you need to cope with the 40hp the engine is making under those conditions?

I'd like to ask her what's happening to all the exhaust that's piling up inside the muffler under this condition. And why does the problem go away at a higher rpm, when you increase both the velocity and the mass flow?

I'm not picking on you, or even her for that matter. I'm just trying to point out a few issues I have with her explanation. In fact it's kind of embarrassing - the company I work for owns Dynomax. I doubt it will surprise anyone that the people who work at the tech line aren't the ones who design the parts...

Anyway, the kicker is, putting a straight-through muffler on the car may solve the problem. But not for the reasons she gave. And it wouldn't be the first thing I'd try, though the money back guarantee is tempting.

 

hahahaha i like that term...i hate the noise of those things...we call em "coffee cans" over here...i guess cus they actually look like coffee cans

 

Craig,

Of course I feel stupid after reading your response. I am also an engineer, so I am supposed to think logically and critically about the information that people are giving me. So much smoke has been blown up my ass during this whole ordeal that I'm getting frustrated.

I'm attaching the best picture I have of my exhaust system. Let me know if any problems jump out at you. I recognize that you can't see halfof the system.


Ed

 

5 1/2 years on my 1959 Ford, 292 Y-block, dual straight pipes, sweet sweet music, no tickets. Its not offensively loud unless you are at high rpm, near redline. My son drove it to college in Texas for 4 years, now I drive it. I am sure it doesnt bother my neighbors, except maybe when I make the pipes rap on purpose.

 

I wasn't trying to make anyone feel stupid. I know what it's like to talk to someone on the phone and then think later "Hey, wait a minute..." It just happened to me a couple days ago. I'm rebuilding an old Ducati right now. I called a Ducati specialist looking for new valve guides. He wanted to know the size. I told him they were "thirteen oh three to thirteen oh four" He said "No, I need the OD, not the ID". That is the OD I said. He said "No, that is surely the stem size. I need the OD". So I told him I'd double check and get back to him. The minute I hung up the phone it occurred to me that there isn't a bike out there running 1/2" valve stems. But since he was the expert I questioned my numbers instead of his. All I can think of was he thought I was talking inches (.1303) instead of millimeters (13.03).

ANYWHOS...

All I can tell from the picture is you have some long pipes, which is where standing waves (resonances) are created. I'm not sure if you have room, but on the pipe pictured, if you could put a 4" round or larger resonator/glasspack somewhere around the midpoint it could work. Even a 4" tube sized down on each end to match your 2 1/4 might work. Length isn't all that critical, change in area is more important. And I wouldn't stress too much about finding the exact midpoint, I'd just error that direction if you find a suitable location. With exhaust work you often have to take what you can get and realize it's not the optimal solution.

And of course this is all still just a guess. ;-)

 

Nothing to offer engineering wise, this is way out of my league.
But I do want to add something that hasn't been discussed. New cars.
In my opinion the best sounding new car today is the V-8 mustangs.
Ford has accomplished with their system a very deep tone but also managed to keep it on the loud side. The tone of their engines can easily be recognized a block away. If I wanted to spend countless hours studying a system that would be the one I would copy. Ford has either done their homework or ran into some dumb luck with those cars.

 

One book that I can really recomend is "Scientific design of Exhaust & Intake systems" by Phillip H. Smith and John C. Morrison. ( ISBN 0-8376-0309-9 )
I calculated a couple of headers using theyr theory and formula's, but those exhaust systems were loud with a harsh raspy tone...
Specially when the RPMs hit the "sweet spot"...

But its a very good book that will probably help you anyway.

 

Q=VA

Fluid mechanics is fun, but it sounds like you need more acoustical information than anything else.

Lot's o' engineers on here, huh. NERDS (just like me).