Help me quiet down a new exhaust

Just finished welding up a new SS exhaust for my '93 MB 400E.  If you're unaware, it has a 4.2L V8, roughly 280hp stock.  The exhaust was original and was on it's way out - a few dime-size holes were present, and the cats by now surely weren't doing much after 30 years and 210k+ miles.  The only thing was, it actually sounded great.  Better as the holes developed.  Quiet most of the time, but when you got on it and went above 4k rpm, it had a real nice sound.  It was a rather strange plumbing setup to me, as it went from 2 to 1 back to 2 cats then 2 into a resonator, then 1 out into the muffler.  The pipe sizes were all over the place.  :shrug:

Anyway, a stock replacement, if you could find one, was going to be something like $2k, and the few aftermarket options were more like $3k+.  So, having fab'd 2 in the past year for other cars that I was quite happy with, I went ahead and made my own.  Based on the previous 2, I used a rough formula to size the main pipe based on hp.  Based on that, I wanted a single 2.75" dia exhaust with a cat and a muffler.  Problem is, cats and mufflers with 2.75" inlets/outlets are pretty hard to come by.  2.5" would have been smaller than stock.  So, with some trepidation I went up to 3".   

Now that it's in there, it's a bit ... anti-social.  And out of character with the car.  It's not Charger loud, but it's more than I want for that car.  The main issue is about 1200-2200 rpm.  It's just too loud / droney in that band.  At idle it's actually very quiet and totally fine.  Above 4k rpm, it sounds angry, which I like 

I was afraid this might happen, so I purposely included a straight section where the stock resonator (1st muffler?) lived.  I've got room there to put in a resonator, or another muffler, or ....  ?  

My question is, what's my best option here?  Another muffler?  A resonator?  A short section of 2.75" pipe?  My Googling hasn't turned up much hard information, mostly marketing spiel. 

Thanks for the help! 

Hard question as sound is very subjective.  What you might find perfect your wife thinks is too loud, etc.  Another muffler will quiet it down more than a resonator, so I guess you need to decide how much volume you want to knock out of it.

The super fast of it is, a resonator won't do anything to quiet down noise. It'll take out resonance / drone in a certain range, but won't knock down the overall noise. 

A muffler will knock down the noise level, but adding a second one may very well indeed still leave you with a drone at a certain rpm, just overall quieter. 

Honestly a single 2.5" would have flowed more than enough.   I've been involved with cars making 300-350 wheel through a single 2.5" with a muffler. NA small blocks.  We run 2.5" duals (different world entirely than a single, but....) on stuff that makes 650-700 at the flywheel, all the time.  The difference in power for a street car is negligible...but the difference in controlling tone and noise is HUGE when you are dealing with a primarily street driven vehicle, especially one in which NVH is a top priority. 

If it were me, I'd try to isolate and figure out if it's a resonance issue or an overall noise issue thats bugging you. If it's a resonance issue, get a frequency meter  and figure out where specifically the issue is. Then you can run the math and either fab a hemholtz style setup where that length of straight pipe is, or start to play with diff resonators or slip in baffles to quell the drone, at that specific frequency. 

Make sense? Again. That's a SUPER fast reply, as there is a LOT that goes into designing a proper system. Especially if you are aiming for a certain tone and volume, both inside the cabin and outside. 

I'd put a small muffler at the back of the system as close to the rear tip as possible.  It seems like the systems that have long extensions of pipe without a resonator or muffler before the tip are more prone to drone, from what I've noticed, and I don't know why so take this all with a grain of salt.  Probably the harmonics involved in that long stretch of pipe.  In any case, that's what I'd be looking at doing.

GaryC83 said:

Honestly a single 2.5" would have flowed more than enough.   

If it were me, I'd try to isolate and figure out if it's a resonance issue or an overall noise issue thats bugging you. If it's a resonance issue, get a frequency meter  and figure out where specifically the issue is. Then you can run the math and either fab a hemholtz style setup where that length of straight pipe is, or start to play with diff resonators or slip in baffles to quell the drone, at that specific frequency. 

Yeah, live and learn.  I think the multiple diameters of the stock system muddied my mental waters.  

The freq meter is a good idea.  Can you clarify what you mean by where it is?  You mean physically along the length of the exhaust where it is?  

In reply to NorseDave :

In other words, if you have a bad drone at say 65 mph, hold the car there and run it with a meter. Once you do that, you'll see where its spiking, frequency wise. If its at say 120-125hz, for example, there are calculations you can run that will tell you what the size and length of a helmholtz resonator you need to fabricate, in order to dampen that specific frequency, thus culling that resonance at cruise.  

In reply to GaryC83 :

Ah, gotcha.  Was already planning on that.  I know the rpm range, but I don't know if the annoying noise is the fundamental (most likely) or a harmonic.  

Seems like a fun little science project!

I'm going to ramble a bit.  Apologies.

An exhaust is a great big tuba under the car.  Just like you vibe your lips in the mouthpiece of a tuba, the engine is making big farty noises at the exhaust valves and they go through the tube.  In the case of the tuba, it's designed to carefully amplify and shape the tone of your lip farts.  The exhaust in a car is designed to attenuate some and preserve others.  My guess is that the 2-1-2 setup in the factory system was a typically over-engineered German thing.  It was a means of maintaining flow that satisfied the engine's needs while keeping it MB quiet.  The good news is that it can be done.  The bad news is that MB likely spent a few million euros and had sophisticated equipment.

The big thing to know is that sound hates change.  This is why we add dynamat to our door panels.  The sound hits the panel (now weighted with butyl gooey stuff) and the panel resists moving in repsonse to the weight.  Then it gets to the gooey stuff and has to fight through it like walking through tar.  Then it gets to the foil backing and has to change again.  Then it gets to the open air in the door and it's finally like "dammit, I give up."

The downside is that flow also hates change... at least when it comes to velocity.  You want to make a system that flows evenly, so suddenly adding a 5" muffler would kill velocity and cost power/torque.

I would suggest a few things.  First, you can usually step down tubing after the 2/3rds point or so.  The exhaust has shed enough heat that it often can make things a wee quieter while actually maintaining flow.  GM did this a lot on their vehicles.  Don't go nuts, but maybe dropping from a single 3" to two 2.25", or a single 3" to a single 2.75" after the muffler could help.

It's also important to realize that sound is a by-product of flow.  The engine needs a certain flow to move the exhaust gasses, but the sound is primarily made by the rapid pulses of the exhaust valve events.  You can add things that absorb the sound energy waves bouncing around IN the flow without hurting the flow.  This is why many exhausts have resonators... which are really just non-restrictive second mufflers.  A lot of people use race bullet mufflers as resonators and they tend to help.  I've only done a few, so I can't really suggest, but any straight-through fiber-packed small muffler should work.

But the big question after that mindless rambling... what kind of muffler did you use?

Edit to add:  Unless you have access to MB-level engineering tools, it's usually best to put your solution nearest the tailpipe.  The piping itself can generate drone frequencies (standing waves) by amplifying the vibes that the engine gives it.  Putting your solution closer to the tailpipe means there is less chance that the tubing behind it can cause another length of tubing which can re-introduce that drone.

Also... where does the exhaust exit?  Are there downturn tips?  downturn tips can make a standing wave between the ground and the trunk floor.

P.S.... I'm thinking about some similar measures myself.  On my 06 Chevy van I swapped the factory muffler for a magnaflow.  The sound outside is incredible.  Not too loud, but a beautiful note.  Inside it drones like mad at somewhere around 2200-2500 (no tach, so guessing).  My first test will be to strap a 1' section of heavy angle iron to the tubing and move it around fore/aft.  If it changes the drone, I know I have a vibrating section of pipe.  The solution there is to add a hanger wherever the drone is most affected by adding the angle iron.  If it doesn't change it, I'll add a resonator in place of the pretty chrome tip.

In reply to Curtis73 (Forum Supporter) :

Well said. 10 thumbs up. I'm no acoustical expert or engineer, but a 1200-2200 rpm drone times 8 cylinders per revolution equals 9,600-17,600 Hz which is where your drone is. Stereo room modes are the frequency that a 3 sided rectangular room will cancel their equivalent frequencies at the exact center of of the room. by picking the correct 3 dimensions of your resonator, you can cancel 3 frequencies in your rpm range if you exit your resonator at it's center. There are online calculators that can do it for you, but for what you want, it will take a lot of trial and error to figure it out. Hopefully you can find 3 dimensions that wind up close to 3'x.9'x .5'.

I want to place a tennis ball hung exactly in the "Middle" of the room embedded with dulled and rubber coated razer blades to remind people that three frequencies of the music from the World's Greatest $1500 stereo are canceled out and don't stand there. 

 I have wondered if there wasn't a way to stuff a great speaker into the intake plenum of a car and with a lot of power, create a ram jet supercharger with sound at a certain frequency.

VolvoHeretic said:

Well said. 10 thumbs up. I'm no acoustical expert or engineer, but a 1200-2200 rpm drone times 8 cylinders per revolution equals 9,600-17,600 Hz which is where your drone is.

I'm no expert either, but I'm fairly certain your math is off.  Base frequency = RPM*(# firing pulses/rev)/60.  So 1800 rpm * (8 cyl/2) / 60 = 120Hz.  

9.6kHz - 17.6kHz would be so high that most ppl over probably 40 wouldn't hear lots of that.  

Curtis73 (Forum Supporter) said:

I'm going to ramble a bit.  Apologies.

None needed.  Good stuff.

I was wondering if necking down to 2.75 or 2.5 might help.  But I have the muffler (a large Magnaflow)  at the very back, about 4" from the bumper, so I can't put anything behind that.  I have to crawl back under and see if I can fit a 1/4 wave resonator in the space I have available.  120Hz (1800rpm) says it should be  28.13".  Which would probably require some spaghetti-ish routing.  

Gonna have to play around with stuff I guess.  I'm assuming for now that the base freq is the prime offender, I'll have to verify that though.  Like I said, seems like a good science project.   

NorseDave said:

VolvoHeretic said:

Well said. 10 thumbs up. I'm no acoustical expert or engineer, but a 1200-2200 rpm drone times 8 cylinders per revolution equals 9,600-17,600 Hz which is where your drone is.

I'm no expert either, but I'm fairly certain your math is off.  Base frequency = RPM*(# firing pulses/rev)/60.  So 1800 rpm * (8 cyl/2) / 60 = 120Hz.  

9.6kHz - 17.6kHz would be so high that most ppl over probably 40 wouldn't hear lots of that.  

Lol, good point. I'm not very good at math either, but still, the theory of sound cancelation is valid. 

NorseDave said:

It was a rather strange plumbing setup to me, as it went from 2 to 1 back to 2 cats then 2 into a resonator, then 1 out into the muffler.  

The 2 to 1 back to 2 sounds kinda like an "X" pipe arrangement. That would have merged the sound pulses and changed how the end result sounds.

Curtis73 (Forum Supporter) 

Edit to add:  Unless you have access to MB-level engineering tools, it's usually best to put your solution nearest the tailpipe.  The piping itself can generate drone frequencies (standing waves) by amplifying the vibes that the engine gives it.  Putting your solution closer to the tailpipe means there is less chance that the tubing behind it can cause another length of tubing which can re-introduce that drone.

All good points. Save for that. With most resonators you want them UPSTREAM. In fact, as far UPSTREAM as you can reasonably get them, for them to be the *most* effective.  Ideally usually under the seating area is most effective, for a variety of issues, especially when dealing with lower frequency drone, as you'll tend to see at cruising rpm. Higher frequency issues can be dealt with further downstream with relative ease, but placing them upstream for low frequency cruise issues works best. Also has the added bonus of killing off the drone and reverb before it makes it through the cabin and can possibly get amplified by a 5' shot of relatively straight pipe before hitting the muffler. 

Like I said, there is a LOT a lot that goes into designing a proper system. I'm by far and away no expert, but I've probably spent way more time working on this junk than most would ever care to even think about. 

In reply to NorseDave :

Which is why I suggested a helmholtz style resonator. You can tag right in and package them typically in a much easier and less "expansive" fashion than quarter wave / j type resonators.  

https://www.faurecia.com/en/newsroom/faurecia-wins-2019-pace-award-resonance-free-pipetm-technology

Those are another option as well. I've had a few sets laying around for a few years now, but have yet to use / try them. I acquired new donor pipes from the dealer and cut the patches out of those. You'd either have to do the same or find them in a junkyard. 

Of note, the reason I haven't tried them is no reason to. I've got a system that has been working so I haven't dedicated the time to making a set of swappable tailpipes to do back to back comparisons. It's a LOT of work setting these exhaust systems up the way we do, and building a second set of pipes just to see if something "may" work... is a pretty big endeavor, with the level we finish off the cars to.  And I haven't cared enough to dive in on my own junk...so they have been sitting.

(based on Gary's recommendation) Google found this. Hard to tell on mobile device if the calculators allow variables but it seems to indicate they do. 

https://wilhelmraceworks.com/blog/killing-drone

More mufflers may help.  When I last rebuilt the Jeep exhaust, I got it a bit quieter and significantly reduced drone by ditching the one huge Magnaflow for 2 smaller (different size) ones in series.  .  That's also on a 3-inch y-piped single.  Interestingly, in that setup, a Flowmaster 70 droned less than the big Magnaflow, although the FM70 was rather loud.  The big Magnaflow was a 5x11x22 case, the smaller ones are 4x9x14 and 4x9x18. 

With the smaller mufflers, the tone isn't quite as deep, but it's quieter, particularly at idle and light throttle.  The big muffler was a bit too deep and boomy sounding unless you were leaning on the throttle, and had a significant drone problem (partly due to the large, unsupported case surfaces resonating, I think). 

In reply to NorseDave :

I'm learning that Magnaflow mufflers tend to sound amazing but also do very little to cancel certain frequencies.  I had the same problem with several of the Flowmaster designs.

Curtis73 (Forum Supporter) said:

In reply to NorseDave :

I'm learning that Magnaflow mufflers tend to sound amazing but also do very little to cancel certain frequencies.  I had the same problem with several of the Flowmaster designs.

Good point on some designs not cancelling certain frequencies well.  Different size mufflers helps with that some, but using 2 different designs in series (such as 1 chambered, 1 Magnaflow type) might do a better job. 

On my S2000, I realized having one of these helped cut the drone out a ton. Not the thing circled but the extension to the right and bottom. I had one welded on to an aftermarket exhaust and it cured a lot of the highway drone. I don't know the science behind it at all.

DirtyBird222 said:

On my S2000, I realized having one of these helped cut the drone out a ton. Not the thing circled but the extension to the right and bottom. I had one welded on to an aftermarket exhaust and it cured a lot of the highway drone. I don't know the science behind it at all.

Resonance again, the dead-end extension will reduce drone at a certain frequency depending on its volume.

OHSCrifle said:

(based on Gary's recommendation) Google found this. Hard to tell on mobile device if the calculators allow variables but it seems to indicate they do. 

https://wilhelmraceworks.com/blog/killing-drone

Good find.  That link is super useful. 

Here's a ton of banter in the S2000 forums about cutting down drone and some solutions. They even get into V6/V8 drone https://www.s2ki.com/forums/s2000-under-hood-22/amuse-helmholtz-resonator-dimensions-510969/page8/