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I'm at it again - fixing drone on a brand new MBRP

Clint Beastwood

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#1
(I still advocate just buying a Thermal from Ron@whoosh - but I was impatient and staring at stitches on my hand so I reflex-purchased whatever was available immediately. I have been toying with the idea of building several resonators for people with mbrp exhausts anyways, maybe a replacement rear section or something, so this instance nudged me further in that direction)

I cut my hand pretty badly while fiddling (again) with my custom exhaust, so I hit up Ron@Whoosh to get something *now*, that was pre-made and would fit without trouble (except leveling the g*****n tips?!). Thermals weren't immediately available, so I grabbed an aluminized MBRP to play with (at that price I won't feel bad drilling holes in it). I've done my acoustic sampling and peak drone is at 3200 RPM - its enough that it causes that pressure-y fuzzy feeling in my ears, so it's definitely causing hearing loss. Until I get over to the fabricators to have my helmholtz chamber or quarter wave (I still haven't decided which. If a quarter wave works for thermal its good enough for me!) built I'll be wearing etymotic er20xs earplugs (*fantastic* ppe's, by the way) while driving.

Anyways, I know that I am way more interested in this stuff than anyone else, but since my first few iterations of the exhaust system I've used a thermocouple to better assess the air temperature inside the side branch resonator (it's basically ambient since there's no exhaust flow). My original calculations were a scant few inches off so I still got a tiny bit of drone, but now I have my exact target temperature to math it out properly.

Drone RPM - 3200 is the "peak" drone, so that'll be my target range. I'll add a bit of steel wool to the resonator chamber (just a bit) because it'll broaden the effective cancellation range by a little.

Input: Drone RPM 3200 and Exhaust pulses per RPM 2
so 3200 x 2 = 6400
6400 / 60 = 106.6666667 hertz (or 107, for the rest of this)

The resulting frequency of 107hz matches my acoustic sampling, so with double confirmation I am ready to proceed.

Input: frequency 107hz, speed of sound 1125.33
so 1125.33 / 107 = 10.5171028 (wave length)
10.5171028 / 4 = 2.629275701 (quarter wave length in feet)
2.629275701 * 12 = 31.55130841 (quarter wave length in inches)

SO - I need a side branch 31.55130841 inches long. Or 32 inches long because its easier :p

My hands aren't deft enough to do the tight 32" coil needed to fit in the ST's exhaust cavity, so I'm having my fabricator/favorite exhaust dude do it as soon as he has time. With July 4 coming up I'm not sure if I'll be able to get to him before he heads out to the desert for the holiday, but I'll update as soon as I get it done.



Notes: With the MBRP, it was nearly impossible to install without it touching the center brace, so I added washers to get ~5/8" of clearance between the mbrp and the center brace/plate/thingy.
 


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Clint Beastwood

Clint Beastwood

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I'll be at my fabricator's getting this welded tomorrow. I'm kinda bummed that I can't weld it myself but I'm not steady enough to do the inside of the coils - since I'm moving I don't have a lift anyways so I guess this works fine.
 


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Once you delete unwanted frequencies from the different exhaust systems, do they end up sounding the same or do they have unique sound characteristics?
 


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Clint Beastwood

Clint Beastwood

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Once you delete unwanted frequencies from the different exhaust systems, do they end up sounding the same or do they have unique sound characteristics?
They’re just a lot cleaner - with the acoustic drone it kinda “fuzzes out” the sound so there’s just a loud boominess. Removing those unwanted frequencies just really... cleans it up. I’ll record video to and back from the shop tomorrow to get an a/b comparison. I have video in a thread here called “identifying and fixing drone” or something like that. I can’t find it right now, but I have a before/after my first round of building my own exhaust with just 3” pipe and a resonator. My “speed of sound” numbers were guesses (needed exact temperatures and I was wrong) so it cancelled about 70% of the drone it would have if I had been accurate with my temperatures (since it affects the speed of sound). A quarter wave resonator is the reason people like the thermals sound so much.
 


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I have a Cerakoted Thermal. I was interested to hear your opinions on how the different systems sounded to you
 


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Clint Beastwood

Clint Beastwood

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I have a Cerakoted Thermal. I was interested to hear your opinions on how the different systems sounded to you
I like the one I built the most, it’s very smooth and mild until you get on throttle, then it sounds like god ripping phonebooks in half. Of the other system I’ve heard:

- muffler deletes. To me, this sounds like garbage without acoustic control, the frequencies are all over the place and it’s physically painful to hear. With a teensy bit of massaging/frequency control it’d sound fine, but people don’t do it because they’re looking to put time/money into it. I do not like at all.

- thermal - awesome every-day exhaust, pretty smooth but savage when needed. Best engineered fist exhaust I’ve heard/seen. Used the same quarter wave resonator that I use. Big fan. If I didn’t love constantly tearing mine apart (or if they’d been in stock after I cut my hand on my exhaust lol) I’d have one on my car now.

- cp-e - sounds really good, drones a little because I think the helmholtz chamber is a little under sized, I think they targeted 120hz which is the secondary resonance instead of the primary so some bottom end drone is still there.

- fswerks- it sounded really good for a system with no active cancellation (helmholtz, quarter wave) I would guess the muffler is a little more restrictive as their noise control is pretty good.

- mbrp. It’s basically straight pipe. There’s no real engineering going on, I bought one because it was available immediately and it’s cheaper than having new 3” pipe fabbed. The whole system was cheaper than buying all the mandrel bends to have one made. I’m adding a quarter wave to one literally right now, they’re almost done welding. It’s pretty savage, really loud, and I know I’ll get tired of it before too long.

- stock with resonator delete. Sounds great from the outside, almost inaudible from the inside but there *is* interior drone, still at around 106hz, so we know what the factory resonator is there for. With a quarter wave it would be back to factory quiet inside, but if you’re cutting off the resonator you probably aren’t interested in adding a different resonator. It does pop a bit but you can’t really enjoy it inside. I have my stock system vbanded for easy reinstall with No resonator for long trips, it’s great for a daily. I wouldn’t run it with a crackle tune, i get negative corrections with a crackle tune on stock w/ resonator delete. With any other exhaust and the same tune, no corrections.
 


Spork1569

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I like the one I built the most, it’s very smooth and mild until you get on throttle, then it sounds like god ripping phonebooks in half. Of the other system I’ve heard:

- muffler deletes. To me, this sounds like garbage without acoustic control, the frequencies are all over the place and it’s physically painful to hear. With a teensy bit of massaging/frequency control it’d sound fine, but people don’t do it because they’re looking to put time/money into it. I do not like at all.

- thermal - awesome every-day exhaust, pretty smooth but savage when needed. Best engineered fist exhaust I’ve heard/seen. Used the same quarter wave resonator that I use. Big fan. If I didn’t love constantly tearing mine apart (or if they’d been in stock after I cut my hand on my exhaust lol) I’d have one on my car now.

- cp-e - sounds really good, drones a little because I think the helmholtz chamber is a little under sized, I think they targeted 120hz which is the secondary resonance instead of the primary so some bottom end drone is still there.

- fswerks- it sounded really good for a system with no active cancellation (helmholtz, quarter wave) I would guess the muffler is a little more restrictive as their noise control is pretty good.

- mbrp. It’s basically straight pipe. There’s no real engineering going on, I bought one because it was available immediately and it’s cheaper than having new 3” pipe fabbed. The whole system was cheaper than buying all the mandrel bends to have one made. I’m adding a quarter wave to one literally right now, they’re almost done welding. It’s pretty savage, really loud, and I know I’ll get tired of it before too long.

- stock with resonator delete. Sounds great from the outside, almost inaudible from the inside but there *is* interior drone, still at around 106hz, so we know what the factory resonator is there for. With a quarter wave it would be back to factory quiet inside, but if you’re cutting off the resonator you probably aren’t interested in adding a different resonator. It does pop a bit but you can’t really enjoy it inside. I have my stock system vbanded for easy reinstall with No resonator for long trips, it’s great for a daily. I wouldn’t run it with a crackle tune, i get negative corrections with a crackle tune on stock w/ resonator delete. With any other exhaust and the same tune, no corrections.
Whats your take on Cobb/Mountune/Ford performance? They seem to be some of the bigger exhaust companies but definitely on the pricier side. Hopefully that means that drone is more controlled.
 


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Clint Beastwood

Clint Beastwood

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Whats your take on Cobb/Mountune/Ford performance? They seem to be some of the bigger exhaust companies but definitely on the pricier side. Hopefully that means that drone is more controlled.
I haven't personally heard any of them so I abstained from having an opinion on them, but from sound bites online the mountune one sounded really good. I'm assuming that the Ford one (isn't it borla?) would be pretty low-drone given that Borla is a pretty decent mid-tier exhaust manufacturer, but from pics I don't see any indication of any active noise control, just passive muffler/resonator stuff. They might have some quarter wave or helmholtz stuff going on that you don't see (like thermal). Even BMW M cars have a bunch of tiny quarter-wave resonators inside the big muffler box (it's not a muffler, it just hides the plumbing on a lot of the newer ones).
 


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Clint Beastwood

Clint Beastwood

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Mission accomplished, my MBRP is nearly drone-free. The only way we were able to fit 31.5" of 2.5" tubing under the car is by coiling it like a trumpet in the hollow where the stock muffler was, with mandrel bends. With the center chassis brace you couldn't run it alongside the main pipe so really you have to fit it in the back. My fabricator is currently designing/building his own series of race mufflers that look really nice, whenever the MBRP muffler blows out I'll probably go back and get one of those. I recorded sound to and from the shop so I'll try to have that uploaded for an a/b comparison later today - I do not know if the camera mic will do it justice but allow me to say, my first round at the sound cancellation was ~70% effective because my speed of sound variable was off, this time I was 100% on the dot and its great. Driving around at part throttle the MBRP is barely louder than stock, when you get on the gas it no longer crushes your ears from ~2900-3500 RPM, it does "moo" for a split second before the inverse wave starts doing its thing but it is *worlds* better. Still crackles, burbles, and you can hear the whistling from the turbo, but even when you just start the car and idle it all sounds "cleaner". 10/10, was worth the case of beer I dropped off to my exhaust dude :p
 


Spork1569

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I haven't personally heard any of them so I abstained from having an opinion on them, but from sound bites online the mountune one sounded really good. I'm assuming that the Ford one (isn't it borla?) would be pretty low-drone given that Borla is a pretty decent mid-tier exhaust manufacturer, but from pics I don't see any indication of any active noise control, just passive muffler/resonator stuff. They might have some quarter wave or helmholtz stuff going on that you don't see (like thermal). Even BMW M cars have a bunch of tiny quarter-wave resonators inside the big muffler box (it's not a muffler, it just hides the plumbing on a lot of the newer ones).
I went with Mountune because I lived the sound online and I'm hoping it sounds even better in person.

I do think Ford performance is just a rebranded borla but from the little snippets I've hear it sounds pretty good as well.

Who knew so much effort went into making sure these exhausts sound great and aren't just loud at all.
 


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Clint Beastwood

Clint Beastwood

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Here's the video! No fancy editing, just a quick A/B - unfortunately the in-camera mic doesn't capture the drone all that well, but you can tell when the mic totally fuzzes around 0:25 (deceleration) 2:37 (3200 RPM under load) 2:42 (3200ish RPM under load). There's a bit of fuzz @ 3200 rpm (3:20) afterwards, but just for a split second and its nowhere near as loud inside the car, despite what the camera actually picked up. Under acceleration onto the freeway there's another quick "moo" right before the cancellation starts (5:14) but you should be able to tell how the sound is more mechanical after 5:14, you can actually hear exhaust pulses instead of just WAAAAAAAAAAAAAAA. At cruising speed it's barely louder than stock afterwards.

MBRP without/with quarter wave resonator (recorded inside)





For comparison's sake, here's my full custom one (recorded outside though)
 


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Clint Beastwood

Clint Beastwood

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I went with Mountune because I lived the sound online and I'm hoping it sounds even better in person.

I do think Ford performance is just a rebranded borla but from the little snippets I've hear it sounds pretty good as well.

Who knew so much effort went into making sure these exhausts sound great and aren't just loud at all.
I think the mountune one sounded very european. I wish more cars sounded like a VW R32 - they were such a garbage car but man did they sound incredible, even stock. I read somewhere that they had musicians working on the exhaust tuning? I don't know if true, just saw it on vwvortex.
 


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MRX turbo, Mountune DP and Thermal. Whatever they did it works with this setup and there are no sounds I don’t like. Even when I get on it I like how it sounds and I wouldn’t call it savage. I call it perfect.
 


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Clint Beastwood

Clint Beastwood

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MRX turbo, Mountune DP and Thermal. Whatever they did it works with this setup and there are no sounds I don’t like. Even when I get on it I like how it sounds and I wouldn’t call it savage. I call it perfect.
Oh I meant savage because at the exhaust outlet it makes a kinda ripping paper sound that is kinda bad-ass :p the one I built in the clip above does it pretty loudly (my older one) - when I get on the gas you can just hear it whooshing more than any exhaust noise. That's the part I like, functional-noise thats happening for a reason.
 


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Oh I meant savage because at the exhaust outlet it makes a kinda ripping paper sound that is kinda bad-ass :p the one I built in the clip above does it pretty loudly (my older one) - when I get on the gas you can just hear it whooshing more than any exhaust noise. That's the part I like, functional-noise thats happening for a reason.
Yes. I just didn’t want the sound misrepresented and turn people away.
 


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Clint Beastwood

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Oh I forgot to get pics of the new MBRP resonance chamber, but here's a photo of one of my early iterations of my custom exhaust showing the resonance chamber. There was more room on the MBRP and the trombone-shaped resonating chamber is ~5" longer. Sorry for not grabbing pics, my fabricator's dog was in the office and I pretty much spent all my time playing with the dog.
 


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flbchbm

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Anyways, I know that I am way more interested in this stuff than anyone else, .
Don't be so sure. I'm into researching a lot of stuff I'll never use. I sit at work and wait for things to break, literally. 10 hours a day for 4 days. At work, Youtube is my BFF. This is like speaker cabinet design.

Edit: I'm watching one guy talk about HOW MANY GOATS PER ACRE and on the other screen MY BIGGEST TURN TO DATE! (wood lathe) and this on my center screen. NEITHER of which I'm going to apply to anything every in my life..lol
 


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Clint Beastwood

Clint Beastwood

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Don't be so sure. I'm into researching a lot of stuff I'll never use. I sit at work and wait for things to break, literally. 10 hours a day for 4 days. At work, Youtube is my BFF. This is like speaker cabinet design.
Yeah it uses the exact same math when you are doing helmholtz chamber design. The problem is I'm not a math whiz so I keep doing quarter-wave as it's easier. Helmholtz is smaller and easier to fit into a system but the quarter wave is MUCH more effective (though over a much narrower range).
 


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Very Interesting research, Thanks for sharing all the valuable data, question? where can I find a muffler similar as yours?
 


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Clint Beastwood

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Very Interesting research, Thanks for sharing all the valuable data, question? where can I find a muffler similar as yours?
Do you mean the trombone shaped resonator I designed for my mbrp? Or the tiny muffler on the full custom system I built in the post above? The tiny muffler on the system pictured is a vibrant ultra quiet, available amazon and other places. If you mean the custom resonating chamber that was custom built. You’re free to take a pic of mine and go to an exhaust shop and say “I want something like this with a total length of xx inches”. Be aware though, to fit the full 31ish inch camber you can’t have a muffler in the rear cavity, unless you use something tiny like the vibrant ultra quiet. The vibrant case is short so it only affects higher frequencies. If you’re behind a car with an mbrp you’ll hear the turbo impeller... whistle? The vibrant ultra quiet would get rid of that whistle And other high frequencies but won’t cure the drone, that’s what the custom trombone looking piece does.
 




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