MBRP 3in cat back exhaust.

[The.Golf.Father]

No - the problem is that the "Drone" is a wave roughly 10 feet long, a short-bodied bullet silencer will only filter higher frequencies (~200-240hz with those tiny ones) which will just make it deeper. You need to attenuate a really long wave, and the only ways to do that without a 10+ foot long muffler are either a helmholtz chamber (like CP-E) or a quarter wave chamber (thermal, my custom one). Unfortunately it's not a matter of just packing in more mufflers. I had read that Thermal sells standalone mufflers, but I do not know if that means they'll sell the one with the built-in quarter wave resonator.

I think there are a few muffler manufacturers out there that build quarter wave resonators into their mufflers, like Corsa and maybe Billy Boat, but I haven't been able to get email replies from either about attenuating a specific range.

Is the 1/4 wave resonator built into the muffler on the Thermal exhaust or the resonator? Thermal does sell resonators and mufflers separately relatively affordable as well.

 

[The.Golf.Father]

Is the 1/4 wave resonator built into the muffler on the Thermal exhaust or the resonator? Thermal does sell resonators and mufflers separately relatively affordable as well.

"To address the drone issues that can arise from the larger 3” tubing we added a straight through resonator on the mid-pipe and a resonator on the rear section. This allows for no restriction when accelerating, but deflects the unpleasant tones that can creep in while cruising." - From Thermal's website regarding the ST catback.
I wonder if simply adding thermal's resonator to the rear would solve the MBRP's drone???

 

[Clint Beastwood]

"To address the drone issues that can arise from the larger 3” tubing we added a straight through resonator on the mid-pipe and a resonator on the rear section. This allows for no restriction when accelerating, but deflects the unpleasant tones that can creep in while cruising." - From Thermal's website regarding the ST catback.
I wonder if simply adding thermal's resonator to the rear would solve the MBRP's drone???

It's their rear resonator that's doing it, it's a really cleverly designed quarter-wave. It's what makes the thermal exhaust magic. If you're able to get the muffler only from them post back here and let me know. It's much cleaner than what I built myself - though I enjoy the DIY process

 

[Clint Beastwood]

This from their website, I doubt the one the use with the Fiesta ST kit is any different, I mean why would they engineer a completely different resonator for one relatively obscure car model. In pictures of the Thermal complete exhaust it looks no different. I have measured the straight pipe distance between the MBRP Y section behind the tips and the curved 3" portion going forward and a 12" resonator would fit welded in. This isn't much more $ than the vibrant resonator and is proven for the car.
View attachment 23086

check out this image from whoosh (where you should buy a thermal from!!)

- the rear section "muffler" - see how it's offset? It's because it loops back and forth 3 times to get the 31.55130841" chamber required to attenuate 106.67hz. The front straight-through resonator is only filtering high-pitch sounds. To get a straight-through resonator to attenuate 106.67hz it would have to be 10.5171028 FEET long. That's what makes quarter-wave resonators so neat, you can attenuate a huge wave in a "relatively" small space. Well, smaller than ten and a half feet

My original custom exhaust used both a vibrant straight-through resonator and a vibrant ultra-quiet rear. Tons of deadening, but all it did was make the 106.67hz tones more noticeable. I had to actively cancel that frequency to make it livable. Now that I added a quarter-wave to my MBRP, it's so quiet in the passenger compartment that on the freeway you don't notice it has exhaust until you floor it, and it's even quieter just sitting there idling.

Here's a before/after of an MBRP with/without a quarter wave resonator. Unfortunately I was only using the in-camera mic, but it gives a general idea - you can hear when the mic "buzzes" before - that's because the built in mic is getting whited out at sub-110hz.

 

[The.Golf.Father]

check out this image from whoosh (where you should buy a thermal from!!)

- the rear section "muffler" - see how it's offset? It's because it loops back and forth 3 times to get the 31.55130841" chamber required to attenuate 106.67hz. The front straight-through resonator is only filtering high-pitch sounds. To get a straight-through resonator to attenuate 106.67hz it would have to be 10.5171028 FEET long. That's what makes quarter-wave resonators so neat, you can attenuate a huge wave in a "relatively" small space. Well, smaller than ten and a half feet

My original custom exhaust used both a vibrant straight-through resonator and a vibrant ultra-quiet rear. Tons of deadening, but all it did was make the 106.67hz tones more noticeable. I had to actively cancel that frequency to make it livable. Now that I added a quarter-wave to my MBRP, it's so quiet in the passenger compartment that on the freeway you don't notice it has exhaust until you floor it, and it's even quieter just sitting there idling.

Here's a before/after of an MBRP with/without a quarter wave resonator. Unfortunately I was only using the in-camera mic, but it gives a general idea - you can hear when the mic "buzzes" before - that's because the built in mic is getting whited out at sub-110hz.

Yes I see how the rear resonator is offset.
I just came across a set of calcs to determine what length 1/4 wave resonator to use, see below, does this jive with your findings?

STEP 1: Use this to find your frequency of drone in hertz:

1. you will need two variables: the drone RPM, and # of cylinders your engine has.
2. divide the RPM by 60 to get the RPS, revolutions per second. You divide by 60 because RPM is revolutions per minute. 60 seconds in a minute so you divide by 60 to get RPS.
3. divide the # of cylinders by 2 to get # fired per revolution.
4. multiply RPS by # fired per rev to get the drone frequency in Hz.

1. 2,000 RPM and 6 cylinders
2. 2,000 RPM / 60 = 33.3333.... RPS
3. 6 cylinders / 2 = 3 fires per revolution
4. 33.3333..... RPS * 3 fires per revolution = 100 Hertz

STEP 2: The formula to get the length of your resonator is:

Wavelength = Speed of sound / Frequency
Resonator length = Wavelength / 4

We divide by 4 because we need a quarter of the wavelength for this to work. These are called side branch resonators or 1/4 wave resonators. I've seen both while doing research on this.

STEP 3: To get the speed of sound you need to find the temperature of your exhaust where you will install your resonator. To do this you will need to buy a temperature gun.

Etekcity Lasergrip 1080 IR Thermometer $15 and I'll never use it again but oh well lol once you get the temperature use this calculator to get the speed of sound.

Speed of sound calculator

STEP 4: Once you get your frequency and speed of sound, this is the easy part. Just plug your data into the formula!

So for example, my exhaust temp was measured at ~240 degrees Fahrenheit and the speed of sound at that temp is 1296.637 feet per second. My above calculations gave me 100 Hz for my drone frequency.

Wavelength: 1296.637 ft per sec / 100 Hz = 12.96637 ft
Resonator length: 12.96637 ft / 4 = 3.2415925 ft

 

[Clint Beastwood]

Yes I see how the rear resonator is offset.
I just came across a set of calcs to determine what length 1/4 wave resonator to use, see below, does this jive with your findings?

STEP 1: Use this to find your frequency of drone in hertz:

1. you will need two variables: the drone RPM, and # of cylinders your engine has.
2. divide the RPM by 60 to get the RPS, revolutions per second. You divide by 60 because RPM is revolutions per minute. 60 seconds in a minute so you divide by 60 to get RPS.
3. divide the # of cylinders by 2 to get # fired per revolution.
4. multiply RPS by # fired per rev to get the drone frequency in Hz.

1. 2,000 RPM and 6 cylinders
2. 2,000 RPM / 60 = 33.3333.... RPS
3. 6 cylinders / 2 = 3 fires per revolution
4. 33.3333..... RPS * 3 fires per revolution = 100 Hertz

STEP 2: The formula to get the length of your resonator is:

Wavelength = Speed of sound / Frequency
Resonator length = Wavelength / 4

We divide by 4 because we need a quarter of the wavelength for this to work. These are called side branch resonators or 1/4 wave resonators. I've seen both while doing research on this.

STEP 3: To get the speed of sound you need to find the temperature of your exhaust where you will install your resonator. To do this you will need to buy a temperature gun.

Etekcity Lasergrip 1080 IR Thermometer $15 and I'll never use it again but oh well lol once you get the temperature use this calculator to get the speed of sound.

Speed of sound calculator

STEP 4: Once you get your frequency and speed of sound, this is the easy part. Just plug your data into the formula!

So for example, my exhaust temp was measured at ~240 degrees Fahrenheit and the speed of sound at that temp is 1296.637 feet per second. My above calculations gave me 100 Hz for my drone frequency.

Wavelength: 1296.637 ft per sec / 100 Hz = 12.96637 ft
Resonator length: 12.96637 ft / 4 = 3.2415925 ft


View attachment 23087

I put a whole host of nerd info in here https://www.fiestastforum.com/threads/im-at-it-again-fixing-drone-on-a-brand-new-mbrp.20873 as well as an older post about "identifying and cancelling drone" - though the older post my speed of sound was off. I didn't realize the chamber temp would stay around ambient, since there's no actual gas flow. Once it's pressurized, it levels off to about ambient temp. It does mean that your drone cancellation is different on hot vs. cold days, but not obscenely so.

 

[The.Golf.Father]

I put a whole host of nerd info in here https://www.fiestastforum.com/threads/im-at-it-again-fixing-drone-on-a-brand-new-mbrp.20873 as well as an older post about "identifying and cancelling drone" - though the older post my speed of sound was off. I didn't realize the chamber temp would stay around ambient, since there's no actual gas flow. Once it's pressurized, it levels off to about ambient temp. It does mean that your drone cancellation is different on hot vs. cold days, but not obscenely so.

Yes thank you, I have read your posts on this in the past.
I am wondering what range of RPM this would have an effect on, like if you calculate based on 3200 rpm and nail the length of the branch resonator perfectly will drone at 3800 be diminished as well? and what if you used 2.5" pipe for the resonator welded to the 3" system?- as space/clearance is an issue on the Fiesta with 3" installed.
I like to tinker with things, maybe I will give this a try.

 

[Clint Beastwood]

Yes thank you, I have read your posts on this in the past.
I am wondering what range of RPM this would have an effect on, like if you calculate based on 3200 rpm and nail the length of the branch resonator perfectly will drone at 3800 be diminished as well? and what if you used 2.5" pipe for the resonator welded to the 3" system?- as space/clearance is an issue on the Fiesta with 3" installed.
I like to tinker with things, maybe I will give this a try.

Mine is 2.5" pipe t'd into a 3" system. The larger the diameter of the side-branch resonator, the broader the effective range of attenuation. There are a few pics of how mine fits on that other thread, it curls back on itself like a trombone or trumpet to fit it all in the muffler cavity. If I had a muffler in the stock location, no idea how I'd ever fit the side-branch.

To find your drone range, double-blind it. First, drive around and identify the RPM where the drone feels the worst - usually the MOST obnoxious point for a FiST is 3200rpm, with a curve down on either side of that range. (If you're getting more attenuation at 3800rpm, its possible your exhaust is touching the body somewhere, probably the mid chassis brace, and it makes the body of your car into a resonator like the body of a guitar). After you identify where the drone feels worst, then do some recordings using a spectrum analyzer app on your phone. Inside the car it might peak at 120hz because of secondary resonance, but if you do several recordings and average them you'll likely find your peak at ~3200.


I can't find it right now but I did post a link here on the forum to a free spreadsheet/calculator I made that has a bit more detail and easy input.

 

[The.Golf.Father]

Mine is 2.5" pipe t'd into a 3" system. The larger the diameter of the side-branch resonator, the broader the effective range of attenuation. There are a few pics of how mine fits on that other thread, it curls back on itself like a trombone or trumpet to fit it all in the muffler cavity. If I had a muffler in the stock location, no idea how I'd ever fit the side-branch.

To find your drone range, double-blind it. First, drive around and identify the RPM where the drone feels the worst - usually the MOST obnoxious point for a FiST is 3200rpm, with a curve down on either side of that range. (If you're getting more attenuation at 3800rpm, its possible your exhaust is touching the body somewhere, probably the mid chassis brace, and it makes the body of your car into a resonator like the body of a guitar). After you identify where the drone feels worst, then do some recordings using a spectrum analyzer app on your phone. Inside the car it might peak at 120hz because of secondary resonance, but if you do several recordings and average them you'll likely find your peak at ~3200.


I can't find it right now but I did post a link here on the forum to a free spreadsheet/calculator I made that has a bit more detail and easy input.

I like how you did that on your custom exhaust version. Now you got me looking for a local exhaust shop! Yet another reason I wish I had a welder. Could just buy two of these from Summit, chop one in half to get the 90 and then the other flip around butt the straight sections and weld, cut down and cap, drill a 2-1/4" hole welderup and done.

 

[Clint Beastwood]

I like how you did that on your custom exhaust version. Now you got me looking for a local exhaust shop! Yet another reason I wish I had a welder.

I bought a welder to do this stuff, but once I realized I'd need the car on a lift to get it all to fit right anyways I decided to just pay my normal fab guy to do it. You can take a pic of my exhaust to give them an idea of what you want to do.

I was toying with the idea of building MBRP-specific rear sections for a while to sell, but it's not cost-effective at small volume. Instead I just put the "how-to" documentation out here

If it wouldn't get him sued by MBRP I'd bet ron could make a "MBRP-drone fix kit" but that's suuuuuuper niche, I doubt many would buy it. I sent my info to MBRP too, but I have a feeling it just went into a nameless corporate inbox and was discarded.

 

[The.Golf.Father]

I bought a welder to do this stuff, but once I realized I'd need the car on a lift to get it all to fit right anyways I decided to just pay my normal fab guy to do it. You can take a pic of my exhaust to give them an idea of what you want to do.

I was toying with the idea of building MBRP-specific rear sections for a while to sell, but it's not cost-effective at small volume. Instead I just put the "how-to" documentation out here

If it wouldn't get him sued by MBRP I'd bet ron could make a "MBRP-drone fix kit" but that's suuuuuuper niche, I doubt many would buy it. I sent my info to MBRP too, but I have a feeling it just went into a nameless corporate inbox and was discarded.

Another issue with a "drone fix kit" is it would be very difficult to build one in that tight space as a bolt on kit, this branch resonator really has to be welded on.

 

[Clint Beastwood]

Another issue with a "drone fix kit" is it would be very difficult to build one in that tight space as a bolt on kit, this branch resonator really has to be welded on.

Oh I meant the entire rear section, like, the entire rear-most part of the MBRP. Un-clamp the stock rear section, clamp on mine. It just wasn't realistically do-able - the shipping would cost to much to accept cores back, so purchasers would be stuck with a big chunk of rear section to store.