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OEM rear muffler with MBRP cat-back?

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Pittsburgh
#1
I'm wondering if anyone on here has ever tried installing an OEM muffler with an aftermarket cat-back exhaust system. I have the MBRP 3" system on my 2017 Fiesta ST and as many on this site have mentioned the drone is pretty bad. I'm looking for a way to make it quieter (ideally as quiet as the factory exhaust) and am wondering if adding a factory muffler would be a good fix. For those unfamiliar, the MBRP cat-back on my car consists of a straight-through rear muffler and mandrel-bent 3" tubing. Between the inconsistent tubing sizes and the design of the MBRP rear muffler, I have no idea if removing the mandrel-bent tubing and attaching the OEM muffler to this system would work well or lead to any issues. The alternative, of course, would be to replace the cat-back altogether, but I'm trying to explore more cost-effective solutions. I don't know very much about any of this at all (I'm just a girl trying to deal with the aftermath of her dad's differing taste in car exhausts), so if anyone had any thoughts they could share, it would be greatly appreciated!!
 


kevinatfms

Senior Member
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#2
I'm wondering if anyone on here has ever tried installing an OEM muffler with an aftermarket cat-back exhaust system. I have the MBRP 3" system on my 2017 Fiesta ST and as many on this site have mentioned the drone is pretty bad. I'm looking for a way to make it quieter (ideally as quiet as the factory exhaust) and am wondering if adding a factory muffler would be a good fix. For those unfamiliar, the MBRP cat-back on my car consists of a straight-through rear muffler and mandrel-bent 3" tubing. Between the inconsistent tubing sizes and the design of the MBRP rear muffler, I have no idea if removing the mandrel-bent tubing and attaching the OEM muffler to this system would work well or lead to any issues. The alternative, of course, would be to replace the cat-back altogether, but I'm trying to explore more cost-effective solutions. I don't know very much about any of this at all (I'm just a girl trying to deal with the aftermath of her dad's differing taste in car exhausts), so if anyone had any thoughts they could share, it would be greatly appreciated!!
Maybe try out Vibrant mufflers. They are very well known in the 4 cylinder world to quiet even the most obnoxious Honda fart can exhaust system.

Speed Academy used it on their K-swap Civic and it gave it a great tone.

We also use one on our Champ Car Elantra to keep it quiet with literally nothing else(no cats, no resonators). It is easily under 90 decibels at full WOT at 25' off the side of the track.

Here is a link to a 3" version that might work.
https://vibrantperformance.com/flat-black-universal-mufflers-v1247/

You can also add a bottle style resonator or muffler in the middle to eliminate any drone.

But buy a quality muffler. Dont cheap out as most of the time the internal packing in the cheap ones is either very crappy or non-existent. Our Vibrant has held up to a TON of race miles at WOT without a single issue, has great tone for a 4 cylinder and fits well into a smaller location on our Elantra.
 


Clint Beastwood

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#3
Maybe try out Vibrant mufflers. They are very well known in the 4 cylinder world to quiet even the most obnoxious Honda fart can exhaust system.

Speed Academy used it on their K-swap Civic and it gave it a great tone.

We also use one on our Champ Car Elantra to keep it quiet with literally nothing else(no cats, no resonators). It is easily under 90 decibels at full WOT at 25' off the side of the track.

Here is a link to a 3" version that might work.
https://vibrantperformance.com/flat-black-universal-mufflers-v1247/

You can also add a bottle style resonator or muffler in the middle to eliminate any drone.

But buy a quality muffler. Dont cheap out as most of the time the internal packing in the cheap ones is either very crappy or non-existent. Our Vibrant has held up to a TON of race miles at WOT without a single issue, has great tone for a 4 cylinder and fits well into a smaller location on our Elantra.
I added a vibrant to an MBRP and all it did was tune out the high pitched sounds, the deep 106hz drone was still there (and worse, since the high pitched was gone).

The issue with the ST's drone is that 106hz range, which is a 12+ foot long sine wave.

I did a write up using all the acoustic measurement and math here https://www.fiestastforum.com/threads/im-at-it-again-fixing-drone-on-a-brand-new-mbrp.20873/

but honestly, with the low overall quality of everything about the mbrp setup, plus the 3" piping, it creates resonance that can't be handled by damping alone it needs some sort of actual cancellation, which is why the thermal works so well.

OP should try to find a stock exhaust system if they want quiet, or save for a thermal. The flowmaster system is surprisingly well damped in that 106hz range - I haven't had the opportunity to take apart one of the mufflers they include with the system but maybe they've engineered in a small quarter wave or helmholtz resonator

OP - there's also a new range of mufflers from magnaflow that have quarter wave resonators built in, but the muffler alone is ~500 bucks.
 


kevinatfms

Senior Member
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#5
I added a vibrant to an MBRP and all it did was tune out the high pitched sounds, the deep 106hz drone was still there (and worse, since the high pitched was gone).

The issue with the ST's drone is that 106hz range, which is a 12+ foot long sine wave.

I did a write up using all the acoustic measurement and math here https://www.fiestastforum.com/threads/im-at-it-again-fixing-drone-on-a-brand-new-mbrp.20873/

but honestly, with the low overall quality of everything about the mbrp setup, plus the 3" piping, it creates resonance that can't be handled by damping alone it needs some sort of actual cancellation, which is why the thermal works so well.

OP should try to find a stock exhaust system if they want quiet, or save for a thermal. The flowmaster system is surprisingly well damped in that 106hz range - I haven't had the opportunity to take apart one of the mufflers they include with the system but maybe they've engineered in a small quarter wave or helmholtz resonator

OP - there's also a new range of mufflers from magnaflow that have quarter wave resonators built in, but the muffler alone is ~500 bucks.
I was suggesting the full body muffler out back in lieu of a resonator. Doesnt the MBRP already have a resonator in the front of the exhaust system?(if OP's dad removed it then i cant imagine how loud it is)

Couldn't you still effectively eliminate any remaining drone using a larger body muffler. More packing = more harmonic wave absorption considering the MBRP has a resonator already built in up front(and if it is still there in OP's case)?

Id think the OP would want a resonator back in the forward portion(if nothing is there) of the exhaust stream under the seating area then a full body muffler out back? That way the resonator can start canceling the drone further up in the stream while the overall decibel level being reduced and remaining drone frequency being elminated by the large body muffler?
 


Clint Beastwood

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#6
I was suggesting the full body muffler out back in lieu of a resonator. Doesnt the MBRP already have a resonator in the front of the exhaust system?(if OP's dad removed it then i cant imagine how loud it is)

Couldn't you still effectively eliminate any remaining drone using a larger body muffler. More packing = more harmonic wave absorption considering the MBRP has a resonator already built in up front(and if it is still there in OP's case)?

Id think the OP would want a resonator back in the forward portion(if nothing is there) of the exhaust stream under the seating area then a full body muffler out back? That way the resonator can start canceling the drone further up in the stream while the overall decibel level being reduced and remaining drone frequency being elminated by the large body muffler?
The resonator on the MBRP is like the malicious compliance of mufflers. If anything its targeting the tinny rasp at high rpms which is not what the MBRP needs. 3" exhaust on the FiST seems to generally drone the worst at ~2700-3200 rpm, 106.67hz is right in the middle there. The problem is the 12+ foot waveform. Either you need something to fake surface area like a helmholtz chamber or a quarter wave resonator, or you need a muffler/resonator roughly 12.5 feet long.

What you really need to do is record sounds of sweeping through the rpms then go through a spectrum analyzer and watch the waveform, look for the drone as a weird peak in otherwise flat frequency range
 


OP
pierogi
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Thread Starter #9
Wow, thank you for all of the responses! I'm sorry I didn't respond sooner--I had given up hope that I'd hear back on this post, so I wasn't checking it.

This info is super helpful, so thank you so much for sharing! I especially enjoyed the acoustics breakdown--I'm a physicist by trade, so that was fun to read! This definitely reaffirmed my ultimate decision, which was to take off the aftermarket cat-back altogether and put on a factory one. I ended up taking it to a muffler guy and he didn't think it was a good idea to add a muffler to the existing exhaust. I just got my car back from him yesterday and I can't believe how quiet my car is now! I know it's an unpopular opinion on this forum, but I love it.

While I have you guys here--I am having one other issue with my car that's worrying me. I've noticed a few times in the past ~years that sometimes right after I put gas in my car when it's sitting at idle the revs will drop and it will feel like it's about to stall. This was a rare enough occurrence (and went away after a few minutes) that it never worried me, but in the past week or so (even before I put the new exhaust on it) I've noticed it happening more at seemingly random times. In particular, I was just driving around and it was happening at nearly every red light I stopped at. My tank was at 50 miles til empty, so I figured I'd fill up the tank to see if that changed anything and it definitely seemed like it had an effect (the revs dropped a tiny bit one time, but it didn't feel like it was gonna stall). Do any of you car people know what could be causing this? I'm worried that there's a tune on my car that I don't know about or something like that and now that I've replaced the exhaust something is incompatible. I just asked my dad and he said he's not sure about the tune, but he's pretty sure that there's a cold air intake. Not sure if any of that is relevant.

Again, I'd really appreciate hearing anyone's thoughts on this. Thanks!
 


FiestaSTdude

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#10
Wow, thank you for all of the responses! I'm sorry I didn't respond sooner--I had given up hope that I'd hear back on this post, so I wasn't checking it.

This info is super helpful, so thank you so much for sharing! I especially enjoyed the acoustics breakdown--I'm a physicist by trade, so that was fun to read! This definitely reaffirmed my ultimate decision, which was to take off the aftermarket cat-back altogether and put on a factory one. I ended up taking it to a muffler guy and he didn't think it was a good idea to add a muffler to the existing exhaust. I just got my car back from him yesterday and I can't believe how quiet my car is now! I know it's an unpopular opinion on this forum, but I love it.

While I have you guys here--I am having one other issue with my car that's worrying me. I've noticed a few times in the past ~years that sometimes right after I put gas in my car when it's sitting at idle the revs will drop and it will feel like it's about to stall. This was a rare enough occurrence (and went away after a few minutes) that it never worried me, but in the past week or so (even before I put the new exhaust on it) I've noticed it happening more at seemingly random times. In particular, I was just driving around and it was happening at nearly every red light I stopped at. My tank was at 50 miles til empty, so I figured I'd fill up the tank to see if that changed anything and it definitely seemed like it had an effect (the revs dropped a tiny bit one time, but it didn't feel like it was gonna stall). Do any of you car people know what could be causing this? I'm worried that there's a tune on my car that I don't know about or something like that and now that I've replaced the exhaust something is incompatible. I just asked my dad and he said he's not sure about the tune, but he's pretty sure that there's a cold air intake. Not sure if any of that is relevant.

Again, I'd really appreciate hearing anyone's thoughts on this. Thanks!
The stalling thing could be evap related. Do you have any codes?
 


OP
pierogi
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Thread Starter #13
Sometimes there’s codes that don’t illuminate the cel. I would put a code reader on it.
Thanks for the advice! I just ordered a code reader, so I'll check back in when it arrives.

I also looked under my hood to try to deduce whether or not there were any signs of tuning. I did see that I have this cold air intake and the only other non-factory component is by Scosche and connected to the battery that leads all the way to the back of the car and is connected to nothing, so I'm sure it was just for powering a sub or something. I know I also have a blow-off valve, but I could not identify it by eye. None of this is explicitly indicative of tuning to me, but what do I know? Lol
 


OP
pierogi
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Thread Starter #15
When you fill the tank do you stop at the first off 'click' of the pump handle, or do you keep trying to get more fuel in the tank to round up to the next even amount of bucks?
I always stop as soon as it turns off. I think I read another thread on here that mentioned that this issue could have something to do with overfilling the fuel tank, but I'm doubtful that that's it in my case.
 


M-Sport fan

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#16
I always stop as soon as it turns off. I think I read another thread on here that mentioned that this issue could have something to do with overfilling the fuel tank, but I'm doubtful that that's it in my case.
Yes, if it is exhibiting that rough idle/almost stalling behavior after a first stop fill-up, it is most likely the EVAP solenoid, or under hood EVAP tubing assembly which is at fault.

The scanner code(s) will tell you that. [wink]
 


OP
pierogi
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Thread Starter #17
Yes, if it is exhibiting that rough idle/almost stalling behavior after a first stop fill-up, it is most likely the EVAP solenoid, or under hood EVAP tubing assembly which is at fault.

The scanner code(s) will tell you that. [wink]
Thank you for the info, I will definitely check back in tomorrow after I get the code reader!!
 


OP
pierogi
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Thread Starter #18
Well, I just got plugged the code reader into the car and it said "No codes are stored in the module!". I also looked under the "since DTCs cleared" section and it said "EVAP System Mon: OK", so it doesn't seem to me like it's picking up any issues.

I'll also note that this problem is so sporadic. I've driven the car a decent amount since I last filled up the tank and haven't had the issue since. I have no clue what's going on!
 


M-Sport fan

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#19
It could be something as crazy as certain pump handles at certain stations allow just that little bit more fuel to fill before their pressure cutoff valve clicks, and that is just enough to trigger the minor 'overfill symptoms'. [???:)] [dunno]
 


Dialcaliper

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#20
Well, I just got plugged the code reader into the car and it said "No codes are stored in the module!". I also looked under the "since DTCs cleared" section and it said "EVAP System Mon: OK", so it doesn't seem to me like it's picking up any issues.

I'll also note that this problem is so sporadic. I've driven the car a decent amount since I last filled up the tank and haven't had the issue since. I have no clue what's going on!
I had the same problem, no codes, idle cutting out after a fill even when I just let the pump stop and sit for 10-20 seconds before removing it.

Changed out my EVAP harness for a new one and the problem went completely away. The only conflating factor was that I went to a 2014-2015 EVAP harness (the problem might be worse on the later models because of the venturi pulling vapor faster, or it may not matter)

In any case, one thing I noticed that was very distinctive is that while the idle was cutting out like the engine was about to stall, the RPM reading on the Tachometer didn’t really drop before the engine picked up again, even though it sounded like it should have.

Leads me to believe the ECU has a programmed cycle after fillup to account for the extra fuel vapor, and a solenoid going bad doesn’t switch properly or quickly enough, leading to either a rich or lean condition causing the interruption. I wish I’d bothered to plug the AP in and monitor the AFR, as it should probably show something odd. Again, no ECU codes were stored for me.
 


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