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Thread: E30 325iS exhaust too LOUD! Magnaflow Cat + B&B muffler - is resonator the answer?

  1. #1
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    E30 325iS exhaust too LOUD! Magnaflow Cat + B&B muffler - is resonator the answer?

    About ~5 years ago I had to replace my cat back exhaust, and always like the sound of B&B mufflers so that's what I went with. I had to replace the Cat as well, and rather than spend the $$ on a stock Cat I decided to cheap out and got a Magnaflow Catalytic Converter for 4.5x less. The first thing I noticed is that it was half the size of the stock unit ...

    Anyway, the combo Magnaflow + B&B is wayyy too loud, and the drone at 2-3k rpm is deafening. I think had I ponied up for the OEM Cat it would have knocked down the volume and drone more to my liking, but this combination just isn't fun anymore. I looked at stock Cats and they've gone up to $2,000 - ouch. That sadi I'm not sure just replacing the Cat alone would get me to where I want to be?

    So now I'm thinking of putting a resonator in line between the Cat and Muffler to knock down the volume and eliminate the drone. Am I correct int that resonators will decrease the volume and drone? I'm after the raspy euro sound, but don't want the volume and droning pain.

    Is this a reasonable approach? If so, any recommendations on resonators? Can resonators be welded in between the Cat and muffler? I'm fairly certain I'd need 2 of them, one for each pipe, or is there a X-connect type alternative?

    Thanks in advance for any insight.

  2. #2
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    I have a Dec cat and an Ireland engineering muffler. The Dec cat was cheap comparatively, and the quality doesn't look great. However paired with the Ireland engineering muffler it is a tad louder than stock. I think I'd start with a new muffler and see how that sounds before you replace your cat and I highly recommend the Ireland engineering of you don't want a lot of noise or drone and it seems very high quality.

  3. #3
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    Hey Thanks for the quick comment Mikealwa.

    I actually don't want to replace anything - not yet anyways. Everything is in great shape as the car only gets driven on sunny days here & there (117k on the clock for a 1990). I just want the most economical way to turn the volume/drone down, which I believe is by adding resonators in line. I assume they'd go right in front of the muffler welded into the straight sections of pipe, but I'm looking for input from someone that knows more about these things than I do.

    I'm learning how to weld (something I've always wanted to do) but not sure I'm up to that task yet. At the end of the day I may just take it to a shop to have it done, but I need to research shops around me too.

  4. #4
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    a muffler used as resonator between cat and rear muffler will help fit the largest that is possible, you do need to make sure that rear muffler is big as far as physical volume goes as well though
    BMW E30 325is M20B33 in the making....... ITB's, roller rockers and stroked to the hilt

  5. #5
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    I was thinking a resonator as a resonator (haha) ... But I was looking at the smallest one possible for ease of fitment. Something like this one from Flowtech.
    Screenshot 2022-09-19 065147.png
    It makes sense however that the larger the resonator the more it'll knock down the volume though, and the relatively small resonator may not do enough. The problem I see is that there isn't a lot of 'straight line' space in there between the Cat and Muffler to mount a large resonator let alone two of them side by side.

    Is there such a thing as a mid-pipe for E30's? Or would I have to go custom fab for an X-pipe into one larger/longer resonator then back to two pipes for the rear muffler? That sounds like more trouble and $$ than just replacing the Billy Boat muffler.

    But then searching for the right muffler to replace the B&B seems to be as much of a PITA as finding a resonator solution for what I already have.

    Maybe ear plugs are the way to go ... Ugh.

  6. #6
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    Honestly, find a used stock catalytic converter at a salvage yard and swap that on. The resonator that you are showing above is going to do next to nothing. The magnaflow Cat you currently have is a much more free flowing design than the stock unit, which is why everything is loud and droning, you've changed the harmonics of the exhaust system. The m20 doesn't push enough power to warrant changing anything except for the catback, and even that is mostly for sound.

  7. #7
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    Just food for thought, at least for the ix the Ireland engineering exhaust was $350, so your resonator solution might approach the cost of a different muffler. However...no garunteed you'll like the sound and you might get lucky with a resonator solution and create a really nice sound you can't buy off the shelf...who knows!

  8. #8
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    Quote Originally Posted by mbonder View Post
    Honestly, find a used stock catalytic converter at a salvage yard and swap that on. ...
    Yeah, the more I think about it this is the right solution, but talk about finding a needle in a haystack. The OEM Cats are NLA and finding one that's worth a damn buying or that hasn't been scraped for the platinum is going to be near impossible.

    That said splicing in two resonators is equally a PITA and also a crap shoot when it comes to the results.

    It seems I got what I paid for with the Magnaflow Cat. I don't want to buy a replacement for the B&B just to end up with the same problem. Anyone know of an aftermarket Cat that would be comparable to the original OEM version? I guess I could look for one that doesn't claim to be free-flow? How many people look for a "more restrictive" cat - haha?

    Thanks for the sounding board folks - this has been a great help.
    Last edited by RobbE30; 09-19-2022 at 01:27 PM.

  9. #9
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    You can get a muffler from Rockauto for less than $200. That's what I would try. With the cat you have, I would get the quietest one, then you might have a chance at a reasonable tone. I would not buy an ANSA again as it rusted out too fast.

  10. #10
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    Thanks, but you have me wondering the logic of replacing my $1200 stainless B&B muffler for a cheap one from Rock Auto? As stated I like the sound of the B&B just not the volume, which is too loud as a combination of the Magnaflow Cat plus the B&B muffler. It makes more sense to replace the free-flow CAT.

    Finding a salvaged ~39 year old OEM CAT is going to be damn near impossible so now I'm looking at the AB Catalytic CAT where they don't claim it to be free-flow but "OEM-like" . Their CAT uses 400 CPSI (cells per square inch), versus the Magnaflow that has 200 CPSI, and they also maintain two separate pipes the whole way, which "may" be the ticket to noise reduction.

    So it's either spend the time & money to fab a custom x-pipe into resonator or just buy a new CAT. Either way the result will be unknown until it's done. I have no idea how much it would cost to have a x-pipe resonator made, but I assume it's going to be the same or more than a new AB CAT.

  11. #11
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    Dammit all to hell ... after several hours of poking around I've come to the realization that my B&B exhaust is THE loudest option out there, and combined with the free-flowiest loudest CAT it's no wonder my car is so screaming loud. It appears that changing the CAT alone isn't likely to get me where I want to be. My options at this point are:

    1. Replace the B&B with a different quality sounding but less loud muffler. I understand the euro brands need to pass TUV sound level standards where the US manufacturers don't - hence the B&B is louder than all of these:
    a. Stromung $689 (special order)
    b. Supersprint $930
    c. Ansa $310
    d. Scorpion $450
    e. Ireland Engineering $350 but not available? Also US (I believe) but not as loud)

    2. Rip out the CAT and muffler and replace with a full Scorpion system (with resonator not CAT) $935

    3. Go to my local exhaust fab shop and see if they can come up with a custom solution using a resonator, and cut out the CAT? At this point this may be the cheapest option.

  12. #12
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    The DEC cat I bought was only like $400, but there seemed to be a huge restriction where they welded the exhaust heade flange which is why I called it poor quality. It's working for me though.

  13. #13
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    Here's the other route you can go.

    1. Install a 2 in 2 out muffler just after the Cat that utilizes the flanges on the B&B exhaust. You'll need a muffler shop that knows how to weld to do it, but this should knock out some volume as well as the drone.

    or

    2.I don't believe you still have any emissions requirements for your car in PA, so why not eliminate the cat all together in favor of a much larger muffler. and a resonator in line with each other. In this case you could put an X-pipe off of the downpipes and go 2 in 2 out or a Y-pipe into single in single out then another Y-pipe to the B&B.


    I don't see the point in swapping catback exhausts, the B&B is a sought-after exhaust for the e30, I'd want to keep that and change the rest.

  14. #14
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    Quote Originally Posted by RobbE30 View Post
    Dammit all to hell ... after several hours of poking around I've come to the realization that my B&B exhaust is THE loudest option out there, and combined with the free-flowiest loudest CAT it's no wonder my car is so screaming loud. It appears that changing the CAT alone isn't likely to get me where I want to be. My options at this point are:

    1. Replace the B&B with a different quality sounding but less loud muffler. I understand the euro brands need to pass TUV sound level standards where the US manufacturers don't - hence the B&B is louder than all of these:
    a. Stromung $689 (special order)
    b. Supersprint $930
    c. Ansa $310
    d. Scorpion $450
    e. Ireland Engineering $350 but not available? Also US (I believe) but not as loud)

    2. Rip out the CAT and muffler and replace with a full Scorpion system (with resonator not CAT) $935

    3. Go to my local exhaust fab shop and see if they can come up with a custom solution using a resonator, and cut out the CAT? At this point this may be the cheapest option.



    had an earlier reply but didn't like the way it read so did not post. BB is known for drone even as just a catback on a stock set up. i wouldn't rip the whole thing out personally. anything that is high flow is going to have some drone

    i've seen resonators both before and after the cat when they couldn't be placed closer to the muffler. the stock cat is too large to accomplish this. may be an answer to try keep the cat you currently have.

    walker is the cheaper oe solution for a mid section. they used to make one with the larger cat, you might have some luck there. a quick look sorta indicates they are using a smaller cat now, but you could get lucky.

    i'm going to run an IE header and will have to solve for a mid as well. i'll be doing an x pipe, then a cat and resonators in the old cat position. leaning toward resonators pre-cat. i have a stromung catback to finish it off.

  15. #15
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    I've had good luck with the eBay stainless exhaust:
    https://www.ebay.com/itm/14184416699...3ABFBMyMCowOtg

    A fair bit more than I paid for them, but as a slightly louder than stock and somewhat more durable option, they've been good to me. B&B is too damned loud. That's your problem right now. Eventually, your problem will be that the Magnaflow cat has failed. They don't last long at all.

    If you're not in california, the best solution is probably to retrofit a newer OE cat.
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  16. #16
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    Thanks for all of the insightful and reasonable input guys - it's been super helpful and much appreciated.

    Quote Originally Posted by mbonder View Post
    Here's the other route you can go.

    1. Install a 2 in 2 out muffler ...
    Wouldn't this be the same/similar to using a resonator? Either way I'm planning to take it to the local exhaust fabricator to see what they think they can come up with along these lines. This seems to be the first 'experiment' as it should be relatively inexpensive while keeping everything else as is.

    Quote Originally Posted by mbonder View Post
    or
    2.... eliminate the cat all together in favor of a much larger muffler. and a resonator in line with each other. In this case you could put an X-pipe off of the downpipes and go 2 in 2 out or a Y-pipe into single in single out then another Y-pipe to the B&B.
    Yeah, I've seen where Supersprint makes this config for the E30 at a reasonable price and am intrigued. I just don't know much about eliminating the CAT though. What happens to the O2 sensor and it's functionality? I've read the exhaust smells, which I'm not too keen on. Keep in mind that I'm not after any supposed 'performance gain' of the CAT delete because it's an M20 after all - I just want the right sound.


    Quote Originally Posted by mbonder View Post
    I don't see the point in swapping catback exhausts, the B&B is a sought-after exhaust for the e30, I'd want to keep that and change the rest.
    That's a bingo! This is what I really needed to hear. Again - I like the sound just not the volume. It's one of the nicest mufflers out there so why get rid of it if I don't have to?

    Quote Originally Posted by 82eye View Post
    ... BB is known for drone even as just a catback on a stock set up. i wouldn't rip the whole thing out personally. anything that is high flow is going to have some drone. i've seen resonators both before and after the cat when they couldn't be placed closer to the muffler. the stock cat is too large to accomplish this. may be an answer to try keep the cat you currently have. walker is the cheaper oe solution for a mid section. they used to make one with the larger cat, you might have some luck there. a quick look sorta indicates they are using a smaller cat now, but you could get lucky.
    Thanks. That's what is so difficult here - it's quite subjective as to what's too much or an acceptable level of drone and volume. I'm an old fart - this was my first car out of college, and I'm well beyond needing to wake the neighborhood up. I talked to the folks at AB Catalytic, and they schooled me on the difference between an OE type and free-flow type of CAT (twice as much porosity on the free flow).

    So for now my first step is to take it to a shop to install a resonator. I'll take their cue for size, placement, x-pipe, y-pipe etc. as I have no experience. The shop I'm taking it to has been around forever and routinely works on all sorts of cool buggys so I'm guessing they'll know what to do.

    If that doesn't work the next step is still a little fuzzy still. I may replace the CAT , or may just get the Scorpion full system with CAT delete ... as soon as I learn what that's all about.

    Of course this is all just temporary until I do that dream S54 swap ...

  17. #17
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    Yes, the muffler will act similarly to a resonator, it will just be larger, so hopefully knock down the noise level some.

  18. #18
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    Quote Originally Posted by RobbE30 View Post

    If that doesn't work the next step is still a little fuzzy still. I may replace the CAT , or may just get the Scorpion full system with CAT delete ... as soon as I learn what that's all about.

    Of course this is all just temporary until I do that dream S54 swap ...
    [/FONT]
    I kind of doubt that a Scorpion exhaust is going to solve your noise problem.

    I am a fan of the S54 swap. I'm also one of a very small number of people who have built S54 E30 exhausts that pass emissions AND have a reasonable volume level. (As a general suggestion: Ignore any exhaust "recipe" that recommends Magnaflow or Vibrant parts.)
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  19. #19
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    This Advance Auto Fabrication AAF M20 Exhaust System sounds AWSOME!! Deep and throaty!! I would add a catalytic converter for street use.

    Last edited by bluptgm3; 09-29-2022 at 06:05 PM.

  20. #20
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    All B & B Mufflers are notorious for Droning, you might want to check into a Corsa muffler, their RSC technology virtually eliminates Droning throughout the RPM range!

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