Yeah, yeah yeah, there are a million threads about mufflers. Not trying to make this one about that.

Have a DT RB with Bonez high-flow. Sounds good on the throttle, but the low speed drone is killing me. Stuff like decel in 3/2/1 gears is really annoying. It makes me feel like a 19-year-old again, but not in a good way.

Are there any options to improve this? I see there are "resonators" that supposedly help on midpipes, but I don't really understand how they work. It is conceivable that you could put one of these behind the cat and in front of the muffler to kill some of that drone?

 

I have the same Bonez combination and have no issues with drone and downshifts sound great.
My DP is wrapped, along with the inlet and outlet on the cat.

The only other thing I can think of for you is to try to source a stock system.

 

I have the same experience as Sgt Blue, and the same Bonez high flow/RB Dual. The cat's can fail over time if you're running way rich or lean. I recently swapped out the Bonez for the RB resonated mid pipe that replaces the cat, but I haven't started the car yet as I'm waiting on a few other things first. I've read anecdotal experiences that it's quieter than the Bonez high flow catalytic converter. I swapped because I needed more exhaust flow for my power goals and will likely be going to a more free-flowing exhaust as well, like the Garage Alpha dolphin tail.

The RB has insulation material that can blow out over time (faster if you're running really lean/hot) and get progressively louder than a new muffler.

 

MKD, what turbo set up do you have?

Stock sequential twins have the flapper door in the exhaust that kills drone big time and quiets down idle/low load.

The RB exhaust is louder than most people think it is IMHO. Don't know if you want to try another exhaust, the Tanabe is a good choice that's well built and conservative sound-wise.

Dale

 

Yeah, wiring that flap open on a parallel set up significantly increases the drone. As does some even mild streetside porting.

I put in a 12"x3" Dynamax (flow through) resonator and that helped. They claim a 4db reduction. It only cost C$200 installed.

Fit between the stock cat and the muffler.

 

Pretty sure he's still spinning the stock twins with BNRs waiting to be installed.
I'm in the same boat as Sarge and F1 guy. Same set-up as you but it's never bothered me. I use to run something louder though, so maybe...I dunno. Maybe I'm still subconsciously comparing the two.

If you don't have your stock exhaust, we can slap mine on and see if that's what you're after.

 

Possible you've lost some of the catalyst matrix.

I have same combo except single tip. Not loud at all until I'm post transition and in parallel turbo operation.

 

What engine mounts do you have? Bad stock mounts or stiff mounts can cause a resonance.

 

My HKS DP and entire cat is wrapped 2x -- I wasn't aware this was a consideration for drone?

The Bonez and RB are only about a year old. I did however run the Bonez very rich for about 1000 lines right after installing it.

Stock twins. I'd love to try the Tanabe but like so many others really like the stock-ish appearance of the BR dual.

What was your experience adding that resonator, and what is the rest of your setup?

I actually still have the stock exhaust but I think it had a small hole in the can. I also assume it is going to be way too restrictive. I want to drive your car!

BR is only ~1yr old.

I wonder if the RB could just be a dud. I'd heard their quality has slipped a bit but don't known how mufflers are constructed so can't really make an educated guess on if something could be amiss there.

Powerflex purple diff mounts, Banzai brace, stock motor mounts. I have no reason to believe the stock mounts are broken (other than maybe this) but haven't looked in a while. FFE "soft" mounts going in with the BRNs over Thanksgiving.

How can bad mounts cause a resonance?

 

Any time, Brometheus. If you blow it up we can trade.

 

I bought a new RB dual tip 2.5 years ago on my '95 I sold. Cat was stock, downpipe was the OEM one from a JDM car. There wasn't much drone. I do think that the stock exhaust is louder than people give it credit for, and the RB is about as loud as the stock exhaust on my old 5.7 liter Challenger (which did not have cylinder deactivation). Running a cat rich might do something to it but the biggest killer of cats is heat, heat from backfires, going lean, or exothermic reactions (air pump malfunctioning, putting too much air in).

If your mounts are original, they are probably broken unless you have confirmed otherwise. Sometimes you have to remove them to check and see if the rubber has separated. It's possible I am wrong but I'd be surprised if they are intact and original. If you are like a modern customer when it comes to NVH (that is, unlike the average FD owner) I do not recommend anything but brand new OEM mounts. The stock mounts in brand new form already vibrate a lot, but at least they are hydraulic like pretty much all new cars' mounts (they use rubber + fluid to dampen). I actually removed some "street" urethane mounts (I won't name the brand) that I bought to replace the stock ones because of the low pitched resonance that made the dash vibrate with the car completely stock except RB catback. It probably would have done it on stock exhaust too. Rx-7's are not quiet and not smooth, even completely stock, if you are used to modern cars. They're just not built like a modern car. My 2015 Mustang (2.3 turbo) and 2017 Challenger (5.7 V8) had much less vibration than my FD with brand new mounts, but the Challenger did drone pretty bad (because Chrysler was too cheap to put 2 mode exhaust in like a Camaro). A large % of high end new cars have active noise cancellation, either with air induction system/exhaust system design or with the sound system.

The other thing you have to realize is that the RB is quiet compared to most exhausts that people run on these cars, so it's not actually drowning out everything with high pitched chainsaw racket. So you notice things that people who drive around with open windows and loud exhausts don't notice. Just like on my Tesla you notice wind noise because you barely hear the electric motors (hence, Tesla switched to thicker glass recently on new Model 3 and Model Y).

The stock mounts are really expensive but if you don't like drone I highly recommend you get brand new ones. You can try urethane ones but you're going to get some kind of resonance, you're just probably trading broken stock mount resonance for urethane resonance. Again most people don't care because their car is loud anyway and they are very different from the normal car buying population's noise tolerance.

 

I wonder just how much extra heat the 2x Ti wrapping all the way down over the cat carries. I don't really think it's getting louder, I'm just getting tired of it.

Lucky me I am not used to modern cars! The newest car I've owned is my current 2004 Tacoma. Having said that, I don't WANT to add vibration; the assumption is that I can beat the car a bit harder with motor mounts, diff mounts, and tranny brace without feeling guilty or worrying as much about breaking things.

I suppose we don't buy cars like the FD for ANC.

I'm really curious now what I will find with the stock & FFE mounts. I'd be super surprised if what I'm hearing is actually vibration from the mounts, but if I find the stock mounts broken or if it changes after installing the FFE's maybe the mystery will be solved.

 

At the time I was full parallel with stock cat and a Kakimoto exhaust. The drone at highway speeds was tiresome. Cruising by sidewalk cafes downtown would cause the china to rattle.

The resonator took out the distracting drone when cruising.

I then went back to sequential turbos for a bit, and the car was much quieter, almost modern on the freeway.

Rebuilt the engine with a mild streetport and back to parallel stock turbos. Car was back to loud and drone. Changed to a Tanabe Medallion (without the resonator) which helped a lot. But I am now back to stock to avoid current strict enforcement by the local police.

Not quite stock quiet, but I can hear the other engine bay noises now, such as the turbos spooling.

 

It doesn't really sound like there is a lot to lose by adding a compact muffler after the cat. Something like: https://www.dynomax.com/mufflers/rac...und-24404.html

I guess I will see what the motor mount situation looks like over TG and then go from there.

Thanks all for the info.

 

Mine was a Dynamax 24222 to match the kakimoto pipe size.

 

mkd,

If all else fails you can always try a Helmholtz Resonator. They are basically 1/4 wave resonators that cancel out the amplitude of sinusoidal waves at certain frequencies. In other words they reduce/eliminate drone within a specific RPM range.

Helmholtz Resonators - Synapse Engineering

With that said, back when my set up's exhaust was comprised of stock twins, RX-7 store DP, RB catback, and a magnaflow resonated MP. I had zero drone.

 

@Montego That is some cool ****! Thanks for sending!

It also occurred to me that I have a 4.44 rear end going in soon, so maybe that will at least improve the freeway aspect of it. Cruising at 60-70 in 4th, for example, is fine. It's 5th and deceleration that are annoying.

 

It could be your exhaust combo just ended up an inch or two too long or too short and its causing drone.

Or as other have pointed out the drone could be something besides exhaust frequency to exhaust frequency 2ndary harmonic (drones are 2ndary or higher order harmonics).

Could be tire noise and exhaust or drivetrain noise and exhaust.

On my FC I had a JIC spec 90 which is 3 1/2" turbo back with just 1 muffler. It was insanely loud and droney. Traded the exhaust and then traded it back after it was wrecked- rebuilt it with all stock hanger locations and a tiny difference in lengths and bend placement from the rebuild process as well as with an oval straight through single muffler instead of its round single muffler and it became completely drone free and improbably quiet on the freeway.

So weird how small changes completely changed its character.

First step is make sure the bonez cat exhaust hanger isnt hitting the body of the car...

 

I don’t think it does anything for drone. I do think it changes the sound so I thought I’d mention it.

 

Probably no worse than the stock precat, or even the JDM catless downpipe with stock heat shields. The stock tune runs pig rich under boost to keep the cat temps under control.

If you can find the time, remove the cat and the engine mounts and inspect both of them.

 

simplistically, noise, is a vibration, and vibration will transmit.

the stock engine mounts are designed to stop the vibrations between the engine and the car, however when/if they go bad, they can transmit noise/vibrations

for the exhaust, sound is a wave, and again simplistically, the diameter and length of the pipe dictate the pitch. think of a pipe organ. the other thing you can do is cancel the wave. the stock car is quiet, partially because the restriction in the muffler has the waves stack up into a steady flow, its why turbo cars get quiet and make a whoosh noise.

straight through mufflers absorb some of the energy, and again the size and shape dictate how much, and what frequencies

its simplistic, as the theory is much easier than the nuts and bolts. i did some experimenting with the P port, and adding mufflers didn't do much, but going from a 3" exhaust tailpipe to 2x2" worked great

 

The Racing Beat exhaust is about as quiet as you can get. In my experience a midpipe with a large Magnaflow muffler in the middle such as Banzai or RX-7 store version is significantly quieter and drones less than most of the high flow cats on the market.

 

I have poly bushings from Juan at j-auto all around on my car. Get the softer ones (the lower the number the softer the polyurethane is..like skateboard wheels) and the NVH won't increase much at all. No need to rattle your fillings out with hockey puck motor mounts.

Anytime you wanna take it for a drive that's cool.

@BLUE TII lol, I barely hear ya about the JIC. I ran a JIC Bullet exhaust on my old Protege5 for years. It fit perfect and the welds were nice. My ears are still ringing though!

 

Catless midpipe is dicey in California though.

 

Seems odd. I have a HKS downpipe, Bonez catted midpipe, and RB DT catback and I don't get any drone.

I'd agree on checking your motor mounts. You may also want to double check that your exhaust hangers are all in good shape still, and that nothing is rubbing against anything it shouldn't be.

I know some people ditch the brace that connects the downpipe to the transmission. I kept mine installed...I am not sure if that would contribute to anything regarding drone or not.