Help me quiet down my Miata

So, my XSB Miata is now officially retired from autocrossing. I got a taste of that sweet E/Street PAX with my MR2 Spyder and I ain't ever going back to Xtreme Street. Which means now, I'm considering making the Miata less Xtreme, more Street. I already have plans for the suspension and wheels and tires to tame it down a little, but the biggest issue for me has always been that it's absurdly loud. Like, way too damn loud. I could see course workers covering their ears as I approached and there always had to be a hold on cone calls until I was through the finish.

The combination: 1999 1.8L engine, Rotrex C30-84 blower, Racing Beat header, ISR resonated test pipe, Good-Win Racing RoadsterSport3 cat-back. Nothing too outrageous. But, even with the Good-Win-supplied baffle bolted in the tailpipe, it still is quite loud (and not terribly pleasant sounding) and it really gets old when driving for longer periods of time. So, what are my options? Different cat-back? Have another muffler welded into the pipe? Putting a catalytic converter in it is tricky, because NY doesn't want anyone buying non-OEM converters. Am I missing something?

On my old Miata champcar, I had a DIY exhaust with a not too large magnaflow muffler. I had it in the OEM muffler location pointed nose to tail, rather than OEM sideways. It was a too loud for anything other than the track without a catalytic converter, but fine-ish with a cat. Ship an aftermarket cat to a friend in a different state?

Years ago in my old RX-7 TII,  I had a small straight through muffler in the cats normal location.

Cat and a better muffler. Also, a resonator helps - the engines tend to drone in the midrange. 

Vibrant Ultra quiet resonators are the ticket for every car that turns out too loud at my shop 

They tame the raspberries and bring the overall volume way down.

They excell at eliminating the "flatulent trombone" sound the kids are into these days.

Trent said:

Vibrant Ultra quiet resonators are the ticket for every car that turns out too loud at my shop 

 

They tame the raspberries and bring the overall volume way down.

They excell at eliminating the "flatulent trombone" sound the kids are into these days.

Agree with this.  A good resonator and a quiet muffler.  Miatas sound best wailing through the intake, not the exhaust.  
 

Also, if quieting it down as a street car is the priority, don't neglect other nvh areas: quieter tires, some small dynamat panels on large metal panels to tame resonance, some rubber trim if any plastic panels or door trim are vibrating, squealing, or rattling.  

Flynlow said:

Trent said:

Vibrant Ultra quiet resonators are the ticket for every car that turns out too loud at my shop 

 

They tame the raspberries and bring the overall volume way down.

They excell at eliminating the "flatulent trombone" sound the kids are into these days.

Agree with this.  A good resonator and a quiet muffler.  Miatas sound best wailing through the intake, not the exhaust.  
 

Also, if quieting it down as a street car is the priority, don't neglect other nvh areas: quieter tires, some small dynamat panels on large metal panels to tame resonance, some rubber trim if any plastic panels or door trim are vibrating, squealing, or rattling.  

Unfortunately my intake also doesn't sound very good. Those Rotrex superchargers may be efficient and make silly power on a stick engine, but they sound like a dying AC compressor. Just a weird rattling, hissing noise.

And, yep, I was already thinking of DynaMat-ing the bejeezus out of the interior.

My car has a Racing Beat header, high-flow cat, and a Racing Beat dual outlet muffler in the stock location and orientation. It's louder than stock but not too much, and has a decent tone.

NickD said:

And, yep, I was already thinking of DynaMat-ing the bejeezus out of the interior.

I could be wrong, but my understanding was sometimes less is more.  If you're going for pure sound reduction, a patch panel to dampen the resonance is what dynamat is used for, but dropping wind/road/tire/etc. noise you really need more backing on the carpet (think fluffy insulation, rather than butyl rubber). 

On a luxury car I'd say go nuts and do both, but dynamat is pretty heavy on a % basis on something like a Miata. 

Based on threads I read here, noise reduction is best achieved with multiple layers of different materials. I used Kilmat, carpet padding, and mass-loaded vinyl in my 2002 to fairly good effect (within reason). If you just use the stick-on stuff, apply enough to change the sound the panel makes when you strike it from a ringing sound to a dull thud.

Something from the back catalog that might help: How to build an inexpensive, quiet exhaust that doesn’t cost horsepower.

My exhaust with a BP engine was RB header, high-flow cat, Vibrant resonator and Walker 'super turbo' muffler.  This was a reasonably quiet setup outside the car, and with a full interior it would have been inside as well.

I think the real key here is a decent resonator, and non-straight through muffler.  You might lose a couple of hp, but it should be quieter.

Supercharged engines are usually going to be noisier than the same engine/exhaust without the supercharger.  Increased cylinder pressure...

In reply to David S. Wallens :

David, 

Is there a different recommendation out there for a muffler? Burns is great but $750 is a little eye watering price wise.

I've got a similar problem that I'm trying to solve with getting a good exhaust that is not choking but provides good sound reduction. My track car is way too loud to the point it's painful to drive without ear plugs.  I've got the Vibrant resonator already but need a muffler. 

Shopping for mufflers is tough because there's no actual information. Just vague descriptors like "a powerful sound".  Size and effectiveness are closely related, though. 

I had a 2000 miata that I fully sound deadened. I stripped the interior and carpet and put a vibration absorbing material (butyl maybe?) covered by closed cell foam. I even filled the cavities by the fuel tank and the trunk. 
 

It made practically no difference in terms of NVH. What it did do was magically get rid of my 65 mph shimmy 

In reply to bmw88rider :

Take a look at Spintech.  They're heavy, but do a reasonable job of quieting things down.  My K24 Miata with 3" exhaust, no cat, Vibrant resonator and a Spintech is under 95dB.  It's got a typical 4cyl sound, but it's not 'fart can' raspy.

Depending on what you want to do, the 18" body length Summit turbo muffler is fantastic. 

Enough noise at WOT to be a bit hot rod, pretty quiet the rest of the time, NO drone. 

I got the aluminized steel one, I think they make SS as well. 

I'm on a header and 2.5 all the way back (NB1).  While I LOVED the WOT sound on the chambered muffler that was on it, doing a couple days a week in office meant that it tried my sanity.

I had built a couple different muffler assemblies figuring I could swap the loud one back on for fun, but car runs a little better on the Summit turbo, so, it's not coming off.

A supercharger makes everything louder, but the Flyin' Miata exhaust may be just what you're looking for.  I personally think it's almost too quiet, but with a supercharger it's probably about right.  A cat definitely makes a difference.  Colorado has the same problem as NY with cats these days, but as long as you're going to install it yourself I find that eBay will still ship non-CARB cats to Colorado.  CARB compliant cats are available from RockAuto for $429.

It's often overlooked that aftermarket exhaust headers are noticeably louder than the stock manifold.  The stock manifolds have heat shielding and insulation that dramatically quiets them down.

bmw88rider said:

In reply to David S. Wallens :

David, 

Is there a different recommendation out there for a muffler? Burns is great but $750 is a little eye watering price wise.

Buy a straight-through Magnaflow oval, cut holes in the ends and repack it.  They have good internals for power, but lousy packing.  On my K-swapped cars with 3" exhausts, I run a repacked Burns in the tunnel and re-packed Magnaflow at the back.  Not loud at all.

There's a GRM story on that repacking...maybe David can find that one.

Berck said:

A supercharger makes everything louder, but the Flyin' Miata exhaust may be just what you're looking for.  I personally think it's almost too quiet, but with a supercharger it's probably about right.  A cat definitely makes a difference.  Colorado has the same problem as NY with cats these days, but as long as you're going to install it yourself I find that eBay will still ship non-CARB cats to Colorado.  CARB compliant cats are available from RockAuto for $429.

It's often overlooked that aftermarket exhaust headers are noticeably louder than the stock manifold.  The stock manifolds have heat shielding and insulation that dramatically quiets them down.

That's not "eBay" shipping cats to CO, it's individual sellers who either don't know the rules (they're only about a year old for CO) or who don't care. 

I think another reason the stock manifolds are so much quieter is their thick walls. The first time I installed a header in my 1.6 Miata, I could immediately hear a "ringing" noise - more high frequency. I wrapped it in insulation (the stock Miata manifold doesn't have any) and that helped. I did burn a hole through it in a couple of years though.

In reply to Keith Tanner :

Sorry, yes, you are correct, that should have read, "eBay sellers."  But eBay does nothing to police the platform or help law enforcement police the platform.  Same with folks selling R-12 to buyers without an EPA license, people selling alcohol "bottles" (that everyone knows just happen to have alcohol inside), etc, etc.

I personally think the CARB rule for Colorado is a bit nuts given what actually goes on.  I routinely follow cars (typically Subarus, sometimes Miatas) in Colorado Springs that, from the exhaust smell, clearly do not have a cat at all.  I'd rather see us actually make sure cars have a cat that can pass emissions standards rather than the ban the sale of all but the best ones in the midst of a catalytic converter theft epidemic.  I suspect actual result of this law is more people driving around without one at all, which is not an improvement.

In reply to Berck :

Well, most of CO does indeed have regular emissions checks that are intended to make sure cars have cats. But there are always going to be those that get around it. The goal is to minimize the number of those people.

I agree that some cat is better than no cat for sure.

In reply to Keith Tanner :

Part of the reason Racing Beat used to make their rotary exhausts out of .125" thickness pipe was for the sound effects.

The newer exhausts made of 16 gauge stainless are much more tinny sounding.  More sound is traveling through the pipe walls.

In reply to Keith Tanner :

Define most.  By population?  Ok.  By area?  Not even close.

When I moved here, El Paso county required emissions testing, but they declared the air quality problem "solved" and removed emissions requirements.  There's a ridiculous number of people even in the required area who illegally register their cars in an address outside the required area to avoid testing.  I know, because they're all bragging about it on social media.

My '91 Miata failed emissions here in 2005, and I bought a Flyin' Miata cat to fix it...  Definitely better than the situation we have now with no testing, and FM can't sell me their cat.

Pete. (l33t FS) said:

In reply to Keith Tanner :

Part of the reason Racing Beat used to make their rotary exhausts out of .125" thickness pipe was for the sound effects.

The newer exhausts made of 16 gauge stainless are much more tinny sounding.  More sound is traveling through the pipe walls.

Back in my "save weight at all costs" days and before SCCA Solo had sound limits, I had a 20 gauge straight pipe on my STS Civic.  Though the cat did some deadening, it...was..OBNOXIOUS.  Not just loud, but super tinny.

Yeah, by population, which is the most logical way. Even California has some emissions-free areas AFAIK. I'm glad I don't have to deal with regular emissions test on my fleet simply due to the size of it. I don't get to follow many cars that are clearly catless around here, although I was following a first gen Mustang the other day that was in serious, serious need of a carburetor adjustment. Either that or it had a leak in the fuel tank.

It's almost impossible to stop 100% of the people who try to get around the rules (see the number of supercars registered to Montana LLCs) but if you make it difficult enough, you can stop most of them. Where the ROI tapers off is always going to be difficult to define. I get it, and I'm glad I'm not in charge of it :)