Normally Aspirated Catback Experiment/Comparison
 

I'm recording this for posterity.

When I bought the car, it had an Enthuza-built 2.5" catback with Magnaflow resonator and muffler (R-Speed GT). The exhaust sounds good and makes good power. The only problem is that it's heavy (25 pounds). With a lot of track driving, it sometimes shifts until it contacts car structure (even though I'm using poly hangers). Every so often, I have to get under the car and adjust it.

I decided to try and find a lighter catback. I still wanted to get reasonable noise control so that an intercom could be used on-track. That ruled out most of the Spec Miata exhausts. Flowmaster's dBX system caught my eye and wasn't too expensive. Online reviews said that it was lightweight and noise control was good. It uses a Flowmaster dBX muffler that is kind of a hybrid reflective/absorptive design with built-in vortex cones and absorptive packing (Flowmaster dBX SERIES LAMINAR FLOW MUFFLERS). It uses 2.25" pipe and doesn't have a resonator, so clearance would be improved over the Enthuza catback.

Upstream of the catback is a Racing Beat header with 2.5" collector and an Enthuza 2.5" test pipe (cat deleted). The catback specs:

R-Speed GT (built by Enthuza):
https://www.rspeed.net/RSpeed_Exhaus.../na0-1016.htm:
https://www.miataturbo.net/attachmen...1&d=1419358834
Pipe Diameter: 2.5"
Material: 304 Stainless Steel
Weight: 25 pounds
Resonator: Magnaflow 14" Absorptive
Muffler: Magnaflow 18" Absorptive
Main Bends: Mandrel 90 degrees in and out of muffler

Flowmaster 819110:
Flowmaster 819110, Flowmaster dBX Series Exhaust Systems | Flowmaster
https://www.miataturbo.net/attachmen...1&d=1419358834
Pipe Diameter: 2.25"
Material: 409 Stainless Steel
Weight: 18 pounds
Resonator: None
Muffler: Flowmaster dBX 12414310 Reflective/Absorptive
Main Bends: Mandrel 60 degrees in and out of muffler

Fit and Finish:
Both exhaust systems fit the car well. No issues at all with installation.

The R-Speed GT, like all Enthuza products, is really high-quality. The pictured exhaust is about 6 years old and has been on the car for thousands of miles and countless off-track excursions. I'm sure it could be polished up (304SS) to look like new if you're into that sort of thing.

The Flowmaster dBX also seemed to be a high-quality piece, although it's finish is OEM (409SS) rather than flashy.

Noise Control:
This is the part where I post a couple of youtube vids and tell you to go listen to the crappy audio, right? Wrong. You guys know me better than that.

Instead, I got the Real Time Analyzer (RTA) App for my iPod and captured audio spectrums. The iPod location is just below the rear view mirror:
https://www.miataturbo.net/race-prep...7/#post1102015

Car configuration was windows closed and hardtop on with a stripped interior. Pretty much worse case for interior noise.

I captured audio in peak-hold mode during my Virtual Dyno pulls. This records the peak frequencies heard at full throttle from 3000 to 7000RPM. Note that the vertical scales are shifted, sorry about that -- operator error.

R-Speed GT:
https://www.miataturbo.net/attachmen...1&d=1419358834

Flowmaster dBX:
https://www.miataturbo.net/attachmen...1&d=1419358834

Like most performance catbacks, both systems let low-frequency audio through. In the middle frequencies, there are significant resonances just below 250Hz and around 500Hz. The Flowmaster dBX really has a hard time with the 250Hz resonance (peaks to about 113dB) but controls the 500Hz resonance better (103dB vs. 107dB for the R-Speed GT). In the higher frequencies, there is no comparison. With easily double the muffled volume, the R-Speed GT kills those high frequencies while the Flowmaster dBX lets them through with another resonance around 7kHz. I would desribe the Flowmaster dBX's sound at full-throttle as "raspier." Both these screen captures were made with the car idling after the Virtual Dyno pulls. At idle, the Flowmaster dBX has the edge (69.2dBA vs. 73.8dBA).

I also made steady-state 70mph audio captures while climbing a slight grade. Again, the vertical scales are slightly shifted.

R-Speed GT:
https://www.miataturbo.net/attachmen...1&d=1419358834

Flowmaster dBX:
https://www.miataturbo.net/attachmen...1&d=1419358834

Results here are similar, although the resonance peaks stand out more. The Flowmaster dBX's reflective technology is cancelling a lot of those peaks but it's lack of muffled volume is letting much more high-frequency audio come through. Again, the Flowmaster dBX sounds "raspy" in comparison to the R-Speed GT, but the overall sound intensity at this speed is less (91.7dBA vs. 96.3dBA).

Power:
And, the moment you've all been waiting for. It's been windy here, so it was tough to get a good VD comparison. I did numerous runs. Overall, the Flowmaster dBX system cost me about 5HP. This is representative:
https://www.miataturbo.net/attachmen...1&d=1419358834

Not sure if the difference is the smaller piping or the Muffler technology. Some say that big pipes on a normally aspirated car cause a torque loss, but that's not what I experienced. My results seem similar to FM's in that regard (http://www.flyinmiata.com/tech/dyno_...ebb_082099.pdf).

Epilogue:
So, in the end I decided that 7 pounds wasn't worth 5HP. I also liked the sound of the R-Speed GT better. It's back on. Anyone want to buy a barely used Flowmaster dBX system?

Awesome review/comparo. Props

Wow, much effort post.

When I saw you were only going to lose 7 pounds by going to 2.25", I was kinda skeptical it'd be worth your while.

Now I want an Enthuza exhaust.... excellent write-up!! Thanks for putting in the work and sharing the results!

I read a bunch of stuff that said 2.25" would be fine for normally aspirated. I was hoping to write a "Great Success" post. Alas, the data doesn't lie.

There's a frustrating lack of data in the websphere. If I see one more exhaust review that says "it sounds great, here's my vid" or "more power according to the butt dyno" . . . I'll scream!!

Jason builds an excellent, Miata-specific product.

The GF has a '95 Miata... with a 2.25 Magnaflow setup almost identical to the Enthuza. (Resonator and muffler).

I've got a '94... with a 2.5" Magnaflow resonator.

Her's put down 117/100. Mine put down 124/101. Both through the stock air box and stock exhaust manifold with nothing more than in exintake cam in both of them.

But, on the street/track it's a much different story.
She's got a lot more midrange TQ than I do. In fact, her car is much faster anywhere under 5k.
Then again, her AFRs look a bit better with her '95 then my '94.

But, up top, the 2.5" shines. And continues to build power up to the rev limiter at 6900, while her's falls off at 6600 or so (I'd need to check the dyno sheets again).

This makes me wonder how the RoadsterSport 3 from Goodwin Racing would compare in weight, noise and power output to these two catbacks.

I could have told you by looking at them that the enthuza exhaust would have been quieter and have better gains.

Tends to indicate that it's the muffler that is more restrictive than the 2.25" pipe. OTOH, I've got a RB Header and test pipe, which probably makes the system more sensitive to the catback.

I was trying to shave weight and maintenance hassle even if the sound went up (track car). The power loss was more than I could accept. Nice to have data, don't you think?

I like it when people just listen to the words of Brain; wrong or right.

:brain: :party: :brain:

:bowrofl:

Maybe I should send mine off to hornetball for testing...

Frankly, I'm not impressed with the Goodwin's RSport 3. Purchased it thinking I would see improvement mid-range tq coming from a 2.5" topseed ebay especial. Butt-dyno feels like the same below 4k, but choked up top. No science involved however. :facepalm: It's sound is much more palatable for around town though. The quiet-ness makes me feel better about priuses passing me then when I was screaming everywhere with the topspeed. :giggle:

Do you want to try a new setup thats lighter and quieter than both of them? Or one thats absurdly light and about as loud as the dbx? I have my magical muffler making machine.

Leafy.... do go on...

My 2.5" OD slightly louder than stock aluminum setup weighs 12lbs. The comparable version to the dbx noise is ~8lbs.