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):
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 dBX Series Exhaust Systems | Flowmaster
Pipe Diameter: 2.25"
Material: 409 Stainless Steel
Weight: 18 pounds
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.
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:
Combined iPod/iPhone and GPS Mount for NAs
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.
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.
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).
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:
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
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?