My 3" custom exhaust with spring loaded flapper valve
 

3" miata exhausts tend to be too loud for my taste despite the use of a large Magnaflow, and for the longest time I was thinking of a butterfly restrictor after the final muffler.

Turns out, Dynomax now make a muffler with a built in spring-loaded flap, the Dynomax VT series:

http://www.dynomax.com/mufflers.php?muffler=vt
https://www.miataturbo.net/attachmen...ine=1325222187

Neat idea, though annoyingly the flapper isn't sold separately.

Here's a vid where you can hear it open up. Fast forward to 0:34:

Because I wanted a bigger muffler, I decided to cut the valve out of a VT muffler and install it just before my tailpipe, and use a bigass Magnaflow.

https://www.miataturbo.net/attachmen...1&d=1325223265

My old 2.5" system had a 5x8x22" Magnaflow, a 16x4" resonator, and a metal cat that interchanged with a louvered resonator.

My new 3" system has a 5x11x22" Magnaflow, a 14x4" resonator, and an FM "hybrid" cat which is 3" behind the cat and 2.5" in front.

Result: At light throttle and cruise the exhaust is slightly louder than my 2.5" system using the resonator instead of the cat. When I get on it, it's very noticeably louder. I also tested the system idling and revving without the flapper valve in place. It was MUCH louder at lower RPMs (<3000).

Peak backpressure, at 15 psi boost, 7000 RPM, measured at top of downpipe:
Old system: 10 psi (270 whp)
New system: 5 psi

I looked at these mufflers and the idea is cool. I like your idea of cutting the valve out even better. You can actually service the flapper with it your way. I was concerned about rust and problems like that.

you still think the 5x11x22 is too loud?

Yes. <ducking>

It seems to be much louder than my 2.5" system cruising below 3000 RPM. (e.g. side streets)

Neat idea.
Why not just go all out and do the whole "throttle body on the downpipe" setup like street bikes have?
Variable backpressure ftw

Not saying I wouldn't rock it :)

The DSM guys have been using these for a while. Be aware that either the springs lose tension over time/ use/ heat, or the little wove steel wire cushion on the flapper tends to wear out/ disappear, and the flapper starts to rattle/ clunk at idle some times. The Mitsu guys claim a 6 month max lifespan before the rattle begins.

I don't know if you'll have as much trouble with it, as 1.) you're not moving nearly the air that a GT35R EVO moves, and 2.) you're a probably not putting the thermal stress on it that the DSM guys are.

Bimmers have these, they use a wastegate.

Can't wait for someone to install one of these backwards, much lulz.... but very quiet

How much was the muffler you cut apart to get the valve???

I'd think the way Jason did it the valve might last a little longer since it should stay cooler being outside the muffler can.

Seriously, you're nuts. My Laguna exhaust for Theseus is the same Magnaflow without the cat/resonator and it's 84dB. I can barely hear it :giggle:

I'm about tempted to have Jason @ Enthuza make me a new rear section with that muffler now....

My mom's 335 has a weird rattle in the exhaust and I traced it back to that. She still has to take it in.

they wont replace it and dunno how to fix it, they'll try to sell you a new $600 muffler. It got disconnected, refit it and put in a new circlip/pin.

I've had to do this on three of his bimmers now.

I've been planning on swapping out my rearmost 5x8x14 3" magnaflow on my suby wagon with one of these dynomax VTs. Wagon is a little drone-y on the highway. I haven't due to the supposed blow out of the little pad and/or worn out spring after a little while, and my brackets are welded to the muffla.

Anyway. I hope it works out for you, keep us updated.

Actually buying the muffler, cutting out the valve and inserting it into the end of the existing magnaflow is a smarter idea. WHen the valve opens I don't want it to be any louder than it already is. The actual muffler section of the dynomax is pretty tiny. Thanks for the idea Jason.

On the dynomax, the exhaust has to flow out through the packing when the valve is shut. Theyre very quiet.

Thats weird, I have 3" straight from the turbo to my 22x5x11 magnaflow and is as quiet as stock below 4000 with wg closed. I was even thinking about making a thread about my setup for people looking for quiet exhaust without sacrificing performance.

Not sure why ours is different, turbo size (doubt it) or muffler design? Mine is offset but I can still roll a golf ball through it.

Anybody know if there is any semi accurate decibel reading apps for iphone or android? Now I'm curious.

I'm surprised they won't fix that under warranty. I guess I'll take care of it myself then. Thanks for the info.

If you want the ultimate exhaust for a Subie wagon, get the Maddad Whisper. I had the 3" dual tip on mine and it didn't drone under any conditions. Other benefits are a clean, almost OEM, appearance, and an excellent sound. The only downside is the price.

I'm running a 3" catback with 3" resonator... Its as loud as stock, just a little bit deeper.

I dont know how much quieter than that you want it but all i hear is engine sound and the turbo spooling.