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Discussion Starter #1
My newly acquired 96 Miata has a huge hole in the stock muffler and the rest of the system is pretty rotten in general, so I'm considering options for replacing everything.

I'm not going for loud or "buzzing bees" but I would like a better flowing exhaust. A good friend of mine (ex-navy welder) does custom performance exhausts with mandrel bends using J/U bends, so I'm thinking 2.25" with a resonator and a reasonably sized rear muffler.

He also asked if I wanted to do the downpipe as well... he and I are both from the VW world and on a lot of those, replacing the factory downpipe can really help exhaust flow and performance. Since I'm new to the Miata world, does this make a difference on these cars as well? Would it be worth it to make a new downpipe in 2.25" (and get a high flow cat / test pipe) or should I just aim to build a cat-back system?

I did some reading in the archives and it seems like the heat shielding on the downpipe is important, so we may have to come up with something if we go that route...

Let me know what you think...
 

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You don't have a turbo so you don't have a downpipe.


Do you mean header.
 

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Discussion Starter #4
The part I'm describing is the pipe that bolts to the stock exhaust manifold / header, goes down the left side of the engine / rear of the subframe and then connects to the cat. It's got a flange on either end and a sheet metal heatshield around it. Maybe you guys just consider that part of the exhaust...
 

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brontosaurus
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Header+exhaust on an otherwise stock miata is probably worth ~7hp.

Personally if I were you I would evaluate your plans for the car. Build for your goal. If you are turboing, just do the catback in 2.5"-3". If you are supercharging or staying n/a a nice header+catback would be beneficial.

7hp ain't much but throw in a timing advance, intake mods, programable ecu, etc... everything adds up.

Planning ahead means you won't be undoing what you just did. Saves you money and a headache.
 

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Discussion Starter #9
Thanks, everyone. I'm staying normally aspirated (stock engine) for the time being, so I think we'll just build a stock style exhaust in 2.25" with a resonator and rear muffler and leave the stock exhaust manifold / cat in place.
 

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Discussion Starter #11
Update... I ended up leaving the front pipe between the header and cat alone and we built an exhaust out of 2.25" tubing / mandrels and a v-band up front.

Version 1: Replaced the cat, used one 14" louvered bullet resonator and a Dynomax turbo muffler (old stock from a VW 16V project I was going to do long ago), stainless tip.

Result: Sounded like a spec Miata. It sings and definitely uncorked it after 4500 rpm but too loud and raspy for me as a daily driver. The louvers in the resonator were pointed "downstream" which probably doesn't help the volume and in retrospect, I should have gotten a perforated style.

Version 2: Replaced the Dynomax with a three chambered Magnaflow XL Stainless muffler (18" oval case).

Result: Cleaned up some of the buzz, quieter / deeper at idle, but still too loud for my goals.

Version 3: Added a triangular / oval case (stainless) resonator in front of the bullet. It's actually a stock part from an Audi, but larger diameter so it shouldn't be restrictive.

Result: Getting there... it's pretty quiet at idle and partial throttle. A touch boomy but sporty on WOT. I think I can live with it although I'd probably prefer it to be just a touch quieter still. Drone on the highway isn't horrible.

Maybe adding a high-flow cat would help? Maybe replace the first bullet resonator with a perforated jobbie?
 
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