Quiet track exhaust for Rotrex
 

Gents,

I purchased a complete Enthuza system for my '99 back in 2012, pre-Rotrex. I got 2.25" tubing, the metal core cat, a frickin' huge resonator, and the Borla Pro XS muffler. She's also got the RB header. A couple of years ago the metal core of the cat separated from the can itself. Long story short, I ended up gutting the cat to get me back on track quickly.

That setup is quiet at startup and idle, but has that stupid rasp because of the gutted cat. It's also pretty quiet around town. Unfortunately, it starts to get loud at highway cruising speeds with the 4.30 rear, and on track it sounds like a hundred KTM 50cc two-strokes waiting for the gates to drop. I need something quieter on track.

First question. The muffler is eight years old and has close to 140 track days on it. Is it spent? That's the first question because if I'm replacing the muffler (and cat), I will switch to 2.5 or 3 inch tubing. Last time I had the muffler off, it had no weird rattles or anything loose inside. Is the muffler recommendation for a supercharged car any different than for a turbo'd car? I've read through all the exhaust threads here I could find.

I assume replacing the cat would quiet down things a bit, but I don't remember the car ever being quiet at track speeds. If all I did was get a high flow metal core cat, what could I expect to happen to the current tune? If all I did was replace the cat, would there be any benefit to getting one with 2.5" I/O and having it neck down before the midpipe? That way, the cat wouldn't have to be replaced if I changed the cat back pieces later on. Can the resonator inlet be changed to 2.5"?

Thanks,

Whether the muffler is spent is hard to say, likely it has lost some of the packing material. Racing Beat exhaust with a resonated mid-pipe is the best choice, I run it with a test pipe on the track and I'm quite happy how quiet it is.

RB Header, test pipe, and RB cat-back was great on my VVT Rotrex. Not too loud, not too quiet.

Tackle the rasp first then re-evaluate sound levels. Cats do a great job of controlling rasp. If it’s primarily a street car I’d start there. If not look at resonator options in place of the cat.

I like to attack sound several ways. Repackable mufflers are great since you can run steel wool type packing in the front muffler to deal with heat and fiberglass in rear muffler. The steel wool packing knocks down overall sound but not great with high frequencies. The fiberglass muffler is great for that.

She's my track car, but I drive her to and from events. My plan is to replace the gutted cat with a 200 count metal core cat and see what happens. The Borla muffler is 14" X 10" X 4". A larger muffler will certainly fit, but I'd like to temper getting it quieter with avoiding adding too much extra weight.

Gents,

I took the Miata to a local shop that's been doing custom exhaust work for decades. The guy spent more time with me than he probably had to spare, talking about what I wanted. However, the bottom line turned out to be that I needed to find a cat that was OBD2 compliant or he couldn't work on it. Maybe it was because I showed up in a track Miata wearing a suit, but he was very adamant about not doing anything that could cause him to get fined, etc., etc.

The Magnaflow Spiral Metallic Core Cat, part number 59956, is pre-OBD2. Are there any high flow cats that meet the spec? I left a message with the Magnaflow support line, but they haven't returned my call.

Thanks,

The shop owners position is understandable. The fines are over 10k from what I recall. Install whatever cat solution you find then have him work on cat back system for you.

I am am not familiar what’s available off the shelf but I recall 100-200 cell cats being borderline for smog. The tune has to be spot on and they have to be really hot to light off the HC’s. You should be able to find the cores themselves and install em in stock cat body. Simple fab job for the right guy. Read not muffler shop guy. Someone who does bench work fab with tig etc,

I am also interested to know how a larger diameter helps with a rotrex setup specifically both in sound and in any potential torque increase.

Has anyone used the MT-approved 12579 muffler on a Rotrex'd car before? I am looking to use it (plus cat plus resonator) but I would love a few data points before committing to it.

It's gonna be a lot louder without a turbine in it.

--Ian

Yup, I know, that's why I wanted a datapoint for someone that had actually used it on a Rotrex'd car.

In 3" or 2.5"? I have both sizes on cars in my garage. I will hopefully be getting video of the 2.5" on a naturally aspirated car soon.

2.5". It would be really helpful if you could share a sound clip. I would expect the sound to be similar at similar power levels (i.e. flow through the exhaust).

Once I get the car started i will work on getting a sound clip. No idea on when exactly as it is being a bitch and not starting.

Currently the exhaust is a Fujitsubo exhaust manifold mated to a custom 2.5" exhaust with 2 resonators with the big magnaflow on the back.

Again here's my old car with RB Header, test pipe, and RB cat-back was great on my VVT Rotrex. It was a great setup.

I've got an FFS with a JR header, magnaflow metallic spun cat and a racing beat resonated. It's sounds the business but it's too loud on the highway and the mid range is boomy.




I've tried a myriad of cats and setups, the OEM one will drop the volume significantly and honestly the power difference is very, very minimal. IMO the sound is still kinda loud, especially when climbing hills and overall the sound is kinda boring.




I'm currently looking for a new solution that will tone down the 3-4k sound and drop the cruise volume but still remain open.




Other setups I've tried, Borla (way too loud, DRONE), Yonaka - almost as loud as the racing beat with a test pipe... with the stock cat in place. Roadster Sport 3, sounds awesome but very loud and boomy in the low and mid range...with a stock cat and no blower.




From a sound perspective the Roadster Sport sounds the best above 4k, but it's simply too loud everywhere. I would take that sound minus about 20db's.