Quiet exhaust options without hindering air flow
 

Hello all,

As the title says I'm looking at exhaust options for a S4 RX7 turbo with a large street port. My local municipality has an absolute hard on for modified cars with exhausts so I'm looking to change it up asap. I have a custom exhaust to a Borla catback and definitely not quiet. What do you guys recommend for a quiet exhaust that won't hinder exhaust air flow and performance as I've heard rotaries need to be free flowing. The quietest one I've found is the Fujutsubo Legalis R exhaust system but I only looks like an axle back system. The HKS Silent Hi- Power still seems quiet loud (99dB @close noise level) and the enlarged tip would still get me into heat with the peace officers. The Racing Beat exhaust is nice but no sound levels so I'll have to contact them. But any experience or knowledge about this please let me know, thanks!

What does the exhaust consist of between your turbo and the Borla cat back? Is it just straight piping or is there any kind of muffler/ resonator in there? Also what is diameter of your exhaust piping?

welcome to the board. :)

everything is a balancing act. your city has dictated what your main priority should be (noise control), so you might lose "some" flow. however, you might also avoid some tickets.

i'm not really sure what they're like these days, but i'm thinking the RB system is probably your best bet for something ready-made. way, way, way back in the day, HKS made a nice, fairly quiet system for the T2, but i'm sure that's been long discontinued - i'm only mentioning it because it crossed my mind.

outside of that, you can probably redo your system with thicker piping and add a presilencer or two. just make sure to use robust parts.

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Sorry I should've mentioned that! The piping is custom is 75mm and missing a muffler, looking to get a new system though since the custom job isn't.... Great aha. The downpipe is a mix of stainless and mild steel and had holes in the weld which I corrected.

Thank you! Yeah unfortunately the car community in my city is full of hoodlums and immature folks so the city has imposed severe exhaust fines, $1000. Basically we can't have many car meets because people were actually drag racing downtown and the city council rightfully so are taking heavy action. I do like the Racing Beat system as it's nice and factory looking, but it just lacks dB readings from what I saw. It seems HKS still sells the Hi-Power but the enlarged tip is unfortunately against the law and they'd try to nail me for it. Custom would be nice as well but it might be a gray area with some over the top peace officers. It sucks with how the exhaust laws are but I gotta make do with what's outlined unfortunately. Thank you for the input!

You might weld a RB universal 3" pre-silencer Universal Presilencer, 3-inch ID for RX7 1986-1992 - Racing Beat in there between the turbo and catback. Then add a couple of resonators into the cat-back before the mufflers if you can find room. Vibrant Performance Bottle-Style Resonators - Free Shipping on Orders Over $109 at Summit Racing. Unfortunately, none of them have rating for the DB reduction.

Fit the biggest straight through offset mufflers you can fit in there. Size matters. And resonators.Studies have shown perf tube absorption mufflers have almost no negative affect on flow. With enough (straight through) muffler you will have almost no high frequency noise. Will still have bass though

I'll definitely consider this thank you! I also appreciate the links to the products it will definitely aid with trying to figure out the best route to go.

ive found, like Wank Factor, you want the biggest mufflers you can find. you do want one in the center of the car. probably dual is better (more muffler)

its tough to beat the RB turbo back downpipe, pre-silencer, its the right size, it fits, etc. after that its up to you (the RB stuff is again fine).

You didn't mention if your cat back is a dual or a single. Dual would be better as that lets you add two resonators before the mufflers along with splitting the exhaust between the two mufflers. I think you're basically trying to replicate this REV TII Exhaust System for 87-91 TURBO II - Racing Beat but with two more resonators or mufflers added before rear mufflers.

Interesting, that's good information thank you!

The Borla catback is duals. The RB system is very appealing however I do like the look of the tips!

What's the good balance between sound and restrictions though? The RB system is definitely hard to beat it is very appealing too.

If you don't have a good starting point (which it doesn't sound like you do) and you're talking about an FC - RB turbo back as many have mentioned. If you end up making/wanting something custom don't stack mufflers, stack a resonator and muffler. Stacking mufflers may reduce some DB however it tends to be a tonal issue with regards to irritating (read: cop inducing) exhausts.

you might have a look at this https://www.rx7club.com/naturally-as...a-13b-1162855/

i did some testing with mine, and a 70mm downpipe into a broken cat and 50mm HKS system was 12psi of backpressure AFTER the turbo, which was silent, but slow
hollowing out the cat drops it to 7psi, response got better, and it was still silent at WOT.

the new system is a 92mm FEED system, and i haven't tested it yet, but backpressure should be really low (its acting like it, boost control went out the window, car feels much lighter). volume went up, but mostly in the low frequencies, you might say that the pitch dropped an octave or two. interestingly its a 100mm tip, so its kind of subtle looking. muffler is gigantic, 200mm x 600mm. its kind of overkill for power, and its not quiet enough, but its actually less showy than the RB system

generally speaking, they rumble, but i have never considered them to be loud. i currently have one on my Gen 1 (and been around countless Gen 1 cars with them) and i have one on my Rx-8. that said, my opinion doesn't count for a hill of beans because i'm not one of your local law enforcement officers. your best bet might be to contact them and see if they have the data you need to make your decision. as you can see, they are also fairly expensive (worth it though) so it makes no sense finding out it's too loud after you've bought it.