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Old 08-12-10, 11:21 AM   #1
N3v
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Best way to make my exhaust quieter (pics)

Click the image to open in full size.

this is a custom made mandrel-bent pipe for my rotary miata. Turns out it's pretty damn loud, surprise surprise. I thought 'ah I had two of these mufflers on my miata, it was pretty quiet.'

It has two 2" pipes that collect after 118 inches. The header is pretty short so I can't really add anything to that. I have a little bit of space here and there but not much.

My current plan is to put a resonator like this on each pipe before each muffler:
Click the image to open in full size.
on the right

But I'm having problems finding them in 2"

So if anyone has any links to a part like that, or any ideas as far as placement to best eliminate noise, or any other devices I should consider I would greatly appreciate it.

My goal with this is to just make it 'regular loud' like a car with an exhaust, but right now when I floor it I'm pretty sure I can be heard for about 800 miles in every direction.
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Old 08-12-10, 12:05 PM   #2
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Elaborating on my questions:

- what do people usually use to eliminate noise but not affect airflow that much on rotaries? Racing beat has that 'presilencer' thing, and I have no idea what's going on inside that thing.

- if my resonator idea is a good idea, will it make the most difference the closer I get it to the beginning of the exhaust?

- can I reasonably expect a decent reduction in noise on dual 2" pipes that have no internal restrictions? Will it always be pretty damn loud? should I sell it and make a short collector?

Last edited by N3v; 08-12-10 at 12:08 PM.
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Old 08-14-10, 02:49 PM   #3
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I'll take a pic today of what I did to my exhaust system to cut drone. No loss in power according to the 1320 dyno. My exhaust system is straight through with perforated-core glasspack mufflers.
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Old 08-15-10, 11:24 PM   #4
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Click the image to open in full size.

Click the image to open in full size.

I took two 1 7/8" ID pipe adapters that fit inside each other and the 2.5"OD exhaust pipe, cut the small pipe off leaving just a ring, welded them together and to the end of the pipe. Cuts the drone fantastically.

I think the adapters were 1 7/8" ID to 2 1/4" ID (which will be 2 3/8" OD, which is the ID of a 2.5" pipe), and the other was 1 7/8" ID to 2.25" OD.

I used to hammerfit them, but sometimes they'd blow out, so I just welded them in place.

The exhaust at the very end is at its lowest velocity and coolest, so the added restriction doesn't hurt power any, but it totally kills the drone below 3500. One ring kills it below ~2800.

Again, the rest of my exhaust system is two straight-through 2 1/2" perf-core mufflers. The drone was awful beforehand.

Next step is to fab a new exhaust system now that I have a RB roadrace header. The plan is two 2" glasspacks in each pipe, then collect at the rear axle, and into my existing rear muffler.
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Old 08-18-10, 08:24 PM   #5
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this is exactly the type of thing i was looking for. that's great! thanks.
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Old 08-18-10, 08:45 PM   #6
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^^
looks like you welded it shut then the extreme power of the rotary blew a hole in it

well if you want the cabin to be quiet by all means shushmat the car, but if its quietness for the neighbors......screw them rev that **** to red and smoke out in front of their houses.

well when i upgraded to a single racing beat catback system it was quieter than the stock system, but i got the roadrace now and its hella loud (still quieter than stock though)
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Old 08-18-10, 09:03 PM   #7
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Quote:
Originally Posted by Roto_Racing View Post
well if you want the cabin to be quiet by all means shushmat the car, but if its quietness for the neighbors......screw them rev that **** to red and smoke out in front of their houses.
You obviously don't drive your car for 2-3-4 hours at a stretch. Headaches and sensitivity to sound after driving mean that you have damaged your hearing. I already have some tinnitus developing (mainly my right ear) and it'd be nice if I didn't make things worse.

Plus, if you get a loud exhaust ticket, they take your driver's license and don't give it back until the car is quiet, plus the $100-odd dollars for the ticket. I'd rather just drive the car and not have to worry about that.

Quote:
well when i upgraded to a single racing beat catback system it was quieter than the stock system, but i got the roadrace now and its hella loud (still quieter than stock though)
I have never heard any aftermarket exhaust system that is as quiet as stock. One of my friends had the RB long-primary system on his car. Mine is slightly quieter.

Stock, the cars were VERY quiet. Mazda called the RX-7 the "Silent Sports Car".
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Old 08-18-10, 09:48 PM   #8
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http://www.youtube.com/watch?v=orG8MIXpHfo
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Old 08-18-10, 09:50 PM   #9
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EDIT:uuum i think if your driving like at 4 revs all day then DUH shift dammit. the car should be able to shift at 3 revs. my car (assuming from the video sound) sounds worse if not alike with the RB off road. and at 3 revs it doesnt sound all that loud, unless your in 5th already then slow down man no wonder you get pulled over so much. and in 5th going on 4 revs on a i assume modded engine if your not going past 90 there is something wrong with that engine. io still suggest the sound dedning.




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You obviously don't drive your car for 2-3-4 hours at a stretch. Headaches and sensitivity to sound after driving mean that you have damaged your hearing. I already have some tinnitus developing (mainly my right ear) and it'd be nice if I didn't make things worse.

Plus, if you get a loud exhaust ticket, they take your driver's license and don't give it back until the car is quiet, plus the $100-odd dollars for the ticket. I'd rather just drive the car and not have to worry about that.



I have never heard any aftermarket exhaust system that is as quiet as stock. One of my friends had the RB long-primary system on his car. Mine is slightly quieter.

Stock, the cars were VERY quiet. Mazda called the RX-7 the "Silent Sports Car".
ehem...firs off my rex is my DD(in P.R where a 30 min. drive can turn into a 2 HR nightmare). i have had it for three years as my DD, and drive it for fun at times(no hearing complaints here). i had a stocker for only the first three months maybe inside the cat and mufflers were done but it was loud.
second the RB single with presilencer, res, ect was pretty quiet even next to stock. i have gotten citations and tickets for loudness before, and all fell though because of the dmv's decible reader( if your in the legal limit you get a certification in the system). of course with the RB off road i will never pass but once it was in the system with the RB single i havent gotten a ticket since.
i work in a rotary and import shop. i havent heard one that was reasonably quiet running(not idleing, or a stock engine with stock exhaust), unless the person had sound dedning done to the car, even then the car was heard in the cabin(just a minor buzzing sound though).
if the rotarys natural sound when modded is affecting your hearing maybe you should drive the car less often and switch to a quieter car, and i doubt you can quiet the car down without sacrificing power. rotaries power comes from the exhaust in the first place.

look all im saying is the engines are loud unless completely stock n/a stock everything, and even then the slight buzzing is heard in the cabin. so if your health is at risk then cut down on driving the car.
good luck with your situation
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Last edited by Roto_Racing; 08-18-10 at 09:57 PM.
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Old 08-18-10, 10:07 PM   #10
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Quote:
Originally Posted by Roto_Racing View Post
EDIT:uuum i think if your driving like at 4 revs all day then DUH shift dammit. the car should be able to shift at 3 revs. my car (assuming from the video sound) sounds worse if not alike with the RB off road. and at 3 revs it doesnt sound all that loud, unless your in 5th already then slow down man no wonder you get pulled over so much.
The GPS said I was going 70. Speed limit is 65, although they only pull you over if you're going over 75 and/or are out of state.

I have a FB trans with the nice .825 5th gear.

I don't get pulled over, because my car is quiet. Funny how that works.

Quote:
if the rotarys natural sound when modded is affecting your hearing maybe you should drive the car less often and switch to a quieter car, and i doubt you can quiet the car down without sacrificing power.
I drive the car daily when there's no salt on the roads. And, as I pointed out at the beginning, I quieted the car down without sacrificing any power. Quarter mile times did not change when I added my homemade silencer.

With the engine's overlap, I can shut the engine off at idle by holding my hand over the exhaust pipe for two or three seconds. Very sensitive to backpressure. The idle vacuum as measured by my Megasquirt did not change one KPa between having the silencer and not having it. The same goes for cruising on the highway. (60KPa and 50KPa, for those keeping track at home)

Incidentally, a lot of the noise coming from my engine is from the INTAKE, not the exhaust. That's next on the list of things to quiet down. I need a better air filter anyway. High overlap engines make a lot of intake noise, but even when I played with stock carb 12As, I noted that removing the choke plate made the engine quite a bit louder.
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Old 08-18-10, 10:27 PM   #11
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im going to take your word on the 70 mph in fith on 4 rpm but imma check with my fb (n/a 13b).
anyway glad you got the car quiet.
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Old 08-19-10, 05:05 PM   #12
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Quote:
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Incidentally, a lot of the noise coming from my engine is from the INTAKE, not the exhaust. That's next on the list of things to quiet down. I need a better air filter anyway. High overlap engines make a lot of intake noise, but even when I played with stock carb 12As, I noted that removing the choke plate made the engine quite a bit louder.
i second that, with the PP the open intake is LOUD, the guy next store comes downstairs and complains i'm vibrating the whole house with the air cleaner off.

with the air cleaner he just complains about the smell =)

i know you used a stock FC intake, but i adapted the stock 12A air cleaner to the weber, it looks kinda goofy and i'm sure it knocks 30hp off the top, but it IS quiet.

oh btw to the O.P. the louvered core mufflers would be bad, if you must, then get some that are 2" ID measured at the louvers, which might make the muffler a 2.5" .

mine, the PP, i'm running 2 thrush straight thru glasspacks, into a borla, and judging by the looks this lady gives me when she's trying to talk on her phone on the sidewalk, its a bit too loud still. will be buying an RB muffler i think
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Old 08-19-10, 05:05 PM
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