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Index > Engine/Drivetrain > Thread: Need exhaust, suggestions?
Thread: Need exhaust, suggestions?
mwpayne1


Redlining
Posts: 239
posted July 05, 2011 09:20 AM

Need exhaust, suggestions?

Hey guys, my exhaust system is...exhausted. Muffler shop of course offered to install one of theirs, my rotary experience tells me to go with an RB setup, however cost is an issue. I could get an RB muffler/presilencer and have them do the rest, what do you guys think? Any good used out there?

       
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mattraver


Revvin Up
Posts: 95
posted July 06, 2011 01:34 AM

i have a road race header with a magna flow 2 in dual in 2in dual out for my pre silencer i cut it open took out the rockwool and put course stainless steel packing which i got from mcmaster carr on line used a dual 2 in inlet single 2 1/2 outlet for muffler .bought 2in stainless manderal j bends (3) and some 2 in stainless tube fabed and welded and it works great on my street port 200hp on the dyno flywheel 166 to the wheels

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mwpayne1


Redlining
Posts: 239
posted July 06, 2011 06:10 AM

Ok, how is the sound? I'm looking for smooth, low tone, quiet, typical of RB setups.

       
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mattraver


Revvin Up
Posts: 95
posted July 06, 2011 10:49 PM

not low tone rb prima flow sound , but not the hefty price. sounds good to me for a rotary ,remember restriction is loss of horse power and rb systems do restrict .maybe you can do everything i did and get a 450.00 street port prima flow that could save some coin and give you the sound you desire .rb pre silencers are just stainless steel packed strait through mufflers.the magna flow dual in dual out makes an x in the center ,the mixing effect does help lower db levels

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Jeff20B


Moderator
Posts: 661
posted July 10, 2011 06:58 PM

I built a system for my REPU last month based on an RB "streetport" exhaust from a 1st gen.

My system consists of:

road race header
dual pipe flange set
two 2" presilencers
powerpulse muffler turned around
magnaflow 5x8 oval 18" center/center

This thing is quiet! But has mountains of torque!

It was kind of a trick to feed the RB powerpulse muffler through the tips and exit out the inlet. It situates perfectly to go up over the rearend. The hangers are even in the right places!

It wound up louder than a regular RB "streetport" exhaust because it eliminates some of the quieting features, so I had to add one more muffler after the rearend.

The quieting features which I couldn't use will be described here, just to make sure I understand what is happening with my modded RB exhaust.

The thickwall tubes of the RB center section are 2" OD or 1.75" ID (they are .120" wall thickness or 1/8" so you lose 1/4" total diameter measuring from outside to inside. Makes sense?). These tubes then bolt to the muffler inlet which uses two medium wall (that's 14 gauge) 1.6" ID tubes. Only a tiny step down on the ID but look like a huge step down on the OD. Nothing to worry about. These two 1.6" ID tubes collect into a single 2.375" tube, which enters the muffler.

The perforated core tube splits apart into two smaller tubes inside the muffler, leading to the characteristic dual tips. The ID of these smaller tubes is 1.5". The splitting apart of the core acts to reduce some noise. The outlet tips themselves incorporate even more quieting tech by first allowing the outgoing exhaust pulses to emidiately expand, thus reducing velocity and also noise. Then they go through a bellmouth trumpet acting like a venturi to give them some speed, then it trumptets back out again at the end of the tips.

For my system I fed the outlets of both presilencers into the 1.5" small perforated tubes described above. As you recall the ID of the thickwall tubing I used was 1.75". The ID of RB's dual pipe (81-82) powerpulse muffler is 1.6". I figured the 1.5" is not drasticly different enough to cause problems. So far it hasn't.

As you recall there is a collector before it enters the muffler in the RB system. Well on my setup there is no collector before the muffler. So it receives a louder exhaust note than a standard-setup RB muffler would normally receive. I'm asking it to do more than it was originally designed for. Will it last a long time? Probably. RB stuff is bullet proof.

By the way, any time you collect an exhaust system, it will have a quieting effect. However the sooner you collect them, the greater the effect. Hence a stock exhaust manifold tends to be quiter than a collected header, while an uncollected RB center section tends to be louder, and the end result is the "streetport" exhaust is louder than the collected exhaust compared side by side.

By the time my RB powerpulse muffler was installed and tested in front of the rearend (in the stock muffler location hehe), I only had enough room for one more muffler after the rearend. It needed to be quiet enough that one more muffler could handle the job. So I test ran it with just the center section (presilencers and powerpulse).

What was the result? It was loud but not horribly so. Louder than a typical RB "streetport" exhaust, obviously because I had eliminated several of its subtle quieting features, but you could probably drive it on the street without attracting too many cops' attention. Well, no thanks. Had one more muffler, will travel.

I set out to make this exhaust subtle, and after the final muffler was added, it was subtle! Maybe a little too quiet! It was so quiet you could stand about 20 feet behind the truck with the hood up and hear the clutch fan pulling air (refleting parabolically off the raised hood) more than the tail pipe! That's quiet! You could hear the carb more than the exhaust!

I could easily replace the final muffler with something smaller for more note out the tail pipe, but why? Infact the engine has one small quieting feature which will go away when the engine is pulled to go in another project. It's the diffuser sleeves in an S4 NA. The next engine is an old school which has regular open sleeves. Mazda added the sleeves so they could share some exhaust features in the FC with the turbo model and not need to come up with much quieter mufflers. As you know a turbo quiets nearly 50%, so the diffusers of the NA engines sorta helped the noise situation but only hurt power a little bit.

So I'm expecting the note of my REPU's exhaust to increase a little when the new engine goes in, which is a good thing. Noise = good. :)

Hey I'm planning another similar exhaust for another REPU. But I'm kinda tired of typing. More later if there's any interest.
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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chuyler1


Hauling
Posts: 140
posted May 07, 2012 09:01 PM

Thanks for the write-up Jeff.

I'll add my experience here to keep it in one place. When I purchased my truck, it already had the Racing Beat Header, 2.5" presilencer, and 2.5" Universal muffler with a few custom bits in between. The sound was fabulous at high rpms but unbearable below 3,200 rpm. I couldn't cruise around town between 30-40 mph without giving myself vertigo from the low frequency drone.

I took the truck to Lou's Custom Exhaust in Manchester, NH and they added another muffler on the back similar to Jeff's setup. They installed their own line of chamber-style muffler similar to a flowmaster with custom welded hangers. It's not a bolt-on solution, obviously, so others would have to seek out a custom shop near them to do something similar. It only cost me $190 total which I thought was a pretty good deal.

The result was amazing. The low rpm drone all but disappeared. I can't comment on how it compares to stock since I've never heard the stock exhaust but I will say it is still louder than my daily driver, a stock Mazdaspeed6, and about on-par with some Racing Beat RX-7 exhausts I've heard. Below 2,000 RPM you can still get the drone as a signal to downshift. From 2,000-3,000 RPM you can cruise just fine but if you go past 1/2 throttle the drone will poke out a little but not enough to over power the radio or wind noise. I stayed in 4th going up a steep hill at 2,500 RPM (~50mph) and it never got unbearable. Above 3,000 RPM the exhaust note is very smooth and linear with no raspy fart can sounds. It still brings a smile to my face when I can run through 1st and 2nd at WOT. On the highway cruising at 4,000rpm there is 0-drone. There wasn't much before but it is silky smooth now.

Here's a photo of the completed work. I'll try to get a video later of the sound.


       
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mwpayne1


Redlining
Posts: 239
posted May 08, 2012 02:56 PM

I ended up buying an RB muffler and presilencer. Once the shop welded it in with new piping, nice, low pitch sound. Not loud at all, just the rotary sound we all love.

       
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Chuyler1


Hauling
Posts: 140
posted May 08, 2012 05:26 PM

quote:
I ended up buying an RB muffler and presilencer. Once the shop welded it in with new piping, nice, low pitch sound. Not loud at all, just the rotary sound we all love.

The same ones in my photo above? Do you also have the RB header?

       
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mazdarx605


Redlining
Posts: 314
posted June 14, 2013 10:06 AM

Well I need to bump this thread up. I have been doing lots of stuff to my truck over the past few months, and exhaust is the most important next step in this process. I have had the truck for over 10 years, and the exhaust has been a cobbled together mess of RX-7 parts, bailing wire, and unicorn tears since I bought it.

What I have on my truck is what looks like a stock FB RX-7 muffler with the tips cut off, a RB header that collects into one pipe at the end of the header which is unlike the RB streetport exhaust system I have on my RX-7, and that is about it. It is barely hanging under the truck due to the hangers being broken(rubber parts), and it rests on the emergency brake ccable in another spot. Also the RX-7 muffler is mounted in such a way that even if I had the spare tire chain/cable assembly I couldn't mount the tire anyway. I have another truck now that I can steal the cable assembly from to mount the tire under the truck, but the exhaust has to be repiared/replaced before I do all of this. Any reccomendations? Note that I also have another RB 13B streetport header with the dual pipe collector flange that is for a GSL-SE RX-7, as well as the whole RB streetport exhaust system.

Oh yeah one other thing. When replacing all of the suspension stuff up front I noticed that with the wheels turned all the way to the right my inner tie rod end with its heat guard would make contact with the header. I massaged the heat shield for the tie rod end a bit, but it still makes contact. Is this a problem with using a none specific header that is probably meant for a RX-7 and not the REPU? I will get pics of it ASAP.

Thanks,

Chris

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