Jeff20B

iTrader: (13)

After hours and hours of research and weighing my options, I've decided to go 100% Borla on my NA 20B. I have no need to fuss with mufflers that will blow out in time. Having had a taste of perhaps the best exhaust system you can get (the RB dual pipe "streetport" system) I suppose I have high standards now or something. lol

Mods, you can scrap the Magnaflow thread if you want, or just let it drop off the first page due to lack of interest.

MosesX605, if you see this, could you post here to let us know how your Borla is holding up? Anyone else too, please, for that matter. dj55b, let us know how the install goes.

Wow, I never knew I could actually feel passionate about mufflers. Especially about one I only first heard about last year in MosesX605's thread and didn't pay it any mind until just yesterday. Borla has appearantly been around for 20+ years, believe it or not. Their stuff is all "high grade" T-304 stainless steel. Should be good enough for a rotary.

I've learned a lot about exhaust flow, velocity etc while designing the exhaust for the 4 rotor recently, and it's all about maintaining velocity in NA applications. All those monster fart tip cannons you see all too often are really comical. They look dumb and function poorly to boot.

I'd rather have something so understated that you hardly know it's there, until you flex your ankle. Part throttle cruise needs to be quiet too. That's why I'm going with thick wall 1/8" (.120) mild steel pipe. Why not stainless? Cost and I can't find it readily in 1/8" wall thickness. Maybe for a turbo system some day I'll get the thinner wall (.063) SS stuff, but for now running NA, it has to be thick, to avoid exhaust noise from eminating through the pipe.

They say you shouldn't locate the muffler below the passenger area because some of the noise the muffler is trying to 'muffle' actually escapes through the shell and resinates up into the passenger compartment. This is less of a problem on a 1st gen where the main muffler is all the way in the back.

So the challange is to find a muffler so well built that not only will it withstand a rotary, but will also prevent the noise problem from entering into the car when used as a presilencer in the cat location. Mangaflow is out (they die quickly so close to the engine). The second choice was an RB presilencer in 2.5". It's pretty expensive though. Some 2nd gen guys were saying a Borla XR-1, which is a racing muffler, used as a presilencer, was actually better than the RB presilencer! Quieter at idle and better tone while reving. Not sure how it responded to part throttle cruise, but I bet it was comparable to the RB. Probably a little quieter too. Maybe? Flow was probably similar at least.

The Borla Pro XS series is a recent addition to their lineup. Most are 14" shell length with a few in 19" length. Other dimensions are 4" tall x 9½" wide. Will it fit in the cat location on a 1st gen? I don't know. Does anyone know?

I believe an offset/center configuration will reduce noise more than a center/center. Any truth to that?

How many Borlas do you think I'll need in 2.5" NA? I'm shooting for just two. Preferably the Pro XS because they're cheaper to buy. If I do need a third, maybe I can fit one of their smal oval XR-1s before the rearend or something.

I highly appreciate any input you guys can share. Thanks!

Jaime Enriquez

When those ozzys make their mufflers they are a bit off-center....I believe they did say they were a little quieter....don't forget the pics when done!!!!!

Jeezus

iTrader: (4)

I cant wait for my borla's to go on. Like you, I am going 100% borla!

EDIT: Just like you but minus a rotor! I got the RB 2.5" piping, Borla XS offset/center 2.5" muffler, and am finishing off with the mazdaspeed protege muffler, which is also borla made. What really sold me was on the side of the tip it has MAZDASPEED laser engraved in it

openshot

if you can get a video of it plz?

trochoid

iTrader: (6)

I still think you should go with 3" pipe. If you have better reasons for going with 2-1/2" due to your research, I would love to hear your insight. Your quite correct going mild steel with 1/8" thick wall. This will make a big difference/help on quieting down the exhaust note. I considered using left over rollcage dom piping for exhaust, but it was too narrow. For true duals on a 12A, it might work, but it's heavy pipe, not to mention pricey.

On reason my 3" is so loud is because my mandrel bends are thinwall galvanized pipe that was used as intercooler piping on a Ford turbo diesel in it's former life. Hey, free is free, lol. If/when I finally get the TII installed in the widebody, I would really like to use a Borla as a presilencer. I do believe it will do a much better job than the round Magnaflow I have now.

dj55b

iTrader: (2)

I'll be agood month before i'm even done my car car the way things are going, but when i do get it done i will post up results too. I think i too might switch to slightly heavier piping. Mine is all SS piping (sorry can't recall what size exactly) but maybe just 1/16" or so and is pretty darn lound with a single muffler. I figured since I'm already down there and might as well redo it all (also didnt't like the fact that I didn't use mandrell bends , just really need the other the axle part.

Jeff20B

iTrader: (13)

Jaime Enriquez, I think I'll focus on the offset/center versions for some noise reduction. Yeah, I'll post pics.

Jeezus, that sounds like a nice looking muffler.

openshot, a video is possible. The plan is to just get what I can afford at first. Probably one muffler and one set of flanges; the bare minimum (too many projects stretches the wallet). Then I'll record some audio clips and/or videos so you guys can hear what one Borla will do for a 20B. Thinking about it, I'm actually very interested now. The ultimate test perhaps.

trochoid, I understand your concern about the pipe diameter. I always knew I needed to use at least 2.5" on the 20B, but can't remember the original reason I came up with. I think it had something to do with math and some flow dynamics I was studying at the time. You basically have three rotors all sort of flowing at the same time. Sort of. When one is just starting to flow, one is just ending, and the third is half way through. with only 120° between rotors, it's like 1.5 rotors are always flowing. Hence the need for larger diameter pipe to handle the 50%greater flow. Compare that to a 2 rotor where only one flows at any one time due to the 180° rotor phasing. This is the reason I don't recomend anything larger than 2.0 or 2 1/4" at the collector if you're NA (even for a 13B). Of course if you're turbo, most of the exhaust energy has already been spent turning the turbine so it's best just to dump the exhaust as quickly as possible. Therefore a 2.5 or 3" is fine.

A recent post by Rotary20B showed that a 2.5" system can make 295 RWHP NA. That's higher than the stock twin turbos in a JC Cosmo. Not bad at all. https://www.rx7club.com/forum/showpo...&postcount=114

There is also the thinwall vs thickwall debate. I do have some thickwall 2.5 kicking around, so that's a plus. It's also easier to quiet a 2.5 system, which I'll need to do since it's NA.

I have local access to 3" stainless, but it's only half as thick (.063, like RacingBeat's stainless 3"). I'd only use it on a rotary if I had a turbo.

So that's where I'm at. If I was doing a turbo, I'd totally use 3" thinwall stainless because I could get away with it. However since I'm NA and will stay that way for now, and since the system has to be quiet and not restrictive, I'll just go with 2.5 and some offset/center Borlas. It is the equivilant to 2.0 on a 13B. At least that's what the math I'm doing in my head tells me (a bunch of horse flies buzzing around). Feel free to swat my math.

dj55b, it'll be a good month before I can start my conversion too. 1/16" I think is .063.

Attila the Fun

iTrader: (1)

In it's 2-rotor days, the RXX-7 had a Borla exhaust. The sound was spectacular. However, at highway speeds there was a harmonic drone that was physically painful. I ended up putting Dynamat under the entire passenger compartment to bring the cound level down to something that didn't hurt.

Jeff20B

iTrader: (13)

George, do you have a post or a website detailing your exhaust? Wall thickness, diameter etc. I've still got my concerns as to whether a muffler in place of a presilencer will be quiet enough for use directly below the passenger compartment.

PercentSevenC

From the Mazdatrix website:

"Another test we did was on rear pipe and muffler diameter vs. power for the 86-92 non-turbo cars. We found that about 2" O.D. was the largest acceptable. The larger diameter pipes were giving very little gain above 7000 RPM, and were killing the power in the 2500 to 5000 RPM range."

This, as Jeff has said, is due to rotor phasing and velocity. The cross-sectional area of a pipe, and thus its airflow potential, goes up as a square of the radius. If we assume that the 20B will require 1.5 times the flow of a 13B after the collector (based on the phasing Jeff mentioned), and assuming a 1/8" wall thickness for both, the 20B will require a pipe of about 2.32" OD. 2.5" is actually on the big end of ideal.

3" would be way overkill, and harder to muffle. Even if that big of a pipe were beneficial in the upper rev range, due to the extra muffling required I'd wager you would see power losses at ALL RPM.

MosesX605

Good job going with the Borla, you won't be dissapointed.

Mine still sounds great, but some of the mellow tone is now drowned out by the raucous noise that the DCD's make at WOT.

FirebirdSlayer666

iTrader: (3)

Jeff, I will defintely be keeping an eye on this one as I plan to do an NA 20B in the FC in the future and since it is a daily driver, good power with good tone and low noise levels will be very key.

Attila the Fun

iTrader: (1)

No. As part of the conversion, PFS made a custom exhaust system. Trusting soul that I am, I never asked any questions about it.

You could try contacting Mitch Piper. At the time he was working for PFS and did almost all the work on my car. He now has his own business. You can get his phone number and other contact info at www.pipermotorsports.com

Attila the Fun

iTrader: (1)

If you haven't already been there, check it out and read the FAQ. Posting your question there will also get you the experience of a number of guys who are more inclined to do their own work than I am.

George

Jeff20B

iTrader: (13)

PercentSevenC, thanks for working out the flow rate. I'd like you to double check it though due to a little miscommunication on both our parts. You know the sentence in the Exhaust Narrative where it says "Another test we did was on rear pipe and muffler diameter vs. power"? They're talking specifically about the dual 2.0" muffler pipes on 2nd gens after they Y apart from the single 2.5" center section. It seems logical to use a smaller diameter after they Y apart. I just hadn't noticed the distinction until now.

I had previously used that paragraph, specifically the part where it says "We found that about 2" O.D. was the largest acceptable." in the design of the SINGLE exhaust system on my REPU. I saw exactly what I wanted to see in that sentence because all I was looking for was a good reason to use the 2.0" OD RB pipe and my RE glasspacks with enough flanges and gaskets to make it all work and still be serviceable in the future. It was perfect. I already had everything lying around.

Since it's a truck, I think I'll be ok sticking with the 2" OD pipe and components. This brings me to the next bit of interesting FYI concerning smaller muffler pipes.

I removed the exhaust center section from my CD Cosmo yesterday. What I discovered is the diameter of the muffler pipe, where it runs from the flange right before ther rear end, all the way to the muffler, is of a smaller diameter than the center section. I had questions on my mind and while rereading that paragraph, the part where it says "Another test we did was on rear pipe" totally jumped out at me. At first glance where they recommended smaller muffler pipe and muffler diameter, the smaller Cosmo muffler pipe seemed the perfect solution.

"So that's the reason why Mazda used smaller diameter pipe back there," I said to myself.

Then of course the realisation that 2nd gens used two mufflers and a Y pipe kicked in and now I'm back to square one (hence the reason for this longwinded post).

Jeff20B

iTrader: (13)

While it's true that a 20B flows 1.5 times as much as a 13B, and a 4 rotor flows 2 times more than a 2 rotor, is a pipe outside diameter of 2.32" really correct for a 20B? I only ask because running a 13B on 2 1/4" is acceptible for the kind of performance (ie non truck use) I'm after.

PercentSevenC, your calculations were based on 2.0" pipe on a 13B, which was inaccurate. RB recommends 2.5" pipe and presilencers on the 2nd gens and 2 1/4" on 1st gens (the Power Pulse dual pipe version Ys into a single 2.25" pipe before it enters the muffler). That reminds me of somerthing else I'd like to discuss.

I wonder why RB decided to keep the smaller diameter muffler pipes on the PowerPulse main muffler for the '81-'82 model, even though they use it in their performace "street port" exhaust system. From header, all the way through the center section, it's all 2.0" OD pipe and presilencers, until it gets to the muffler flange, at which point it squeezes down to something like 1 7/8" OD. Whatever it is, the smaller size is quite noticeable just looking at it. Why why why?

For those that don't know, the '81-'82 model year had a partial dual pipe exhaust system. It's single as it leaves the exhaust manifold, goes through a cat or two (not familier with the front section) then Ys apart into two small diameter pipes. I know not why they did this. It was the first model to get a cat, and this must have been their solution to some newly introduced problem or something. They must have solved it by '83 because the '83 to '85 all had single pipe in approximately 2.0" OD.

So why did they choose to 'cripple' their flagship system on the first gen? Why couldn't they have kept the 2" OD pipe and collected it into a 2 1/4" or maybe even 2.5? Would it hurt velocity too much? They do state that it is compatible with stock ported 12As all way to bridgeported or I guess even peripheral ported 13Bs. It's certainly compatible with a superchargers and even Steve's S5 T2 conversion, although he figured out a way to add a 3" pipe to the muffler (not sure how that works out for flow).

Anyway, it had to be compatible with the smaller, unported engines, and seeing the smaller diameter pipe on the Cosmo muffler sort of answered that question. I just thought I'd bring you all up to speed on the rear pipe size issue.

Jeff20B

iTrader: (13)

Getting back to the 20B exhaust, since we know that 2.0 and 2.25 are ok on a 13B in single exhaust format, I'm convinced that 2.32 and 2.5 on the 20B are acceptible sizes where 2.32 = 2.0 and 2.5 = 2.25. Does that make sense?

Think of it this way. The smaller diameter would be more for low end driving like a typical luxury car or a rotary truck, and the larger diameter would be more for higher RPM spirited driving in a sports car like a 1st gen. I could technically go with either size, but I'll stick with 2.5" because I already have some lying around and it's easier to find. Plus I plan to do a port job during the rebuild and it will certainly be driven in high spirits.

Speaking of luxury car VS sports car, I'm replacing the thermal reactor with a header and presilencer on the Cosmo, but I'm keeping the center section and muffler. Why? Because the back of the heat exchanger has a very familier stud pattern. It looks like the presilencer will be compatible with the existing flange already on the car. Less work for me.

Jeff20B

iTrader: (13)

Thanks MosesX605. Glad to know your Borla is still holding up. I wonder how mine will do in the cat location.

FirebirdSlayer666, I'm doing my best to build a quiet yet powerful system. There is a chance you could use your existing exhaust system, if it is already 2.5 then into dual 2.0, as mentioned in the Exhaust Narrative. We'll see.

George, I have a very stong impression that your system was fabricated from the more commonly available and bendable .063 wall thickness (1/16") stainless steel pipe. Since it's a turbo you're not going to need nearly as much muffling as I will, as the turbo is good for around 50% of the noise reduction by itself.

So then what's the reason for the annoying cruise noise problem? Take for example my friend's REPU. It has a piece of crap exhaust system that somebody fabricated from an exhaust manifold (presumebly to keep the O2 sensor), which they cut and welded (in cast iron!) a 3" flange. It then has a downpipe that flows into a single RB universal muffler http://www.racingbeat.com/resultset....rtNumber=16030 directly below the cab, and then extends all the way to a 2.5 or 3" supertrapp. It was quietish and noticeably gutless with the spark arrestor. I suggested we remove it for more power and hopefully not a great increase in noise. We did (broke a couple allen head screws in the process) and the power really picked up, as did the noise! It is now quite painful on the freeway. It just drones on and on. There are so many things wrong with that system it's rather comical. When I build my 2" exhaust for my REPU I'll show him that smaller is better. It'll be nice to hold a pleasant conversation at highway speeds.

The point in all of this is his truck was not all that noisy while it still had the spark arrestor. It seems removing it made the whole truck louder, inside and out. Perhaps it's a combination of thinwall pipe and not enough muffling? Remember his truck has an exhaust manifold. It does keep things a little quieter than a header. I recently proved this out quite conclusively on my white REPU.

I recently replaced an RB header with an exhaust manifold from a nitrided R5 13B on my white truck. It's the 13B that was carbed right before Mazda switched to the RE-EGI engines. It had an exhaust manifold quite similar in design to the '81-'83 type, only heavier. No O2 sensor. The exhaust system on that truck is a total piece of crap. The RB header was the only good thing about it, but was going to waste. It consisted of thinwall 2" low grade stainless steel pipe directly after the header, then a cruddy chambered 3" muffler, with reducers/expanders at either end, then back down to 2" thinwall pipe, then into a long glasspack "hotdog" that sounds as hollow as an empty coffee can. The muffler does too. Knock on them and they sound blown. Then from the back of the hotdog extends some 2 1/4" aluminized pipe, obviously bent at a muffler shop, so it's obviously thinwall, and into a dual tip under the bumper. You know the velocity and flow of this system is quite bad.

It was always loud in the cab. The passenger especially got the full brunt of the noise, sitting directly above the chambered muffler.

If you read this part of the Borla FAQ, it explains why chambered mufflers are so loud.

Q. How does a muffler’s sound and performance differ based on its location in the exhaust system (i.e.

Muffler design is the main factor. The chambered mufflers send the sound waves back and forth in the muffler so the muffler is very loud. Many of us recall the ‘69 Camaro with the hot street muffler of the day. We didn't care if it was loud inside the car or out; we wanted people to hear those horses. Today, enthusiasts like to hear the CD player or carry on a conversation while still enjoying a muscle car. And, we have to deal with both legal issues and the integrity of “do onto others as we want them to do onto us”. If we live in a gated community, the neighbors will probably not be willing to deal with a 100 db machine rumbling into the garage at 11pm. Hence the recent muffler relocation to the rear on most vehicles - an effort to keep resonance down inside the vehicles. Breaking down the sound, however you do it, will be quieter for the driver done as far away from them as possible. Effectively speaking: mufflers placed within about 18” of the header collector make the most power.

Now that I replaced the header with a manifold, and a length of RB 1/8" wall pipe, it is noticeably quieter in the cab but also noticeably down on power above 2000RPM. However below 2k it actually jumped in power. It's actually easier to drive in stop and go. The whole system needed to be replaced (even the header since it exploded in a small section before, and required a patch job). I think a carb swap to match the new flow capabilities is in order. Power will probably pick up too.

PercentSevenC

Hmm, good point, I hadn't noticed that wording either. And you're right, RB uses 2.5" stuff for the second gen. But here's something else: Racing Beat also said that there is no performance gain from replacing the stock 2" OD midpipe on first-gens with a larger one. Now, Racing Beat sells a complete bolt-in single-pipe exhaust system for those cars, so they could just as easily have used 2.25" or 2.5" pipe after the collector and simply added a replacement midpipe, but they didn't. Why? Here's my theory: in a short-primary system, when the primaries collect there is still a high amount of kinetic energy in the exhaust gases. Meaning, when the pipes join, the exhaust pulses are still relatively discreet, allowing them to more readily blend together. Thus, a big diameter pipe is not necessary to maintain flow. However, in a long-primary system, much of the exhaust gas energy has already been expended by the time the primaries are collected. The pulses have smoothed out, and they will create more turbulence when collide in the collector. So a more free-flowing pipe is needed to maintain some semblance of smooth flow. Again, Racing Beat could have used anything here, but they chose 2.25", so I will assume that that was the size they found with all their testing that yielded the most power.

So why then would they use a 2.5" pipe for second gens? I think the answer to that can be found in a post by Rotarygod in the old Borla thread. Here's part of it (it was a long post):

Remember that Racing Beat doesn't sell a collected header for second-gens. Their complete exhaust system is an uncollected "true dual" system. Meaning that the 2.5" stuff is intended to be used with a stock exhaust manifold. What is a stock exhaust manifold, really? It's a relatively large, hollow chamber. Starting to make sense now? Since all the "tuning" (or lack thereof) is already done when the pulses enter the manifold, a more free-flowing pipe can be used; it won't become restrictive and won't cause a power loss since velocity isn't important anymore.

Jeff20B

iTrader: (13)

Ah, that sort of makes sense. Also notice that regardless of whether it's turbo or NA, the 2nd gen midpipe is still 2.5. Now the reason for the diffusers in NA housings becomes clear. The diffusers help quiet the pulses a bit, and the large hollow chamber allows, as you said, the 'tuned' pulses to enter, mix for some more noise reduction, and then exit into the 2.5" downpipe. Mazda went with a single exhaust port size in NA and turbo housings to save money. The diffusers and manifold wind up bringing the noise character more inline with the reduction in exhaust noise a turbo affords. Would you agree?

Jeff20B

iTrader: (13)

Oops. I just realized that George's loud exhaust on the RXX-7 was during its time with a 2 rotor engine. He didn't say whether it was NA or turbo. Since it's a 2nd gen, they probably used at least 2.5 if not 3" in the midpipe. Probably stainless steel, and probably (most likely) thinwall since it can be bent by muffler shops.

That, I'm pretty sure, is the reason for the painful drone on the highway.

Considering your old system along with my friend's REPU with its 3" system, my REPU with its booger bash system, I mean the chambered muffler directly below the cab system, and the fact that none have the necessary thickwall pipe where it's needed, you know, like BEFORE the muffler (especially the first in the system), I'd say I'm pretty safe going with thickwall 2.5" pipe and Borla since it's not chambered and is rather well built. The 2nd and 3rd gen guys use them as presilencers, most likely on their turbo systems, in 3", probably thinwall etc, and say they actually sound better than RB presilencers.

Wow, this exhaust system is sounding better and better all the time, and it's not even built yet. I think I've given enough reasons to go 2.5" to satisfy even trochoid.

PercentSevenC

Possibly, yes. North America was the only market, IIRC, that got NA FCs, so Mazda would have liked to make them as compatible with the turbo cars as possible to reduce extra costs. That means that all the expensive stuff (like the exhaust system and rotor housings) are the same. (And since they were already making 6-port side plates for the GSL-SE, they could just reuse those for the FC.) Since Racing Beat's downpipe and things are supposed to be compatible with the OEM components, that's another reason why they would use 2.5".

In short, I still think that 2" (or 2.25" depending on the system) is enough for a 13B and that 2.5" should be plenty for the 20B.

Jeff20B

iTrader: (13)

Your reasoning is sound. I'm curious what others think about all of this exhaust talk.

Attila the Fun

iTrader: (1)

The old exhaust system was spec'd by Mariah Motorsports. The motor was a street-ported N/A. The system started with a Racing Beat header, a Hi-flo cat, and the Borla cat-back. I had a 3"-diameter pipe. The combined mods to the motor produced a car that pulled strongly throughout its range, with more-than-stock power down low that built rapidly all the way to 9000 rpm. It would probably have gone higher, but I didn't push it past that point intentionally.

FWIW, after the 3-rotor conversion, I sold the Borla-based system to John Becker, who had an '88 convertible. He really enjoyed the increased performance. Like me, he had to line the car with Dynamat to save his eardrums.

Jeff20B

iTrader: (13)

Ah, 3". Thanks for that. I can't wait to find out how 2.5" performs on the 20B.