79RX7GUY

so i am looking into some kind of dual exhaust system on my 85 GS and i don't even know where to start or whether or not its a good idea. does anyone have a true dual system that could give me some advice? i am also open to all kinds of ideas if anyone wants to help me figure out what to do with my exhaust. thanks guys!

cfamilyfix

iTrader: (15)

I have seen a few done....usually 2in id tubing all the way back to to mufflers turnd up on their side. you also have the option of collecting the pipes just before the rear muffler. You can use a racing beat muffler for 81-83 (i believe) With the RB muffler you will prob not need any presilencers, but if you use anything else its gonna be a bit loud without them. Hope this helps

j9fd3s

iTrader: (3)

yeah 2" piping to 2 mufflers of your choice.... its simple.

patmac6075

iTrader: (26)

Why not just go with the R/B "race port" exhaust? Isn't it a true dual all the way up to the rear muffler? Can't imagine there would be an enormous gain (or even a perceptive gain) in power?
It's not so much that I'm against trying new things, but it just seems like that wheel has already been invented. Not sure there would be any real cost savings either...but then again, can't hurt.
Good luck ...be sure to post results.

PercentSevenC

I doubt a true dual system would increase power at all over a properly collected system. More likely you'd lose power.

79RX7GUY

what would be the best setup for power gains then?

PercentSevenC

An exhaust built on a dyno.

patmac6075

iTrader: (26)

Please correct this misconception, but I was under the impression that the Racing Beat "Race Port" exhaust was superior to the Racing Beat collect exhaust. From what I remember the further from the exhaust port that the two are collected results in better flow (obviously you must come to a point of diminishing returns).

And yes....an exhaust built with the help of constant dyno testing would provide the best results.

Siraniko

iTrader: (13)

Stock motor/carb with header = barely noticeable power gain
Stock motor with modded carb and header = yes

but to answer the real question, run dual pipes no smaller than 2 1/8 with collector over or under the axle (or single 2.5) but the rest is your call (if you want it quiet or loud)

PercentSevenC

The RB "streetport" long-primary system is better than the short-primary system, but that's mainly because it collects to a larger diameter pipe. It's a very complex subject, but theoretically, a short primary system will have more peak power and a short-primary system will have a broader powerband, assuming both are actually built to match the engine. Most aren't, including Racing Beat's.

zaridar

iTrader: (3)

anyone have any pics with dual exit exhaust? ive only ever really seen on the pass side if its a 1st gen

j9fd3s

iTrader: (3)

that was the experience i had on my GSL-SE.

RW-7

iTrader: (56)

Heres a pic of my setup on the GSL-SE.
It is a racing beat road race header with 2 1/4" pipes, 2 cherry bombs and a super turbo muffler per pipe. All from Summit racing. The mufflers had to be set in sideways as you can see from the 1 pic I have right now.
Even with 4 silencers, and 2 muffler it is still a little loud, but I like the sound of it, and after all it is a rotary.

BTW this mod was an increase in power over what I had before. Stock exhaust with a cat delete pipe.
Same system on my old 81.

zaridar

iTrader: (3)

so does anyone have any pics of dual exhaust with dual exit? on opposite sides of the vehicle?

kgray

The tank kind of prohibits this from what I can recall.

nofords

iTrader: (9)

I have my old FB out back I can try and snap some pics of.

I had it split behind the rear axle to 2 mufflers.

The stock tank can't be used. I had done two different setups for this. One, i purchased a new steel tank and cut it down. I didn't like the way it sat under the car so I went with a cell.

If you go with a cell that fits in the spare well (such as an ATL for example) then use an F-Body style muffler. Clean and simple.

rotor-motor

iTrader: (7)

just sounds over weight....

snowbreaker

iTrader: (1)

dude im doing a dual exhaust on my car as well cause i recently purchase a rotary engineering header uncollected and its 2 1/2 pipe so what me and my buddy rotor-motor thought of was to do the header to a dual inlet and dual outlet magneflow muffler and pipe it off to the side like the corvettes have, so the pipe will come out right before the rear wheel so it would kinda look like a corvette not sure how that would sound or anything, just an idea
but i also was thinking for a dual exhaust you have to put the muffler before the gas tank and just pipe it back undernethe the tank?

zaridar

iTrader: (3)

this will be very loud!!!! personally i dont think i like that look... just my opium... you will probably want two mufflers or a presilencer and muffler i think

13BT_RX3

I had a dual system on my 13B Bridgeport rx7 back in the day. Well kind of. It was dual 2-1/2" all the way back after my custom short primary header. It was an equal length design based on specs in Racingbeat's exhaust tips. After the collector I megaphoned it to 3-1/2" then split into dual 2-1/2s under the axle to two Edelbrock RPMs.



From what I understand, the goal of a high performance exhaust system is to increase volumetric efficiency in the RPM range that the intake and engine porting are tuned to support. Unless you have positive pressure like from a supercharger a true dual exhaust system will not work as well as a properly designed collected system. Collected exhausts create standing waves that have a pressure impact on the exhaust port that varies with RPM. If designed correctly you will have negative pressure on both ports in the RPM range that your engine needs it. It sucks the residual exhaust gas out, scavanges, increasing VE. The rule of thumb is a short primary system has a narrower power band, but larger pressure changes which make it more tolerant of mufflers than a long primary system. On long primary systems mufflers placed before the collector can destroy the wave tuning.

One additional bad thing about true dual system is the slugs of exhaust from each rotor only have one muffler to flow through at a time. The slug will pop out of one muffler then a lag while the other side does it's thing. If the exhaust is merged and run through these same two mufflers the slugs can stack one behind the other and then each slug will exit through both mufflers acting virtually twice as big. Muffling should work better too due to the increased muffler volume the slugs see. What V8 guys have done for years to combat this issue is run an H or X pipe. I have heard an intesting way to find where a car with dual exhaust wants an H added is to run a stipe down each pipe with a crayon. Then do a run and where the crayon burns will help you know where to cut in your H. Never tried this, but I thought it was interesting.

snowbreaker

iTrader: (1)

so then what your saying is that my idea of running one or two dual inlet and dual outlet magneflow mufflers then pipe it off and out of the sides of the car like the corvettes wont have the pressure need for it to work or will it be just like a single exhaust?

DriveFast7

iTrader: (1)

13BT_Rx-3 that is the best post I've seen on the subject in a time, thanks.

Here's my favorite shot of my exhaust.

rotarygod

iTrader: (1)

The H or X pipe in a V8 exhaust is nothing you can compare to on a rotary exhaust system. It is used because on a V8 you have 2 cylinders on a bank firing 90 degrees off from the other 2. You hit a point where the waves from these "off" cylinders interfere with the other waves and build up in the pipe. It is that location that you install and X or H pipe so that some of that extra pressure can bleed over. A rotary does not have this problem. If you have a set of 180 degree headers on a V8, you wouldn't have this problem nor would you with a flat plane crank V8.

I've had all of the rotary exhaust configurations. For street use the true dual is actually quite nice. Fortunately we have direct pressure waves and acoustic waves in the exhaust. A long true dual system works just like an organ pipe and in fact tunes based off of organ pipe resonance. A collected exhaust does this as well since the pipe effectively ends at the collector. The acoustic tuning wave will reverse phase and direction wherever there is a large area increase. The collector will allow you to use the velocity of the gas to help scavenge, or pull air from the other side. The both tune but they do so in different ways. I like to consider a true dual as a long primary that is collected at infinity.

For street use I noticed that the true dual had a very nice powerband. It was great off idle and had good low to mid range power. Top end was improved over the stock box like system but wasn't what you'd get out of a collected system. It got some pretty nice gas mileage.

When people say they don't feel much gain from a header it's because the gains are very different than say from a true dual system. The biggest gain is up top with any other gains coming on smoothly as rpms rise. Of course design and pipe length play huge roles so just consider it a general rule.

My personal favorite is the long primary that collects each pipe at the rear going into a single muffler. This one is a fantastic compromise between the true dual and a short primary. You get very nice noticeable improvements in power over most of the rpm range of the engine. Top end power "may" not be as high ultimately at the very highest spot of the rpm range but the overall average is far nicer. I personally think all n/a rotaries should be run this way.

A neat thing happens to sound depending on where you collect them. The closer you collect the exhaust to the engine the easier it is to muffler. The higher pitched the sound will be as well. The farther back you collect it, the deeper it will sound but the louder and harder to muffle it will be. A true dual sounds like two 1 rotor engines next to each other and it should.

The nicest sounding exhaust in my opinion is one with a short primary length. The worst in my opinion is the true dual which sounds something like a cross between a Harley, a lawnmower, and a poorly running riced out Civic. Some people love it. I just threw up a little in my mouth remembering their sound. The short primary with a good muffler can almost sound like a sport bike.

If all you care about is building a peak power dyno queen that lacks everywhere you will use it but will not impress people who think a peak dyno number rules the worlds than I'd say go with a short primary. If you live in the real world and intend to drive it then I'd say do a long primary like Racing Beat's street port system. When I had it, I got used to it. I sold it and replaced it with a standard header. I wondered where all my power went. It was a huge disappointment after that. I sold the car shortly after. It was no longer fun. Yes that exhaust is that noticeable!

zaridar

iTrader: (3)

i agree on the collecting it closer to the engine... also smaller diameter pipe in my experience is a bit louder and higher pitced ( more raspy) ... larger diameter is very slightly quieter but is usually quite a bit deeper

13BT_RX3

That’s a good point. I was wondering when you were going to help on this subject.

Long or short primary systems are tuned on the idea that one leg of the pipe to the collector is exhausting and open while the other is not so that leg is closed. They act like branched pipes with one branch closed. You should be able to get a true dual system to tune on the same numbers as a merge collected system by adding a closed pipe of the appropriate length teed in at the appropriate length. So the dual exhaust version of an 18" merge header would be to put a closed 18" tube of the same diameter teed in at 18" from the port opening on each leg. Please keep in mind that a test is worth a 1000 theories and experience is worth a 1000 tests. I have no experience with this and have not tested it.

Another way to tune a dual exhaust system is with open pipe length as mentioned by rotarygod. Determining this length will be the trick. Some theory:
http://en.wikipedia.org/wiki/Acoustic_resonance
http://en.wikipedia.org/wiki/Speed_of_sound
Once you decide to go down the unmerged pipe direction you may want to follow what 2-cycle engine folks do. Here’s an interesting article: http://www.motorcycle.com/how-to/how...care-3423.html
I like their explanation of the evolution of the expansion chamber. It started with a tuned single open pipe. Then they added a megaphone… Then it got more complicated. Now you need software.

Thank you. RB’s specs are the only respected published numbers I know of. It is nice to start with experience as your guide. There is still a lot to be left to interpretation though.

It might work. It will depend what the length to the merge muffler is. I don’t know what that length would need to be either. I would imagine it should act like an open pipe with the open pipe length ending at the merge muffler inlet. It should be a pretty free flowing system, but if you can get it to tune is TBD.