787B

OK, finally I have decided for sure. I dont care what anyone says, I am going single. I have thought about this many MANY times and it seems like it changes each week, but now I am positive.

MY EXHAUST SYSTEM IS GOING TO BE SINGLE.

here are my reasons...


1. it is less restrictive
2. it is cheaper (1 less muffler and no y-pipe)
3. it is a bit lighter


the only downside i see is that visually its not as cool but....i care about performance more than i do looks.

anyway, here is my next question. what side do i place the muffler on??? driver or passenger??

stinkfist

GOOD FOR YOU!!!!

Passenger. First and third gens have it on passenger. You sit on the driver side so you don't want to add more weight to the DS do you?


Mike

787B

thats a good point. i will do that. thanks

NZConvertible

Read my lips: No it's not.

mazdaspeed7

Wanna bet? 2 pipes have more sirface area, and anybody who has ever taken a pysics class will tell you that flow is slower over a surface. As long as the collector is properly designed, a single exhaust WILL flow more than a comparable dual exhaust.

BTW, Im ditching my true dual exhaust for a tuned primary, single exhaust system. I will be dyno tuning it for the best powerband.

jspecracer7

Place it on the passenger side...and trust me...you'll be happy that you went single...I could tell in my FC that when I went single, there was some power gain. Plus it was lighter and easier to work on when I had to get something fixed down under

NZConvertible

Only if the combined area is the same, and why would you do that?
Are we talking turbo? If we are, twin 2.5" pipes have 39% greater cross-sectional area than a single 3". With that much reduction in restriction, surface area becomes irrelevant. Only half the gas passes down each pipe but there is much more than half the original area to flow through.
If it's a NA the same theory applies, just different pipe sizes. In fact if the headers are tuned properly (unlikely), those pipe sizes are fine.
There's nothing stopping you using bigger pipes after the Y. You've got the room, why not use it?
And I've been to more physics classes than I care to remember!

Erik

i'm pretty sure this is an N/A

glad to hear you're going single man! the ONLY way to go!

Evil Aviator

If you start with a header with 27" primaries, a long taper (15deg or less) 2-1 collector, into a 2.25" ID pipe, followed by a megaphone or resonator, you will not need much dyno time.

For you physics types, a collected single exhaust and uncollected dual (what you guys call "true dual") have about the same restriction with everything else being the same. This is because the exhaust gas pulse from each rotor travels through basically the same length and number of mufflers in each configuration. A split exhaust (Y) has the most restriction, but also the most sound deadening because the exhaust gas pulse from each rotor travels through TWO mufflers as well as a bit of additonal piping. Of course, the amount of restriction depends a lot on your mufflers, cats, tubing diameter, etc.

Placing the muffler on the passenger side will probably allow for the least amount of bends.

mazdaspeed7

Thanks for the suggestions. One thing though, this is going to be on a muffled street car, and I want to tune for a usable powerband more so than peak power.

What about stepped tubes? They seem to make more power in piston engines. Theres isnt much competition in the header market for rotaries(n/a's), so there isnt really a need to make the best design, just something that works reasonably well and is easy to produce. I was goint to step from 2" to 2 1/8" 12-15" after the engine, then have the outlet of the collector be 3", and then step to 3 1/2 about 12" before the muffler. BTW, my system is going to be full custom, imcluding the header, and Ill be finishing it off with a Supertrapp muffler.

Felix Wankel

3.5? good god, I put dual 2.5 on my NA back in the day, and my riding mower had more low end power.

mazdaspeed7

FYI, it will be 3.5" only for the last 12" before the muffler. The majority of the single pipe will be 3". The flare up to 3.5" is to make it quieter, as well as using a larger core on the muffler, which will also make it quieter.

This car is getting retired fromt he daily driver routine within a few months as long as everything works out well. I have a true dual exhaust now, and my VDI wired on the high rpm setting(no air pump), and I am used to not havign much low end.

But I heard of a nice little trick for more low rpm power. Run water injection in the exhaust. It slows the exhaust pulses down, and adds volume, so it has the same effect as smaller pipes, and the collector being farther from the engine, both of which increase low end power at the expense of high end. I know the water would need to be refilled, but I would only use it when I needed the extra low end. Casually driving around town, my car still has more than enough low end to get me in trouble in anything other than 5th gear.

Barwick

definitely passenger side.. offset the weight of the driver. And having a dual exhaust just adds more weight if you ask me, I don't think the flow benefits of splitting the pipes really makes enough difference to offset the extra weight.

Evil Aviator

The shorter header may be better for your application, but I have it on good authority that 27" is the magic number for road racing on a street-ported 13B. Yes, stepping works very well on rotaries, but I think you should start the stepping after the header collection point.

mazdaspeed7

I would think it would work better on the primaries before the collector. Also, whats a good distance between steps, not too short, but not too long? Im curious as to why you think it would work better after the collector. Not that I dont believe you, you are very knowledgable about this stuff, I just was looking for a reason if you had one.

BTW, 27" is VERY close to what Ive been told is a good starting point for the collector.

speedjw

I think I posted a fairly in depth response to this topic only a few days ago. So, to reiterate, single exhaust has the advantages of, lighter weight and scavenging.

By tuning the diameter of the pipe you can control exhaust velocity and backpressure. By rapidly increasing the diameter of your pipe you will increase backpressure, simple fact, the divergent section of a nozzle increases pressure. This may or may not be a good thing, depending on what type of power you are looking for. I haven't found the perfect answer to that one yet, so I couldn't tell you what absolutely is right and what isn't. But for that matter, I doubt there is more than 5 people in the world who could tell you that. Of course, some may have a better idea than others.

But, good luck with your exhaust, I applaud your decision to increase the power of your RX7 while reducing weight.

Oh yeah, on a side note, Dynomax mufflers are some of the highest flowing, quietest mufflers I've ever seen tested. Just a thought, if sound level is an issue.

Evil Aviator

Hehehe, if I posted where I got the info I would be spammed by PM. And no, I'm not really all that knowledgeable. It may just be best to read my posts just like any other opinoins on this forum. You said that you were going to dyno tune your car, so go ahead and try different configurations and see what you get.

mazdaspeed7

I dont care where you got the info, I wouldnt want you to get spammed for telling where. You didnt answer my question though. Is there a reason for not stepping the primaries?

And you definately have your areas where you really shine with your knowledge, and this is looking more like that than not.

speedjw, do you have a link to that in-depth response, or perhaps a thread name so i can find it?

amemiya

yeah he is right it is actualy fluid dynamics called the boudry lare. the air malicuals on the pipe surfice are not moving it proportionaly speeds up to the rest of the flow in the center.

baix2

hm...
well what i think is that:

the exhuast flow is a constant quantity at an instant...flow rate doesn't increase if u put an bigger pipe exhaust....mass has to be conserved

and also, i really think single give a more efficient flow, when flow is experiencing change in diameter of pipe or got split into 2 pipes, energy will be lost...therefore single is less restrictive

and also it's true that boundary layers developed due to the "no slip" condition at the wall of the pipe.... but there might be pressure gradient in the flow, plus i think it's almost impossible to understand the flow structure since it's a turbulent flow (high Re), turbulent flow is one of the mystery stuffs that still couldn't be completely solved in mechanical engineering. and like above said, probably only a few ppl can answer this question, because the question here is a particular case for a car, for example Turbo vs NA will be different

but definitely go with single for performance, it has advantage over dual, except the fact that if u like the look of it

1) weight less
2) cost less

can't really say much about the weight balance, because we don't know about the exact weight distribution, so far we only know that it's almost 50/50 front to rear, what about left to right?? only mazda knows...maybe that's why they design dual stock exhuast to make the car balance

supergoat

I'm always gonna stay dual for 2 reasons.

1) Looks. I LOVE dual exhaust.
2) It's quieter.

That's my only reasons for staying dual.

Yea, not much to add to the convo, but I thought I'd add my $0.07

fuzzi

This may be a little redundant.. but .. what size piping is good for an N/A car thats streetported with headers?? I mean performance wise would putting a pipe with a huge diameter give you an advantage ?? Dont N/A cars need more backpressure to make power then Turbo's.. after a certan point wont you lose power ??

von

Heres a question for mazdaspeed7. Why do u want less top end and more low. Doesent your top end the reason why your so fast. I cant imagine being faster with less top and more low.

I thought a single exaust increases velocity and has alot to do whith length. The longer the more torque. Isnt dual y desing like cutting off your exaust at the y and letting it travel in thin air. Because at that point the velocity is not going to speed up. My single is really quite... Even thou thiers one muff the gas coming throu it is going faster thierfore the muff is doing more work for sound supression. Doesent this make sense to anyone??? I kind of just figured this stuff out on my own so I can be wrong. I never took a physics class.

Perry Gehenna

my friend's 87 turbo had a 3" exhaust that splits into 2 x 2.5", he changed it for a straight thru 3" and LOST 10rwHP (verified by dyno)
so on went the twin system again.
but perhaps that particular single system wasn't well made.

the dyno result seemed logical to us at the time, going on the premise that rotary engines love to breathe, as well as the discernible loss of power when going single.

btw, is "stepping" the exhaust system also referred to as an "expansion chamber"?

rhubarb,
Perry Gehenna

wai

mine is on the pass. side, it sounds super loud,
even at idle.