ericgrau

iTrader: (1)

<delete me plz>

ericgrau

iTrader: (1)

A 3" single exhaust is the same as a 2" double. Simple as that. Sure, 2 lines are less restrictive, but so is a bigger single line. Actually most of the restriction comes from the cat and mufflers, so get a nice muffler if you go single.

lupin

iTrader: (3)

well after reading this I think I am gonna do it guys. And I won't really do it for performance but because I want to save money on gas. I might just keep the rear storage bins and put the rear carpet down(just to hide my batt and amp).

ericgrau

iTrader: (1)

175 lbs. => 1mpg city (in theory) => 20 gallons saved per year (driving 12,000 mi./year, 50% city). Weight reduction doesn't affect freeway mileage much.

Roen

iTrader: (2)

Are we talking 2" true dual or y-pipe dual? I mean, with turbo motors, bigger is usually better, until you hit things like boost creep, but with an NA motor, it's not as simple as that, because of things like backpressure and abrupt changes in exhaust velocity when exhuast volume is very small. (e.g. going WOT at 2k rpm after a bit of cruising)

GTU_FAN

The piping is 2.5" not 2".

As far as NA's go the Racing Beats true-dual road-race exhaust is the best... period. You will not get better gains with a single. As far as the weight issue is concerned yes the Racing Beat exhaust is heavy but the power gain makes up for it. Say you make a single exhaust out of titanium that weighs 10lb (for arguments sake even the header could be titanium) vs the Racing Beat stainless exhaust that weighs 80lb...

...Remember my point about power-to-weight? It applies here. For every 17 lbs you remove from an RX7 you gain the equivalent of 1hp so the titanium exhaust will net you an extra 4hp because of weight savings. However I haven't heard of a single netting more then a 20hp gain whereas the road-race exhaust is known to net over 30. So ultimately the road-race exhaust will have at least 5hp over a single. As for cost I'm sure it would be about the same since a titanium header would be very expensive. I would love to see a titanium road-race exhaust though

Now as far as 3" exhaust on an NA flowing more then a true-dual 2.5" that's not necessarily true and it's certainly not the best setup for an NA. Turbocharged cars will benefit from less backpressure so the Racing Beat turbo exhaust uses a 3" downpipe but lessens to 2.5" at the y-pipe.

The reason Racing Beat uses smaller diameter piping then say an Apexi catback is because their exhaust is the only one designed to take advantage of a rotaries unique exhaust pulses. Its a common misconception that the less backpressure you have or the shortest exhaust possible (aka dropout tube) will be more effective on a non-turbocharged car. Perhaps on a pure dragracer when they are at max rpms at all times it is better but not on a daily driver street car. Also rotories without mufflers are very very loud.

What I mean by exhaust pulses is the rythm that exhaust flows. The high pressure exhaust gasses want to enter the low pressure exhaust piping because as we all know nature abhores a vacume. The reason the true-dual setup is so effective on a rotary is because it is the only engine other then a motorcycle with only 2 exhaust ports. A seperate pipe for each rotor makes perfect sense because as we all know the rotors are not perfectly alligned they have slightly different port timing. If both rotors were to share a pipe the exhaust flowing through that pipe would be more turbulant which makes for inneficient flow. On a turbocharged car the exhaust is fed through the turbo so it makes no difference but on a non-turbocharged car seperated headers will be more efficient.

Now the reason you want an exhaust to pulse instead of dump is again because of vacume. As we know hot air has less density then cold air. Once hot gasses are pushed through the exhaust fresh air is sucked into the vacume left behind. When you floor it and then lift off abrubptly what happens? Exhaust flow is lessened and the air around your car gets sucked further back into the exhaust sometimes causing backfiring because obviously the air outside the car has more oxygen to burn then in the exhaust gases...

...Now the reason 2.5 piping is good for most small motors is because its a good balance between flow and density. Large piping allows for greater flow but the exhaust gas expands more and is less dense which allows the outside air to flow further back into the exhaust causing resistence or back-pressure. The pulsing action in the RB exhaust is designed so that the exhaust has enough density and flow to keep gasses flowing efficiently out of the exhaust reducing backpressure which creates more vacume which pulls more exhaust out of the exhaust port. And since each rotor has its own pipe the flow is less turbulent and optimally efficient for each rotor.

Thats my theory of it anyway...

Bluewind

iTrader: (16)

My eyes hurt

Really interesting though. When you think about it, it really makes sense. So accustomed are we to the conventional combustion engine with generally (Honda) inline 4s. Thanks for the info. It's this type of information that intrigues me, please point me in the direction of other technical threads for tuning these FCs'.

GTU_FAN

^I'm going to preemptively give the nit-pickers the finger for arguing about exact weights or how many other motors have 2 exhaust ports per header... I can think of one right now... the porsche 912. Its a flat motor so it has 2 headers. And when I said, "Its a common misconception that the less backpressure you have or the shortest exhaust possible (aka dropout tube) will be more effective on a non-turbocharged car.", I meant to say that having no mufflers or silencers is bad for a rotary because it's too loud for the street.

GTU_FAN

Check out the archive for starters... Otherwise your guess is as good as mine just search and be real specific about what you want to learn. I'm sure at least a thousand other threads have been written about exhaust. Somethings though your better off researching on your own...

sc0rp7

iTrader: (3)

Have you, personally gained or witnessed a car tested with a true dual exhaust and a single exhaust? I have heard/seen reports of some 7's getting 20+ rwhp out of tuning with a SAFC, but not everyone gets those gains. therefore based on the only experience I have had with a single vs a dual exhaust (which if you do more research is normally not recommended for S5's as its runners are effectively too long for top end power), the power vs weight argument is not even worthy of being brought up. Im not trying to argue, but I did see all of these HP claims about a true dual ehaust, convinced my friend to replace his stock exhaust (with gutted cat) with a RB true dual exhaust to find very little hp increase. not enough to warrant the extra weight above a single, and definitely not enough to come close to 30 hp gains unless your stepping up from a completely stock partially clogged exhaust.

- Chris

also, especially if you are talking about stock port motors (what the RB dual system was designed for), then why do most ITS and etc track cars run a dual header into an expansion chamber, then 3" out the back of the expansion chamber? I do realize this setup is supposed to be for high end only, considering they are race cars, but if thats true you are gonna tell me that this type of exhaust produces more than 30 hp? There may be some kind of rule against dual exhaust or something more I am possible missing, but I thought I would share my knowledge.

GTU_FAN

MMMk well first of all I have no personal experience to vouch for horsepower gains with my dual other then seat-of-the-pants I admit that but I stand by my claim that it produces more gains then any other setup. Maybe not as much as RB claims especially on an older motor but I felt an improvement over the strait-pipe w/downpipe setup I used to have.

And pardon me if I'm wrong but why are the "runners" on a single any shorter then a dual? In a traditional setup the muffler still comes out beneath the rear bumper on one side or the other its the same distance.

As for the dual exhaust/expansion chamber argument have you looked at the Racing Beat catalogue lately? Their new design for the true-dual uses one presilencer/expansion chamber instead of the older one that had 2. (I have the older one) I dont know why they did this but I dont believe they would design it this way if it ruined the pulsing effect so it must somehow be as good or better in that way. As for how many pipes is better behind the presilencer a dual is quieter and the piping can be smaller and when split like that maintain the same or better overall flow while remaining denser reducing backpressure.

I dunno what ITS racecars your talking about but if it aint a rotary then why does it apply? Rotaries have different exhaust characteristics then piston engines why do the think they sound so different?

The hp vs. weight argument is worthy of being brought up in a thread about weight reduction dont you think? Especially one so hard-core they are talking about the weight difference in glove-box hatches lmao

SChema

I'm sorry, this may have been answered already, what is the dry weight of a TurboII?

Nihilanthic

Are there more people here saying you should use 'true duals' with a rotary? Ugh.

If you dont have them collect within a good distance to correspond to the powerband of your motors porting and general tune, you will have NO scavenging. Also, if you're doing heimholtz tuning based off of just the distance of the pipe, youd have to have both dump basically out of the fender - having the pipes go the full distance of the car would be so long, the soundwaves would have to travel so far the resonant frequency would be about idle.

The best exhaust for a rotary you could find is something similar to what a 2-stroke 2-cyl motorcycle, because the exhaust comes out of the two exhaust ports on a 13b every 180* - just like a two stroke. But, well, its NOT a 2stroke, so Im only talking about header lenghts, you dont need that big funny water-bottle shaped resonant thing sticking out for controlling reversion.

Or, more accurately, after pairing off the pairs of cyls 360* apart on a I-4, look at how those two pairs (and from where they collect each one fires 180* from eachother, every 180* of crank revolution) are then paried off to the midpipe in the exhaust on a 4-2-1 exhaust. I forget the exact math for what rpm range and what exhaust port opening/closing = what distance you'd want for pairing off, but its out there. And as far as speed of sound, its faster on the exhaust side than on the intake side, so youd want longer runners regardless.

Also, the stubby 'runners' for a paired up turbo manifold up to the flange are just to get the exhaust pulses to the turbo as fast as you can. Turbocharging makes scavenging almost impossible and also negates the need for it becuase you have this yummy thing people call 'torque'.

Cliffsnotes for people who dont care to read all that ****: if you dont pair off, it doesnt scavenge. How far away the collection joint is from the ports determines the rpm range - closer = higher rpms, because the speed of sound is generally the same in the exhaust once the car is up to temp, so... you change the distance it has to travel when you change how long it has to travel to scavenge (by revving up)

Oh, and a single pipe weighs a lot less than twin pipes. Ideally you'd just dump it out the side or straight down, through the bodywork. Once you pair off the pulses to scavenge, you really want to get rid of it as soon as you can. Longer pipes cool it, slowing it down, and that means youd have to taper it down towards the exit to keep up velocity so you dont have the exhaust 'stall' in the pipes and create backpressure by not evacuating fast enough. Also, if you know the low pressure areas of the cars bodywork and underside when under way, try to put the opening in one of them to help evacuate the exhaust a little more if you can.

Nihilanthic

#2800 range. The lightweights are the bases and sports IIRC, with the 86 sport being the lightest supposedly - #2600 range. After 86 chassis reinforcement was put in for crash safety IIRC.

Verts are in the #3000 range.

Wh1t3 C0m3t

speaking of alum hoods and light weight parts (from pages ago)i have an NA alum hood hood in perfect shape waiting to be purchased. hit me w/ a pm.

GTU_FAN

MMMk well I dont see how anyone could run a motorcycle exhaust out the fender without some kind of silencer it would be too loud for the street. But for a race car sure whatever works...

You may be right about the pulse/scavenging argument in regards to length of piping but there really is no way around it if you want it to be streetable.

Seems like your condoning single exhaust over dual though for the street or am I wrong? Do you agree that it shouldn't be any larger then 2.5" for a stock motor? Do you think RB exhausts are overpriced and poorly designed?

sc0rp7

iTrader: (3)

the ITS race cars I was referring to are rotaries bud. I was trying to be rather polite in saying you are WRONG. now I understand you dont get the concept of that. I love RB exhausts personally but everything I said was based on the reading of multiple threads, then based on persoanl experiences on a dyno, real... not ***...

mmmk, so in reply to your first paragraph... how are you soo sure it produces more than other setups when the only setup you have tried other than the dual is straight pipe/ downpipe? *D'OH*

when I refer to runners I am referring to them in total length before collection (as someone above mentioned). so yes, the runners of a true dual header, with true dual exhaust are in fact MUCH longer. this is because the exhaust runners would only be the length from the engine till they collected instead of the length of the entire car...

as for your third paragraph, RB now uses a non-collected (I believe unless you can show me otherwise as it states in the description it still has 2 2" tubes, and does not say anything about them collecting) single silencer probably because its cheaper than building two seperate ones, and possible because it makes it a little quieter. I know dual exhaust is quieter, because of smaller piping diameter, and two mufflers... I never said it was not quieter or inferior. As for the last sentence... where did denser come from? how would it make anything "denser," you are correct about 2 pipes being able to be smaller and maintain flow, I never disputed that.

I already covered your little ITS mistake.

as for the HP to weight argument, they wanted to know how to make it lighter, not "more horsepower" as you claim this HEAVIER exhaust will do in a weight reduction thread *D'OH*

sorry if I came off as a jackass, but before disputing what I said, you should have done a little research and determined if I was correct or incorrect. this woulda saved a little bickering and alot of thread space.

yes the glovebox weight difference is a little silly... but every little bit helps I guess...

here... i'll even contribute a single thread that will give you some information.

https://www.rx7club.com/showthread.p...ning+resonance

- Chris

Nihilanthic

"Dual pipes" are for engines that already have two groups of paired off runners, and even then you want to have them paried (x pipe or h-pipe) for more scavenging, and to balance out cross-plane v8s (which most dual piped cars tend to have up front). The reason youd do that is because a y-pipe up front can be more restrictive than two main pipes and an X-pipe.

Running "true duals" on a rotary is like running 8 pipes on a v8. It adds weight, has no scavenging, and would kill performance, especially with cams with overlap by robbing them of their scavenging effect. 13Bs have a peripherial exhaust port and thus do have some overlap and benefit from scavenging, especially ported ones!

So no, true duals are just a bad, more expensive, heavier idea for any application for a 13b, period. The only rotary I Can thnk of that would benefit from it, at all, would be for a 26B (after youve already paired off the right pairs of exhaust ports), but even then you'd probably want to have at least an X-pipe if not pair them off totally anyway. Come to think of it a 26B would be more like a single-plane v8, and Im not familiar with their exhaust design. I can say a single pipe would save weight, however.

But yeah, for a 13b, pair it off, then dump it as soon as you can in a low pressure part of the exhaust for racing. For the street, get a cutout!

SupraMikesRX7

iTrader: (1)

Right.... So, more weight reduction....
The car is super easy to roll around now.








All the sound deadening is out of the car now. The rear floor is going to be cut out and the fuel tank removed. It will be replaced by a 10 gallon fuel cell.

PB Album: http://s49.photobucket.com/albums/f268/rmikemcd/RX7/

ericgrau

iTrader: (1)

My job involves a lot of fluid engineering, so maybe I can simplify things.

There is a fixed amount of horsepower lost in the stock exhaust. Your goal is to regain part of it. Regardless of whether you go single or double, the limit is the same. A single exhaust will save metal because the rotors share it (they take turns every revolution). It only takes a small upsize of all exhaust components for a single exhaust to match a double. There is a fixed amount of horsepower lost in each exhaust (or intake) component. And even a small upsize will greatly decrease losses. There is no point to going extra-large. It is better to move on to a different component. If, however, you increase exhaust flow (more boost, etc.) then horsepower lost will greatly increase. In that case making the entire exhaust larger may be worth it.

adrock3217

While looks may not be important to all of you, my take on removing all the interior..

Sand everything down to bare metal, smooth but with a little bit of a "grit". Primer everything, wetsand the primer, and then hit it with some high-gloss paint, of whatever color you choose. Black will hide the "ugliness" of some of the lines of the interior..and its so much nicer to look at. Easy to clean, too.

GTU_FAN

I wasn't really disputing what you said. I never said your exhaust design would be inferior in performance just too loud for the street. I appreciate your clarification about what you meant by "runners". Yes in that sense my dual never collects but I assume the new design does in the single presilencer. It seems to me that I've seen exhaust designs with "cross flow" tubes from Borla and Flowmaster why haven't I seen any from japanese exhaust manufacturers?

The designs you and Nihilanthic mentioned aren't offered for the Rotary. Perhaps they should but as for everything that is sold and not custom-made RB is the only one besides Pacesetter and Mindtrain that makes headers. Everything else is just catbacks for NA's. Turbo's of course have all kinds of different manifolds, downpipes and full exhausts.

I never said the dual is the best design possible just that its the best currently made and sold. If someone made a custom exhaust with a tuned header that collected into a single pipe that might indeed be more efficient and obviously lighter. My point with the power-to-weight wasn't which exhaust is the lightest its which exhaust offers the best power-to-weight. Obviously the dual weighs more but if it makes the most power it makes up for its added weight dosen't it?

And still you haven't been able to give an example of a single-exit exhaust that is specifically designed and premade for the rotary available for sale that produces more gains then the dual. If there isn't then your advice shouldn't be dont buy a RB dual. It should be Have a custom exhaust made with these specific characteristics... blah blah blah. Otherwise its like saying the dual isn't the best exhaust on the market even though there isn't a better one being sold. Whats the point of saying that?

Obviously the research in that thread you linked is a good education for everyone to make a more informed decision about exhausts. But like you said without having a dyno available to test every design your considering you can only go off of personal experience and the manufacturers claims. Both of which are usually exaggerated.

Dont get me wrong, I'm glad someone out there has done all the research and tested things personally to prove that all the exhausts for the NA currently sold are both poorly designed and inneficient. That in itself is a great help to those who wouldn't have thought so otherwise... myself included

ericgrau

iTrader: (1)

Ok... the point of my simplification was to show that there is a fixed amount of horsepower you can get from an exhaust, no matter how good it is. You cannot get more than this, and in fact many exhausts are overkill for the N/A, whether single or double. You can hire NASA to make the best N/A exhaust imaginable, but you won't get more than a couple horsepower over a halfway decent exhaust because there is only so much horsepower available: the horsepower lost in the stock exhaust. Removing your exhaust would give you the greatest horsepower gains possible (nothing lost).

It's like buying bread. You come in with $3 for "stock bread". But then you look for something cheaper. No matter how much money you save, no matter how great of a deal you get, you won't walk out of the store with more than $3.

The goal then is to get an exhaust that comes reasonably close to the limit for a good price, light weight and reasonable noise.

GTU_FAN

Now that's good common sense.