2" pipes

2 1/2" pipes

3" pipes

VARIABLES:
laminar flow of a gas inside a tube

turbulent flow of a gas inside a tube

compression waves in a gas exiting through a tube

CONSTANTS:
pi = 3.14159

Circumfrence = pi x D ....also equals weight

Area of a circle = pi x R to second power..... also equals flow




a two pipe long primary exhaust system of 2" pipes
has a total Circumfrence (pi x D)
2 x 3.14 = 6.28 in 2 pipes used = 12.56 in

What about using one 3" pipe
has a total Circumfrence = pi x D
3.x 3.14 = 9.42 in

9.42" x 33% = 3.11"
9.42" + 3.11" = 12.53"

Area of a circle

2” ID = pir2 area is 3.14sq in. for 2 pipes = 6.28 sq in.

2 ½” ID = area is 1.25 x 1.25 ( pi ) = 4.906 sq in.

3” ID= area is 1.5 x 1.5 ( pi ) = 7.065 sq in

6.28sq in. x 12.5% = .785sq in.
6.28 sq in. + .785 sq in. = 7.065 sq in.


The 2 2” pipes weigh 33% more than 1 3” pipe
But the 3” pipe has 12.5% more cross sectional area

wouldnt it have been better to design a one 3" pipe exhaust all the way back instead of a 2 2" pipe design all the way back?
of course hooking up 2 presilencers means you need 2 Y connectors or the design could be to put the pre's in series to one another.


ANOTHER TOPIC
a respectible larger than stock diameter exhaust but in this example drop down to using 1 standard 2" ID tube at the inlet and outletof a presilencer. Lets assume its a good free flowing presilencer so the restricions are acutally just at the inlet and outlet of said part.
On a parrallel idea the intake gasket between the intake manifold and the intake ports of the engine does the same thing. the gasket creates a smaller passageway than the port size at that juncture.

this is a good general observational and mathematical side of this topic. But there is an art that is gained by experience in building these systems as well



anybody that has done alot of work in this area care to offer info?

 

Too much reading...so whats the real question.

 

^^word.

dude, if this is a street car youre caring WAAYY too much. id only put this much effort into it if it was an all out race car and you eneded every last HP you could get.

you wont notice that big of a difference on the street. i HIGHLY advise against 3" on a NA on the street.

 

We could start with:
1. Based on the above background I offered, wouldn’t it have been better to design a one 3" pipe exhaust all the way back instead of a 2 2" pipe design all the way back?

2. Do small lengths of restrictions in an exhaust or intake really matter

3.what are the effects of:
laminar flow of a gas inside a tube
turbulent flow of a gas inside a tube
compression waves in a gas exiting through a tube
And how can this be used to ones advantage to tune and intake or exhaust.
I guess this gets into Paul Yaws flow bench testing abit.

To reduce variables consider only 12a or 13b stock or street ported engines without NOS or turbo issues.

 

Sometimes, smaller exhaust is installed INTENTIONALLY for daily drivability - yes, least restriciton is best for max power, but how about when you are putting around town and want that little bit more torque? Well, if you dont want to modify the engine or driveline, you make a more restricitive exhaust.

As far as the rest, most people on this board arent going to flow bench their street cars, for this, you may even try contacting a vendor, but they may not as well. Yes, a smother flowing exhaust will result in better scavenging and performance, but will you notice the possibly mere 10ths of a horsepower? If you really want a smooth tunnel, you could hydroform your exhaust, but I dont think anyone does that for the Rx-7.

 

aws140

why? the 3"pipe will weigh 33% less than two 2 " pipes in pipe material and it should flow at least 12.5% more too. ground clearance is determind by the dimension of the presilencer in that area. of the car.

just above I posted some starting questions that wackyracer asked for

 

Yeah drittens when you're equating smaller and more restrictive to more torque, you're thinking about piston engines.

Restricting a rotary's exhaust isn't going to increase torque.

two79rx7s, yes you're right. 1x3" pipe is lighter and will flow more than 2x2" pipe, however it will potentially be louder as well, and you have to consider the effect of the exhaust pulses from each combustion chamber on each other. If you try to tune your exhaust so that the trailing edge of one pulse causes a low pressure "scavenging" effect on the next pulse, I could see how a single pipe would be beneficial, but I think you'd have to take rotor phasing into consideration for that. IE: how many degrees apart are the rotors spaced. 12a vs. 20b is where you'd see a difference there (and of course the third rotor, but I digress).

Past this point, I start to get lost, so that's about as much as I've got for ya. Balancing the difference between keeping each rotor's exhaust separated vs. trying to "tune" is where I'm left wondering. With 2nd gens, Mazda went to a dual-exhaust setup. Was it for looks, or was it because the rotor phasing made the pulses run into each other at a less-than-optimal time when in a single pipe?

Jon

 

drittens more restriction + more torque? I thought less restriction = more HP and more torque.

I was asking these questions because, for example Paul Yaw went to the trouble to ask quetions about the efficency of the stock nikkei and came up with a great mod for this carberator and it made a huge difference not just 1/10ths

I dont have the members name right off hand but he is getting over 32 MPG or so because he changed the advance in the ignition and leand the overly rich carb. he made big gains here too.

as an example suppose you wanted to build a DD and always drove 65 MPH to and from work and you ran the car with a tuned intake and exhaust set for that speed
It may be possible to get great power and gas milage at the same time in that sweet spot. then you came home from work and drove around in your other RX7 swet up for drifting and flame throwing.

sorry guys by the time I post an answer someone can get there ahead of me as in the case about torque above

 

for a street car theres a couple more considerations....

1. noise! this is the biggie, its why everyone runs RB stuff (either 1x1.75id or 2x1.75id), until recently its been hard to find a straight thru muffler that actually makes the car quieter in 3".

2. space. on the pro7 cars (2.5") going under the axle vs going over it was about a 5-6hp difference. 3" is just hard to fit either way, 2x2" is easy over the axle though.... and under...

3. power. what engine are you running? on a stock 12a going from a 1.75" to a 3" doesnt really give you much power, theres other restrictions, that make a 2.5" a better choice, imo

 

viper- i read on either RB, mazdatrix, or somewhere that they did a lot of testing with the size of exhaust piping and they said when you go up to 2.5 all it does is kill the low end and doesnt really help the top end. they said with a 2" pipe the low end was much better and it still had the same power up top.

as stated above, NOISE. with 3" youre going to be loud as hell. like i said, it all depends on what you want to use this car for. so, what are you intentions for the car? DD? weekend fun? track car? what?

 

2" OD is fine on a 2 rotor. You could also go as large as 2.25".

On my 20B I'm going with three 2" into a single 2.5" collector and 2.5 all the way. The 2" primaries are because they're easier to come by than 1.75 OD in thickwall pipe (1/8"). You don't want to use thinwall on a rotary.

On the 4 rotor I'm using four 2" primaries, joining each 180° pair of rotors into a single 2" pipe for dual secondaries. That's four to two. Then these two 2" secondaries will join to a single 2.5" collector and then expand to 3" (linear taper like a megaphone).

I could go on to explain my reasons but they're posted elsewhere on this forum.

 

sorry player on that 3inch theory. If you find the right guy to do the job right going under the axle is not big deal and not a real problem. I have a three inch exhuast going under my axle. Not only that Im running the 40 series flow master in the tranny tunnel. It all looks good!! 2.5 is very restrictive no matter what motor your running. The revs suck, don't go past a certain rpm, and the performance is shot to hell. And that the problem with racingbeat exhuast systems. There made for easy installation, and sound, not performance even if it's claimed on there page.

 

I find that last sentence strange. I just installed a full RB system and it did wonders for the performance. With two people, two toolboxes and the old exhaust system in the car it was still a very noticeable difference. The revs climb extremely fast. I've slammed into redline multiple times because I wasn't used to it. I personally can't see any reason why I'd ever get rid of it. So I have to ask. Have you ever used an RB system? The restriction is usually necessary unless your sitting at the upper echelon of your rpm limits for 98% of the time. Its all application dependant. So for a car that sees far more street than track, the smaller diameter piping isn't so much an option as a necessity. You can go bigger. And most people are searching for the top end without thinking too hard about the low. But sitting at a light and slipping the clutch because you've got no low end isn't a whole lot of fun. Rotten eggs anyone? Yum yum. I'm sure going under the axle has it's advantages, but the lower the exhaust is, the more likely it is to get hit by misc. road debris. I've had air lines on my 53' trailers cut by small pieces of junk laying on the road, and they hang about a foot from the ground, minimum. On top of that you have to leave space for the axle to drop on the potholes and whatnot. So again if you don't have to worry about that kind of stuff happening. Go nuts. But alot of the people here drive street cars, that see minimal track time. We all have budgets and better things to spend our money on than fixing things that could have been done so as not to worry in the first place. But of course, to each his own. G'day folks

 

This is all fine and dandy but is lacking in some important ways. First of all worrying about size and laminar flow only is completely ignoring the benefit of the exhaust in the first place and that is with velocity in relation to flow. Too large a pipe is little velocity which means it will hurt power. Too small a pipe will hurt flow on the top end but keep low end up. Back pressure NEVER makes low end power! Ever! Smaller pipes do but it's not backpressure. It's velocity. I've gotten into this before.

You also need to realize what the exhaust ports flow in relation to the exhaust pipe. There's not really a whole lot of point on flowing more than the exhaust ports are capable of. There are some exceptions to this but I'm not going to write a book on it here. Even a streetported 13B doesn't need to have more than a 2.5" exhaust system so if you have a 12A or anything less capable than a sp 13B, you don't even need that large.

I have a set of exhaust sleeves that I make for 13B's. It hold the oval port shape all the way out of the engine and then you make a header to go with it. The area of the ports leaving the engine are equivalent to a 1 5/8" pipe each. That's it. The problem is the stock sleeves expand in area to almost that of a 2" pipe and everyone uses something larger than optimal. I've found that with the sleeves and a smaller set of primaries on the header that I can pick up power everywhere so don't think it's only about size. I don't care what women say!

 

I have two exhaust setups, first being a 2'' rb road race header - dual 2'' straight pipes - rb muffler before, perfect if you are going for no low end torque and louder then a R6 crotch rocket wot. sounds really annoying ricer to me, but has the most ear splitting backfires.

My 2nd exhaust which is on the car atm uses the above road race header - custom 2'' y-pipe - bonze hi flow 2'' cat - rb muffler. I really like this setup more then the one above in the week since I had it made, the cat quiets the engine so much more I feel like I can whine the engine out to 6-7k without alerting every cop in a five mile radius, I can drive on the freeway without instegating street races I don't even wanna participate in by every integra/focus wanna be car out there. Also with the new back pressure created by the cat the engine feels smooth and silky like a rx-8 motor.

just my .02 on the completly different exhaust I have had on my 1st gen

 

thanks for everyone input so far I have alot of information I have gathered on this but I may not have a chance to post it for a few days

 

There is no sadder sight in the world than a beautiful theory killed by a cold hard fact.

If you want the facts, then look and see what exhaust systems wins races in the ITS and ITA SCCA road racing classes.

Your initial theories also ignored collectors and the timing of where the collector is located vs rpms

 

RB exhaust does wonders for increasing power. My butt dyno detected it on the first drive, and it remains so years later. Excellent build quality, too.

Exhaust pulse tuning is equally important. RB headers are designed to ensure proper pulsing which aids scavenging, helping the engine 'breathe' properly and minimizing exhaust-go-round diluting the incoming fuel/air charge. A Dynamic Effect Chamber in the intake to tune the incoming charge, combined with proper exhaust to tune exhaust flow, does wonders.

Remember kids, fart cans != power.

 

Hello, Tom.
...and any reduction in impedence from turbulence by reinventing the wheel will yeild nill compared to the gains from proper exhaust pulse tuning; -again, only ever helpful at a very specific RPM range.