xplikt

From information I've collected in old posts and from online, this appears to be my best setup for autocrossing.

I made this rouch sketch as an example, let me what you think.

The MBRP muffler is a SS straight-through. I have one on my truck and it produces the best note I've ever heard. I'm hoping for a good sound quality from the RX-7 too.

PPC-Racing

MBRP is great stuff, all hand made, seamless and polished till it's a mirror.. I'm an hr from their shop and have been doing business with them for years.. they are pretty big trunk/ski doo hicks but they do very sweet car exhaust parts..

One question, why go to the trouble of dual piping all the way back to the muffler, your not changing anything in terms of pressure or velocity excpet when you first fill the pipes with exhausts gas after that it's the same pressure and velocity as if you were running a single pipe to the muffler from the header flange..

if this is wrong someone put up the physics of why, it's so much easier to talk about it in those terms. Becuse from my mind except for the volume of the dual pipes, thier is no difference..

bcty

the longer you can keep the 2 pipes separate the better flow and more hp you will gain and your torque curve will be flatter... i dont know about the distances on the 2nd gen but on the 1st gen to have a perfectly balenced exhaust system they should collect 3 feet after the rear of the car

Tyler

PPC-Racing

yes you can 'tune' a header to perform but that's only for a specific , narrow band of power. I build headers for piston engines and tunning a header is a pain in the ***, but keeping primaries apart for that long is a bit odd, from a piston point of view, but regardless you still have specific volumes and flow rates. if the connection point is 3 feet from the exhaust port or 3 yards. they come together at the muffler to create a loss in the flow it looks like you were trying to gain with the twin pipes.

xplikt

The Racing Beat headers are uncollected, so the dual pipes 'should' act as part of my header, or so I've read (many times). Think of them as really long primaries.

The car is also being fully ported real soon, including a bridgeport, I just need to pick up a service manual and a couple things first.

"they should collect 3 feet after the rear of the car"

I think that's about the same as the second gen too. Takes care of tailgaters, I guess.

PPC-Racing, I have had an MBRP on my truck for about a year now and I am very happy with it. Excellent muffler, their sound and build quility are unmatched. I don't know what it'll sound like with the RX-7, so before I buy another muffler, I am just going to take the one on my truck off and put it on the RX-7.

Do you have a pricing of mufflers you could PM me?
Also, do you know what the difference in sound the 2.5" in/out, 12" long, 4" round opposed to the 6" round is?

NanaimoRx-7

"they should collect 3 feet after the rear of the car"

That would be for stock port 1st gen's which I beleive the recommended distance for collection is 120". With porting the distance begins to shorten....I can't remember exacly how much for each kind of porting right now but it does easily get that distance inside the rear bumper. Although with the flames that extra couple of feet would really help to nail those tailgaters.

PPC-Racing

I guess my brain is in piston mode, making primaries that long is suicide for any sort of torque hell even secondaries should be no more then 3" aftyer the exhaust outlet.. seems that rotaries don't take into account back pressure and excape velocity like a piston eng does.. would an X pipe bridge in the middle of that lenght help to shorten the required distances ? also how does that first gen tuning compare to say a A5 6 port? IF I'm going to make a header for my streep ported 91 NA then I want to do it right .. and it looks like taking piston/4 stroke dynamics and allying them here does not come close..

No pricing I have all mine custom built, I had them use a 4" resonator and stuff it with a straight through so I could run side pipes on my very heavily boosted zx2. chances are though, that any price you get will be cheaper then something from say .. R1 or vibrant ..EW!

from my experience the 4" round will be a sharping hitting tone.. but I don't have any expeirience with the rotaries and the MBRP...

rotarygod

iTrader: (1)

That exhaust setup is going to be perfect with a bridgeport! You'll love it!

The exhaust flow characteristics of a rotary are extremely different from that of a piston engine. A piston engines exhaust valves open much more gradually that of a rotary (excluding the Renesis). The energy released by each exhaust cycle is much less intense. Add to this that there is only exhaust flowing through that port on every other revolution of the engine. On a rotary since we are using multiple rotor faces in the same chamber we have an almost constant flow of gasses rushing through the port. The peripheral exhaust ports also open very suddenly making the pulses leaving them very intense. This is why unmuffled rotaries are so loud. The sheer difference in volume and velocity of the rotary exhaust changes the rules somewhat in how you design an exhaust in relation to a piston engine. The same general rules do apply such as how the engine power band is affected with different exhaust lengths but they are highly exaggerated. I will argure that a good exhaust on a rotary is just as fundamental to power as a good exhaust is on a 2 stroke engine. Study them and you'll understand. A piston engine may have good results with headers over stock manifolds and indeed get a horsepower gain but you'll never see just an exhaust give a piston engine 30%+ more horsepower which is possible in the case of the rotary.

PPC-Racing

So, as a comparison, a streetport on a 6port would require a shorter primary do to the less extreme intake porting. But how much shorter I know in aircraft, which is where most of my rotary experience is from, they have the shortest, fastest out primaries this side of a funncar. Should I take the racig beat or similar as being the best way to go or are they the best they can do as a mass market product..

rotarygod

iTrader: (1)

Obviously depending on porting there will be different requirements for collector legth. There are always 2 really good locations from which to choose to collect a rotary exhaust. One is the short primary. The other is the long primary. The properly designed long primary typically gives a few percent more power than a properly designed short primary system. Exactly where the best point to collect it is determined by where the desired horsepower peak is. Typically the more radical the porting then the shorter the system is going to be. An uncollected system (collected at infinity) works fantastic on a stock port car but on a peripheral port car a stock RX-7 cast manifold make more power! A long primary (optimally designed) exhaust will collect farther back on a stock port than it will on a streetport or bridgeport. You get the idea. The only way to detemine to best point is to obviously build the header on a dyno.

Racing Beat makes some of the best readily available mass produced stuff in terms of durability but it leaves much to be desired when it comes to optimal. Headers should never turn immediately out of the engine like theirs and everyone elses does. The Racing Beat stuff is very heavy but built to last. Their collected header is too long to be a proper short primary on any style of porting and too short to be a long primary. They know this but they just don't publish it. They build their headers around what is easiest to construct and still be a good upgrade. Damn near anything is better than a stock exhaust. Many people use their stuff though and love it so base your own opinions on what you find out through research.

xplikt

Minor modification. Has oneone had any experience with megaphones, Racing Beat's in particular. I'm considering just forgetting the muffler and y-pipe and going to Racing Beats collector-megaphone pipe.

rotarygod

iTrader: (1)

A megaphone works fantastic on the exhaust. It is possible to get the benefits of a megaphone but still have a muffler unless you just like screaming, deafening loud rotaries At the point where the exhaust collector is, it is a good idea to use some sort of megaphone or expansion with which to expand the area of the exhaust to. The SCCA spec-7 cars use such a setup. They are required to use the road race header from Racing Beat but the only other stipulation is that the exhaust can be no larger than 3" through a muffler. After the header they immediately collect the exhaust and then gradually expand the pipe to 3" with a megaphone. It is then sent back to a 3" straight through 3rd gen. muffler. Many people argue that 3" is too big for an exhaust on an n/a and if we were strictly talking airflow potential then that may be true. Go look at the properly designed headers for 12A's on Paul Yaw's website. They are for stock port 12A's that just use a wing which extends into the port opening to reduce turbulence. His pipes collect fairly close to the engine but they collect and then the area expands very rapidly to a total of 3" in size. This is for a stock port 12A! There is a phenomenon called acoustic ramcharging in addition to inertial ramcharging. While these are normally phenomenon which are referred to in intake design it is also an important part of exhaust design. The expansion in exhaust area acoustically tunes the exhaust differently by tuning not to just one frequency but to a high frequency, a lower frequency, and everything in between. You need to understand horn speaker acoustics to fully grasp this. Remember acoustic sound is energy too and exhaust energy is what we are trying to use to its fullest potential. The other benefit of the expansion is that we are changing the internal pressure inside the pipe. The air is slowing down as the area expands. The best exhaust would be one that is infinitelly long and constantly expands in area from the port area to an infinitely large size at the rate of a few degrees. Obviously we can't do this. Our goal is to get the exhaust to mix with the outside air as smoothly and efficiently as possible. Go for the megaphone. The rate at which it expands makes a difference as well but I say just weld an exhaust pipe on the end of it and run it through a really big muffler. The air will be going so slow through here that the muffler won't be a very big restriction. I believe that megaphone expands to a 4" size. A 4" exhaust and your engine won't know it because you fooled it with the megaphone.

Rx7carl

Great info! Lemme ask a question here. If Yaw is designing a short primary header for stock port motors, but uncollected makes more power, what gives? Im confused? It seems the new wave in N/A race exh is going to short primaries, while the old school said go long. WTF?????

rotarygod

iTrader: (1)

His design is based around a certain racing class that only uses stock ports and 12A's.

Uncollected doesn't always make more power. As I stated earlier it is only best suited for stock port engines or ported engines with no increase in port overlap. The greater the porting the more important a collected exhaust becomes due to scavenging. On an engine with port overlap, the exhaust port and the intake ports are "open" at the same time. If you have a really efficient scavenging exhaust then one side of the engine will help "pull" the exhaust gasses from the other rotor. When this happens, the low pressure on the exhaust side that is getting "pulled" is actually helping to pull in more air on the intake side. This increases the engines overall volumetric efficiency in the range where this happens best. While our engines aren't 2 cycles, the 2 cycle engines rely heavily on this phenomenon. Those guys know how to properly design an exhaust!

With a true dual there is NO exhaust scavenging. Since a degree of backpressure is important to an engines tuning the true dual has its best effect at lower rpms. I know that Mazdatrix shows a dynochart that has the engine gaining power with the true duals all the way to redline but you have to understand that Mazda had just about the worst possible exhaust manifold on those cars and anything is better. When I had the true dual setup I noticed an impressive gain in power over the stock exhaust. Low end was fantastic but it slowly subsided the higher the rpms went. When I've had collected systems (like I do now), the low end isn't quite as good as with the duals but the top end has always been killer. If I were just going to drive around town all day or do autocross on a stock port motor I'd probably do the duals. For real performance I like the collected (long primary).

xplikt

Thank you for that excellent response. The admin might want to archive this one. It seems we are verging on the realms of bike and atv exhausts. It reminds me a lot of expansion chambers and reverse cones.

Maybe one day we'll be as advanced as bikes, haha.

KneeDragonR1

How so?

Scavenging is only waves. They hit the something and bounce back. This could be the collector or it could be the end of the pipe. If you tuned taking into consideration the length of the entire pipe it is easy. You would probably tune for a later wave inflection, but the principal is still the same.



KneeDragonR1

Rx7carl

So what would be best for an N/A stock port racing engine for road racing type applications? Thats what Yaw is designed for right? Im thinking optimized for mid-hi rpm operation, sacrificing low end since its not that important.

rotarygod

iTrader: (1)

Scavenging is waves, this is correct. However, you are thinking only of acoustic sound waves which do bounce back and forth (at the speed of sound). There are also direct pressure waves from the exhaust gasses themselves (traveling at the speed of the exiting exhaust gasses). These are the pulses we can see. In a true dual setup there is no pressure wave scavenging from the other rotor. In between each exhaust pulse from one rotor is the exhaust pulse from the other rotor. With a dual exhaust these never interact. With a properly collected exhaust, the opposite side pulse will fill in the gap between the other rotors pulses and help maintain exhaust velocity through the system. Each side helps the other sides gasses to keep moving and in essence pulls the other sides gasses out. Depending on the length of the primary tubes the effect can be more pronounced at different rpms. Acoustic waves also interact with the other side in a collected system but the main bassis for collecting the pipes is to keep inertia through the exhaust high.

If you are running a stock port 6 port engine that is retaining those terrible exhaust sleeves with the insert you are at a performance disadvantage (unless everyone else is using the same thing). You would be pretty limited to just using a proper length header of 2" internal diameter tubing exiting straight out of the engine for at least 3" before turning. Collect somewhere between 20-24 inches for a short primary or around 120" for a long primary (good luck getting this under the car!) to a 2 1/2" collector. Attach this to a meagphone and expand to at least 3" and run through a straight through muffler. Either build out of stainless or get it ceramic coated. Heat is energy too and we want to keep as much energy as possible inside the exhaust. This will work much better than a Racing Beat header but this is still only a general idea on what would be close to optimal. For the best results you'd need a dyno and alot of development time.

xplikt

I am seriously liking the idea of using a megaphone. I know I can get a 3.5" SS straight through from MBRP, so I can just cut the megaphone a bit short. The other possibility would be to just exit from the megaphone right out back (end of the exhaust system).

I am thinking the megaphone will be too loud -- even for Montana. So, I'll be nice a put a muffler on there.

Looks like I got a sound plan for an exhaust. Now, I just need to take a trip to Spokane and have a competent welder give me a hand.

The engine is stock, but it wont be very much longer. I am currently in the process of removing all the emissions garbage off the vehicle. Next week I am looking at porting the TB and manifold. After that it's pulleys and engine porting.

I wont be able to make it down there untill later this month, but that's not too far away.

Thanks for the help guys. Any other small things I should be away of?

KneeDragonR1

You can tune exhaust and/or intake to use these pressure waves. See the 2nd gen dynamic intake.

Do an internet query. I do not remember the exact formula to determine the optimal length to tune for exhaust pulse scavenging.

You plug in rpm, exhaust tube length, speed of sound, and duration.

Typically you tune for the 3rd or 4th wave reflection at a desired rpm for headers.



KneeDragonR1

KneeDragonR1

xplikt

Nevermind, it's not going to be for another month. I have a BMW driving school I'm looking to attend next month. No time to take the entire engine apart and back together on my schedule.

xplikt

I just ordered a 6" round 12" long 2.5" in/out MBRP muffler, road race headers with 6-port actuation and a Flowmaster Scavenger Series dual 2" to 2.5" y-pipe. I decided that I'm going to go true dual all the way from the headers and then collect right before the muffler. It should be collecting right where the pipe meets the stock muffler. The MBRP has an awesome sound that is unmatched, I am hoping for a real unique sound with the rotary. If it's any good I'll have a friend of mine video tape it. I need to get my truck's online anyhow.

Thanks for the help guys!

rotarygod

iTrader: (1)

You'll love it. I hate my short primary setup that I just made. It's going back to a long primary. The 2.5" pipe will be just fine for street use.

breesej

You may have bought the wrong tubing diameter for your "optimal" muffler and "Y" pipe.

If you do the math....
(2) 2" primaries are 6.28 sq. in.
and (1) 2.5" outlet is 4.90625 sq. in.
al loss of 1.37375 sq inches in exhaust tubing area.

ITS and ITE guys who run only stock/street ports use a long 2" primary header connected to a 2" to 3" "y" pipe/megaphone.

Good luck with your build and post your results...